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{{short description|Science of genes, heredity, and variation in living organisms}}
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'''Genetics''' is the study of [[gene]]s, [[genetic variation]], and [[heredity]] in [[organism]]s.<ref name="griffiths2000sect60">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.60 |chapter=Genetics and the Organism: Introduction}}</ref><ref name="Hartl_and_Jones">Hartl D, Jones E (2005)</ref><ref>{{Cite web|url=https://www.dictionary.com/browse/genetics|title=the definition of genetics|website=www.dictionary.com|language=en|access-date=25 October 2018}}</ref> It is an important branch in [[biology]] because heredity is vital to organisms' [[evolution]]. [[Gregor Mendel]], a [[Moravia]]n [[Augustinians|Augustinian]] friar working in the 19th century in [[Brno]], was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

[[Phenotypic trait|Trait]] inheritance and [[Molecular genetics|molecular]] inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded to study the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the [[Cell (biology)|cell]], the organism (e.g. [[Dominance (genetics)|dominance]]), and within the context of a population. Genetics has given rise to a number of subfields, including [[molecular genetics]], [[epigenetics]], [[population genetics]], and [[paleogenetics]]. Organisms studied within the broad field span the domains of life ([[archaea]], [[bacteria]], and [[eukarya]]).

Genetic processes work in combination with an organism's environment and experiences to influence development and [[behavioural genetics|behavior]], often referred to as [[nature versus nurture]]. The [[intracellular]] or [[extracellular]] environment of a living cell or organism may increase or decrease gene transcription. A classic example is two seeds of genetically identical corn, one placed in a [[temperate climate]] and one in an [[arid climate]] (lacking sufficient waterfall or rain). While the average height the two corn stalks could grow to is genetically determined, the one in the arid climate only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment.

== Etymology ==

The word ''genetics'' stems from the [[ancient Greek]] {{lang|grc|γενετικός}} ''{{lang|grc-Latn|genetikos}}'' meaning "genitive"/"generative", which in turn derives from {{lang|grc|γένεσις}} ''{{lang|grc-Latn|genesis}}'' meaning "origin".<ref>{{cite web |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2321880&redirect=true |title=Genetikos (γενετ-ικός) |work=Henry George Liddell, Robert Scott, A Greek-English Lexicon |publisher=Perseus Digital Library, Tufts University |access-date=20 February 2012 |url-status=live |archive-url=http://webarchive.loc.gov/all/20100615000649/http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=#1980&redirect=true |archive-date=15 June 2010}}</ref><ref>{{cite web |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2321873&redirect=true |title=Genesis (γένεσις) |work=Henry George Liddell, Robert Scott, A Greek-English Lexicon |publisher=Perseus Digital Library, Tufts University |access-date=20 February 2012 |url-status=live |archive-url=http://webarchive.loc.gov/all/20100615000649/http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=#1980&redirect=true |archive-date=15 June 2010}}</ref><ref>{{cite dictionary |url=http://www.etymonline.com/index.php?search=Genetic&searchmode=none |title=Genetic |dictionary=Online Etymology Dictionary |access-date=20 February 2012 |url-status=live |archive-url=https://web.archive.org/web/20110823020616/http://www.etymonline.com/index.php?search=Genetic&searchmode=none |archive-date=23 August 2011}}</ref>

== History ==
{{Main|History of genetics}}

The observation that living things inherit [[Phenotypic trait|traits]] from their parents has been used since prehistoric times to improve crop plants and animals through [[selective breeding]].<ref name="Publishing2009">{{cite book |title=Science: The Definitive Visual Guide |url=https://books.google.com/books?id=sFiJFuzRVFQC&pg=PA362 |year=2009 |publisher=Penguin |isbn=978-0-7566-6490-9 |page=362}}</ref><ref name="Poczai and Santiago-Blay 2022a">{{cite journal | vauthors = Poczai P, Santiago-Blay JA | title = Themes of Biological Inheritance in Early Nineteenth Century Sheep Breeding as Revealed by J. M. Ehrenfels | journal = Genes | volume = 13 | issue = 8 | page = 1311 | date = July 2022 | pmid = 35893050 | pmc = 9332421 | doi = 10.3390/genes13081311 | doi-access = free }}</ref> The modern science of genetics, seeking to understand this process, began with the work of the [[Augustinians|Augustinian]] friar [[Gregor Mendel]] in the mid-19th century.<ref name=Weiling>{{cite journal | vauthors = Weiling F | title = Historical study: Johann Gregor Mendel 1822-1884 | journal = American Journal of Medical Genetics | volume = 40 | issue = 1 | pages = 1–25; discussion 26 | date = July 1991 | pmid = 1887835 | doi = 10.1002/ajmg.1320400103 }}</ref>
[[File:Festetics Imre-Oelenhainz.jpg|thumb|Portrait of [[Imre Festetics]], the first [[geneticist]] and [[Ethology|ethologist]]. His concepts of [[Selection (biology)|selection]] and [[evolution]] were later formulated in [[Natural selection|Charles Darwin's theory of evolution]].]]
Prior to Mendel, [[Imre Festetics]], a [[Hungary|Hungarian]] noble, who lived in Kőszeg before Mendel, was the first who used the word "genetic" in hereditarian context, and is considered the first geneticist. He described several rules of biological inheritance in his work ''The genetic laws of nature'' (Die genetischen Gesetze der Natur, 1819).<ref name="Poczai and Santiago-Blay 2021" /> His second law is the same as that which Mendel published.<ref name="Poczai and Szabó 2019">{{cite journal | vauthors = Szabó AT, Poczai P | title = The emergence of genetics from Festetics' sheep through Mendel's peas to Bateson's chickens | journal = Journal of Genetics | volume = 98 | issue = 2 | page = 63 | date = June 2019 | pmid = 31204695 | doi = 10.1007/s12041-019-1108-z | hdl-access = free | s2cid = 174803150 | hdl = 10138/324962 }}</ref> In his third law, he developed the basic principles of mutation (he can be considered a forerunner of [[Hugo de Vries]]).<ref>{{cite journal | vauthors = Poczai P, Bell N, Hyvönen J | title = Imre Festetics and the Sheep Breeders' Society of Moravia: Mendel's Forgotten "Research Network" | journal = PLOS Biology | volume = 12 | issue = 1 | article-number = e1001772 | date = January 2014 | pmid = 24465180 | pmc = 3897355 | doi = 10.1371/journal.pbio.1001772 | doi-access = free }}</ref> Festetics argued that changes observed in the generation of farm animals, plants, and humans are the result of scientific laws.<ref name="Poczai 2022">{{cite book | vauthors = Poczai P  |title=Heredity Before Mendel: Festetics and the Question of Sheep's Wool in Central Europe |date=2022 |publisher=CRC Press |location=Boca Raton, Florida |isbn=978-1-032-02743-2 |page=113 |edition= |url=https://books.google.com/books?id=QJRwEAAAQBAJ&dq=info:maQOFGaQPfYJ:scholar.google.com&pg=PT6 |access-date=30 August 2022}}</ref> Festetics empirically deduced that organisms inherit their characteristics, not acquire them. He recognized recessive traits and inherent variation by postulating that traits of past generations could reappear later, and organisms could produce progeny with different attributes.<ref name="Poczai et al. 2022">{{cite journal | vauthors = Poczai P, Santiago-Blay JA, Sekerák J, Bariska I, Szabó AT | title = Mimush Sheep and the Spectre of Inbreeding: Historical Background for Festetics's Organic and Genetic Laws Four Decades Before Mendel's Experiments in Peas | journal = Journal of the History of Biology | volume = 55 | issue = 3 | pages = 495–536 | date = October 2022 | pmid = 35670984 | pmc = 9668798 | doi = 10.1007/s10739-022-09678-5 | s2cid = 249433049 }}</ref> These observations represent an important prelude to Mendel's theory of particulate inheritance insofar as it features a transition of heredity from its status as myth to that of a scientific discipline, by providing a fundamental theoretical basis for genetics in the twentieth century.<ref name="Poczai and Santiago-Blay 2021">{{cite journal | vauthors = Poczai P, Santiago-Blay JA | title = Principles and biological concepts of heredity before Mendel | journal = Biology Direct | volume = 16 | issue = 1 | page = 19 | date = October 2021 | pmid = 34674746 | pmc = 8532317 | doi = 10.1186/s13062-021-00308-4 | doi-access = free }} [[File:CC-BY icon.svg|50px]] Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}.</ref><ref name="Poczai and Santiago-Blay 2022b">{{cite journal | vauthors = Poczai P, Santiago-Blay JA | title = Chip Off the Old Block: Generation, Development, and Ancestral Concepts of Heredity | journal = Frontiers in Genetics | volume = 13 | article-number = 814436 | date = 2022 | pmid = 35356423 | pmc = 8959437 | doi = 10.3389/fgene.2022.814436 | doi-access = free }}</ref>

[[File:Blending Inheritance.svg|thumb|[[Blending inheritance]] leads to the averaging out of every characteristic, which as the engineer [[Fleeming Jenkin]] pointed out, makes [[evolution]] by [[natural selection]] impossible.]]

Other theories of inheritance preceded Mendel's work. A popular theory during the 19th century, and implied by [[Charles Darwin]]'s 1859 ''[[On the Origin of Species]]'', was [[blending inheritance]]: the idea that individuals inherit a smooth blend of traits from their parents.<ref name="Hamilton2011">{{cite book | vauthors = Hamilton H |title=Population Genetics |url=https://books.google.com/books?id=ng85sd1UR7EC&pg=PT26 |year=2011 |publisher=Georgetown University |isbn=978-1-4443-6245-9 |page=26}}</ref> Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with [[Quantitative genetics|quantitative effects]]. Another theory that had some support at that time was the [[inheritance of acquired characteristics]]: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with [[Jean-Baptiste Lamarck]]) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children.<ref>Lamarck, J-B (2008). In [[Encyclopædia Britannica]]. Retrieved from [http://www.search.eb.com/eb/article-273180 Encyclopædia Britannica Online] {{Webarchive|url=https://web.archive.org/web/20200414173437/http://www.search.eb.com/eb/article-273180 |date=14 April 2020 }} on 16 March 2008.</ref> Other theories included Darwin's [[pangenesis]] (which had both acquired and inherited aspects) and [[Francis Galton]]'s reformulation of pangenesis as both particulate and inherited.<ref>[[Peter J. Bowler]], ''The Mendelian Revolution: The Emergency of Hereditarian Concepts in Modern Science and Society'' (Baltimore: Johns Hopkins University Press, 1989): chapters 2 & 3.</ref>

=== Mendelian genetics ===
[[File:Sexlinked inheritance white.jpg|thumb|Morgan's observation of [[Sex linkage|sex-linked inheritance]] of a mutation causing white eyes in ''[[Drosophila]]'' led him to the hypothesis that genes are located upon chromosomes.]]

{{main|Mendelian inheritance}}

Modern genetics started with Mendel's studies of the nature of inheritance in plants. In his paper "''Versuche über Pflanzenhybriden''" ("[[Experiments on Plant Hybridization]]"), presented in 1865 to the ''Naturforschender Verein'' (Society for Research in Nature) in [[Brno]], Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.<ref name="mendel">{{cite web |title=Mendel's Paper in English |url=http://www.mendelweb.org/Mendel.html | vauthors = Blumberg RB |url-status=live |archive-url=https://web.archive.org/web/20160113051202/http://www.mendelweb.org/Mendel.html |archive-date=13 January 2016}}</ref>

The importance of Mendel's work did not gain wide understanding until 1900, after his death, when [[Hugo de Vries]] and other scientists rediscovered his research. [[William Bateson]], a proponent of Mendel's work, coined the word ''genetics'' in 1905.<ref>genetics, ''n.'', [[Oxford English Dictionary]], 3rd ed.</ref><ref>{{cite web |url=http://www.jic.ac.uk/corporate/about/bateson.htm |title=Letter from William Bateson to Alan Sedgwick in 1905 |publisher=The John Innes Centre |access-date=15 March 2008 |vauthors=Bateson W |archive-url=https://web.archive.org/web/20071013020831/http://www.jic.ac.uk/corporate/about/bateson.htm |archive-date=13 October 2007}} The letter was to an Adam Sedgwick, a zoologist and "Reader in Animal Morphology" at [[Trinity College, Cambridge]]</ref> The adjective ''genetic'', derived from the Greek word ''genesis''—γένεσις, "origin", predates the noun and was first used in a biological sense in 1860.<ref>genetic, ''adj.'', Oxford English Dictionary, 3rd ed.</ref> Bateson both acted as a mentor and was aided significantly by the work of other scientists from Newnham College at Cambridge, specifically the work of [[Edith Rebecca Saunders|Becky Saunders]], [[Nora Darwin Barlow]], and [[Muriel Wheldale Onslow]].<ref>{{cite journal | vauthors = Richmond ML | title = Opportunities for women in early genetics | journal = Nature Reviews. Genetics | volume = 8 | issue = 11 | pages = 897–902 | date = November 2007 | pmid = 17893692 | doi = 10.1038/nrg2200 | url = http://www.nature.com/reviews/genetics | url-status = live | s2cid = 21992183 | archive-url = https://web.archive.org/web/20080516070928/http://www.nature.com/reviews/genetics/ | archive-date = 16 May 2008 | url-access = subscription }}</ref> Bateson popularized the usage of the word ''genetics'' to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in [[London]] in 1906.<ref name="bateson_genetics">{{cite conference |vauthors=Bateson W |title=The Progress of Genetic Research |editor=Wilks, W |book-title=Report of the Third 1906 International Conference on Genetics: Hybridization (the cross-breeding of genera or species), the cross-breeding of varieties, and general plant breeding|publisher=Royal Horticultural Society |location=London |year=1907}} :Initially titled the "International Conference on Hybridisation and Plant Breeding", the title was changed as a result of Bateson's speech. See: {{Cite book|vauthors=Cock AG, Forsdyke DR |year=2008|title=Treasure your exceptions: the science and life of William Bateson|url=https://archive.org/details/treasureyourexce00cock |url-access=limited |publisher=Springer|isbn=978-0-387-75687-5|page=[https://archive.org/details/treasureyourexce00cock/page/n265 248]}}</ref>

After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1900, Nettie Stevens began studying the mealworm.<ref name="net">{{cite web |title=Nettie Stevens: A Discoverer of Sex Chromosomes |url=https://www.nature.com/scitable/topicpage/nettie-stevens-a-discoverer-of-sex-chromosomes-6580266/ |website=Scitable |publisher=Nature Education |access-date=8 June 2020}}</ref> Over the next 11 years, she discovered that females only had the X chromosome and males had both X and Y chromosomes.<ref name="net" /> She was able to conclude that sex is a chromosomal factor and is determined by the male.<ref name="net" /> In 1911, [[Thomas Hunt Morgan]] argued that genes are on [[chromosome]]s, based on observations of a sex-linked [[White (mutation)|white eye]] mutation in [[Drosophila melanogaster|fruit flies]].<ref>{{cite journal |doi=10.1093/icb/23.4.855 |title=Thomas Hunt Morgan – The Geneticist |year=1983 | vauthors = Moore JA |journal=Integrative and Comparative Biology |volume=23 |pages=855–865 |issue=4|doi-access= }}</ref> In 1913, his student [[Alfred Sturtevant]] used the phenomenon of [[genetic linkage]] to show that genes are arranged linearly on the chromosome.<ref>{{cite journal |vauthors=Sturtevant AH |year=1913 |title=The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association |journal=Journal of Experimental Biology |volume=14 |issue=1 |pages=43–59 |url=http://www.esp.org/foundations/genetics/classical/holdings/s/ahs-13.pdf |doi=10.1002/jez.1400140104 |bibcode=1913JEZ....14...43S |url-status=live |archive-url=https://web.archive.org/web/20080227183131/http://www.esp.org/foundations/genetics/classical/holdings/s/ahs-13.pdf |archive-date=27 February 2008 |citeseerx=10.1.1.37.9595 |s2cid=82583173}}</ref>

=== Molecular genetics ===
{{Main|Molecular genetics}}
[[File:DNA Overview2.png|thumb|upright=0.6|[[DNA]], the molecular basis for [[Heredity|biological inheritance]]. Each strand of DNA is a chain of [[nucleotide]]s, matching each other in the center to form what look like rungs on a twisted ladder.]]

Although genes were known to exist on chromosomes, chromosomes are composed of both [[protein]] and [[DNA]], and scientists did not know which of the two is responsible for inheritance. [[Griffith's experiment|In 1928]], [[Frederick Griffith]] discovered the phenomenon of [[Transformation (genetics)|transformation]]: dead bacteria could transfer [[genetic material]] to "transform" other still-living bacteria. Sixteen years later, in 1944, the [[Avery–MacLeod–McCarty experiment]] identified DNA as the molecule responsible for transformation.<ref name=Avery_et_al>{{cite journal | vauthors = Avery OT, Macleod CM, McCarty M | title = STUDIES ON THE CHEMICAL NATURE OF THE SUBSTANCE INDUCING TRANSFORMATION OF PNEUMOCOCCAL TYPES: INDUCTION OF TRANSFORMATION BY A DESOXYRIBONUCLEIC ACID FRACTION ISOLATED FROM PNEUMOCOCCUS TYPE III | journal = The Journal of Experimental Medicine | volume = 79 | issue = 2 | pages = 137–158 | date = February 1944 | pmid = 19871359 | pmc = 2135445 | doi = 10.1084/jem.79.2.137 }} Reprint: {{cite journal | vauthors = Avery OT, MacLeod CM, McCarty M | title = Studies on the chemical nature of the substance inducing transformation of pneumococcal types. Inductions of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type III | journal = The Journal of Experimental Medicine | volume = 149 | issue = 2 | pages = 297–326 | date = February 1979 | pmid = 33226 | pmc = 2184805 | doi = 10.1084/jem.149.2.297 }}</ref> The role of the nucleus as the repository of genetic information in eukaryotes had been established by [[Joachim Hämmerling|Hämmerling]] in 1943 in his work on the single celled alga ''[[Acetabularia]]''.<ref>{{cite book |title=Cell and Molecular Biology | vauthors = Khanna P |publisher=I.K. International Pvt Ltd |date=2008 |page=221 |isbn=978-81-89866-59-4 }}</ref> The [[Hershey–Chase experiment]] in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.<ref>{{cite journal | vauthors = Hershey AD, Chase M | title = Independent functions of viral protein and nucleic acid in growth of bacteriophage | journal = The Journal of General Physiology | volume = 36 | issue = 1 | pages = 39–56 | date = May 1952 | pmid = 12981234 | pmc = 2147348 | doi = 10.1085/jgp.36.1.39 }}</ref>

[[James Watson]] and [[Francis Crick]] determined the structure of DNA in 1953, using the [[X-ray crystallography]] work of [[Rosalind Franklin]] and [[Maurice Wilkins]] that indicated DNA has a [[Helix|helical]] structure (i.e., shaped like a corkscrew).<ref>{{cite book |title=The Eighth Day of Creation: Makers of the Revolution in Biology | vauthors = Judson H |author-link=Horace Freeland Judson |year=1979 |publisher=Cold Spring Harbor Laboratory Press |isbn=978-0-87969-477-7 |pages=51–169}}</ref><ref name=watsoncrick_1953a>{{cite journal | vauthors = Watson JD, Crick FH | title = Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid | journal = Nature | volume = 171 | issue = 4356 | pages = 737–738 | date = April 1953 | pmid = 13054692 | doi = 10.1038/171737a0 | url = http://www.nature.com/nature/dna50/watsoncrick.pdf | url-status = live | s2cid = 4253007 | bibcode = 1953Natur.171..737W | archive-url = https://web.archive.org/web/20070204110320/http://www.nature.com/nature/dna50/watsoncrick.pdf | archive-date = 4 February 2007 }}</ref> Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what look like rungs on a twisted ladder.<ref name=watsoncrick_1953b>{{cite journal | vauthors = Watson JD, Crick FH | title = Genetical implications of the structure of deoxyribonucleic acid | journal = Nature | volume = 171 | issue = 4361 | pages = 964–967 | date = May 1953 | pmid = 13063483 | doi = 10.1038/171964b0 | url = http://www.nature.com/nature/dna50/watsoncrick2.pdf | url-status = live | s2cid = 4256010 | bibcode = 1953Natur.171..964W | archive-url = https://web.archive.org/web/20030621051153/http://www.nature.com/nature/dna50/watsoncrick2.pdf | archive-date = 21 June 2003 }}</ref> This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for [[DNA replication|replication]]: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand.<ref>{{cite journal | vauthors = Stratmann SA, van Oijen AM | title = DNA replication at the single-molecule level | journal = Chemical Society Reviews | volume = 43 | issue = 4 | pages = 1201–1220 | date = February 2014 | pmid = 24395040 | doi = 10.1039/c3cs60391a | url = https://pure.rug.nl/ws/files/14412201/2014ChemSocRevStratmann.pdf | url-status = live | s2cid = 205856075 | archive-url = https://web.archive.org/web/20170706055534/https://pure.rug.nl/ws/files/14412201/2014ChemSocRevStratmann.pdf | archive-date = 2017-07-06 }}</ref>

Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of [[Protein biosynthesis|protein production]].<ref name="Betz2010">{{cite book |vauthors = Frederick B |title=Managing Science: Methodology and Organization of Research |url=https://books.google.com/books?id=1ARRexcXgAgC&pg=PA76 |year=2010 |publisher=Springer |isbn=978-1-4419-7488-4 |page=76}}</ref> It was discovered that the cell uses DNA as a template to create matching [[messenger RNA]], molecules with [[nucleotide]]s very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an [[amino acid]] sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the [[genetic code]].<ref name="Rice2009">{{cite book | vauthors = Rice SA |title=Encyclopedia of Evolution |url=https://books.google.com/books?id=YRcAVvmE6eMC&pg=PA134 |year=2009 |publisher=Infobase Publishing |isbn=978-1-4381-1005-9 |page=134}}</ref>

With the newfound molecular understanding of inheritance came an explosion of research.<ref name="Sarkar1998">{{cite book | vauthors = Sarkar S |title=Genetics and Reductionism |url=https://books.google.com/books?id=7lzpDHFw-40C&pg=PA140 |year=1998 |publisher=Cambridge University Press |isbn=978-0-521-63713-8 |page=140}}</ref> A notable theory arose from [[Tomoko Ohta]] in 1973 with her amendment to the [[neutral theory of molecular evolution]] through publishing the [[nearly neutral theory of molecular evolution]]. In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic [[evolution]] occurs.<ref>{{cite journal | vauthors = Ohta T | title = Slightly deleterious mutant substitutions in evolution | journal = Nature | volume = 246 | issue = 5428 | pages = 96–98 | date = November 1973 | pmid = 4585855 | doi = 10.1038/246096a0 | s2cid = 4226804 | bibcode = 1973Natur.246...96O }}</ref> One important development was chain-termination [[DNA sequencing]] in 1977 by [[Frederick Sanger]]. This technology allows scientists to read the nucleotide sequence of a DNA molecule.<ref name=sanger_et_al>{{cite journal | vauthors = Sanger F, Nicklen S, Coulson AR | title = DNA sequencing with chain-terminating inhibitors | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 74 | issue = 12 | pages = 5463–5467 | date = December 1977 | pmid = 271968 | pmc = 431765 | doi = 10.1073/pnas.74.12.5463 | doi-access = free | bibcode = 1977PNAS...74.5463S }}</ref> In 1983, [[Kary Banks Mullis]] developed the [[polymerase chain reaction]], providing a quick way to isolate and amplify a specific section of DNA from a mixture.<ref name=saiki_et_al>{{cite journal | vauthors = Saiki RK, Scharf S, Faloona F, Mullis KB, Horn GT, Erlich HA, Arnheim N | title = Enzymatic amplification of beta-globin genomic sequences and restriction site analysis for diagnosis of sickle cell anemia | journal = Science | volume = 230 | issue = 4732 | pages = 1350–1354 | date = December 1985 | pmid = 2999980 | doi = 10.1126/science.2999980 | bibcode = 1985Sci...230.1350S }}</ref> The efforts of the [[Human Genome Project]], Department of Energy, NIH, and parallel private efforts by [[Celera Genomics]] led to the sequencing of the [[human genome]] in 2003.<ref name=human_genome_project /><ref>{{Cite journal|title=The sequence of the human genome|journal=Science|volume=291}}</ref>

== Features of inheritance ==

=== Discrete inheritance and Mendel's laws ===
{{Main|Mendelian inheritance}}
[[File:Punnett square mendel flowers.svg|right|thumb|A [[Punnett square]] depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms]]
At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called [[gene]]s, from parents to offspring.<ref name=griffiths2000sect199>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.199 |chapter=Patterns of Inheritance: Introduction}}</ref> This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in [[pea]] plants, showing for example that flowers on a single plant were either purple or white—but never an intermediate between the two colors. The discrete versions of the same gene controlling the inherited appearance (phenotypes) are called [[allele]]s.<ref name="mendel" /><ref name=griffiths2000sect200>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.200 |chapter=Mendel's experiments}}</ref>

In the case of the pea, which is a [[diploid]] species, each individual plant has two copies of each gene, one copy inherited from each parent.<ref name=griffiths2000sect484 /> Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a given gene are called [[homozygous]] at that [[Locus (genetics)|gene locus]], while organisms with two different alleles of a given gene are called [[heterozygous]]. The set of alleles for a given organism is called its [[genotype]], while the observable traits of the organism are called its [[phenotype]]. When organisms are heterozygous at a gene, often one allele is called [[Dominant allele|dominant]] as its qualities dominate the phenotype of the organism, while the other allele is called [[Recessive allele|recessive]] as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have [[Dominance relationship#Incomplete dominance|incomplete dominance]] by expressing an intermediate phenotype, or [[Dominance relationship#Co-dominance|codominance]] by expressing both alleles at once.<ref name=griffiths2000sect630>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.630 |chapter=Interactions between the alleles of one gene}}</ref>

When a pair of organisms [[Sexual reproduction|reproduce sexually]], their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as [[Mendelian inheritance#Law of segregation|Mendel's first law]] or the Law of Segregation. However, the probability of getting one gene over the other can change due to dominant, recessive, homozygous, or heterozygous genes. For example, Mendel found that if you cross heterozygous organisms your odds of getting the dominant trait is 3:1. Real geneticist study and calculate probabilities by using theoretical probabilities, empirical probabilities, the product rule, the sum rule, and more.<ref>{{Cite web |title=Probabilities in genetics (article) |url=https://www.khanacademy.org/science/ap-biology/heredity/mendelian-genetics-ap/a/probabilities-in-genetics |access-date=2022-09-28 |website=Khan Academy}}</ref>

=== Notation and diagrams ===
[[File:Pedigree-chart-example.svg|thumb|left|upright=1.1|Genetic pedigree charts help track the inheritance patterns of traits.]]

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual, [[Wild type|non-mutant allele]] for a gene.<ref>{{cite web|url=http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm|title=Genetic Notation| vauthors = Cheney RW |publisher=Christopher Newport University|access-date=18 March 2008|archive-url=https://web.archive.org/web/20080103021518/http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm|archive-date=3 January 2008}}</ref>

In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the [[Punnett square]].<ref name="Müller-Wille-Parolini_2020">{{cite book |chapter=Punnett squares and hybrid crosses: how Mendelians learned their trade by the book | vauthors = Müller-Wille S, Parolini G |publisher=[[British Society for the History of Science]] / [[Cambridge University Press]] |date=2020-12-09 |series=BJHS Themes |volume=5 |title=Learning by the Book: Manuals and Handbooks in the History of Science |pages=149–165 |doi=10.1017/bjt.2020.12 |s2cid=229344415 |url=https://www.cambridge.org/core/journals/bjhs-themes/article/punnett-squares-and-hybrid-crosses-how-mendelians-learned-their-trade-by-the-book/18A1CE37A6EE536CC1CE1D4FF6FF3174 |access-date=2021-03-29 |url-status=live |archive-url=https://web.archive.org/web/20210329111650/https://www.cambridge.org/core/journals/bjhs-themes/article/punnett-squares-and-hybrid-crosses-how-mendelians-learned-their-trade-by-the-book/18A1CE37A6EE536CC1CE1D4FF6FF3174 |archive-date=2021-03-29}}</ref>

When studying human genetic diseases, geneticists often use [[pedigree chart]]s to represent the inheritance of traits.<ref name=griffiths2000sect229>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.229 |chapter=Human Genetics}}</ref> These charts map the inheritance of a trait in a family tree.

=== Multiple gene interactions ===
[[File:Galton-height-regress.png|thumb|right|Human height is a trait with complex genetic causes. [[Francis Galton]]'s data from 1889 shows the relationship between offspring height as a function of mean parent height.]]

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "[[Mendelian inheritance#Law of Independent Assortment (The "Second Law")|Mendel's second law]]{{Broken anchor|date=2025-07-23|bot=User:Cewbot/log/20201008/configuration|target_link=Mendelian inheritance#Law of Independent Assortment (The "Second Law")|reason= The anchor (Law of Independent Assortment (The "Second Law")) [[Special:Diff/604468613|has been deleted]].|diff_id=604468613}}" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Different genes often interact to influence the same trait. In the [[Omphalodes verna|Blue-eyed Mary]] (''Omphalodes verna''), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called [[epistasis]], with the second gene epistatic to the first.<ref name=griffiths2000sect644>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.644 |chapter=Gene interaction and modified dihybrid ratios}}</ref>

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and [[Human skin color|skin color]]). These [[complex traits]] are products of many genes.<ref>{{cite journal | vauthors = Mayeux R | title = Mapping the new frontier: complex genetic disorders | journal = The Journal of Clinical Investigation | volume = 115 | issue = 6 | pages = 1404–1407 | date = June 2005 | pmid = 15931374 | pmc = 1137013 | doi = 10.1172/JCI25421 }}</ref> The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called [[heritability]].<ref name=griffiths2000sect4009>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.4009 |chapter=Quantifying heritability}}</ref> Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and [[health care]], height has a heritability of only 62%.<ref>{{cite journal | vauthors = Luke A, Guo X, Adeyemo AA, Wilks R, Forrester T, Lowe W, Comuzzie AG, Martin LJ, Zhu X, Rotimi CN, Cooper RS | title = Heritability of obesity-related traits among Nigerians, Jamaicans and US black people | journal = International Journal of Obesity and Related Metabolic Disorders | volume = 25 | issue = 7 | pages = 1034–1041 | date = July 2001 | pmid = 11443503 | doi = 10.1038/sj.ijo.0801650 | doi-access = free }}</ref>

== Molecular basis for inheritance ==

=== DNA and chromosomes ===
{{Main|DNA|Chromosome}}

[[File:DNA chemical structure.svg|thumb|right|The [[molecular structure]] of DNA. Bases pair through the arrangement of [[hydrogen bonding]] between the strands.]]The [[Molecule|molecular]] basis for genes is [[deoxyribonucleic acid]] (DNA). DNA is composed of [[deoxyribose]] (sugar molecule), a phosphate group, and a base (amine group). There are four types of bases: [[adenine]] (A), [[cytosine]] (C), [[guanine]] (G), and [[thymine]] (T). The phosphates make phosphodiester bonds with the sugars to make long phosphate-sugar backbones. Bases specifically pair together (T&A, C&G) between two backbones and make like rungs on a ladder. The bases, phosphates, and sugars together make a [[nucleotide]] that connects to make long chains of DNA.<ref>{{Cite web | vauthors = Urry L, Cain M, Wasserman S, Minorsky P, Reece J, Campbell N |title=Campbell Biology |url=https://plus.pearson.com/courses/gregg91165/products/GTP1DPWIL20/pages/ac865b14db19976dfd6054de245cd8d8e65000756?locale=&key=2790626781132109428282022&iesCode=5VEW6xrTXI |access-date=2022-09-28 |website=plus.pearson.com}}</ref> Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.<ref name=Pearson_2006>{{cite journal | vauthors = Pearson H | title = Genetics: what is a gene? | journal = Nature | volume = 441 | issue = 7092 | pages = 398–401 | date = May 2006 | pmid = 16724031 | doi = 10.1038/441398a | s2cid = 4420674 | doi-access = free | bibcode = 2006Natur.441..398P }}</ref> These chains coil into a double a-helix structure and wrap around proteins called [[Histone]]s which provide the structural support. DNA wrapped around these histones are called chromosomes.<ref>{{Cite web |title=Histone |url=https://www.genome.gov/genetics-glossary/histone |access-date=2022-09-28 |website=Genome.gov |language=en}}</ref> [[Virus]]es sometimes use the similar molecule [[RNA]] instead of DNA as their genetic material.<ref>{{cite book |title=Microbiology |vauthors=Prescott LM, Harley JP, Klein DA |year=1996 |publisher=Wm. C. Brown |edition=3rd |isbn=0-697-21865-1 |url=https://archive.org/details/microbiology0000pres/page/342/mode/2up |page=343}}</ref>

DNA normally exists as a double-stranded molecule, coiled into the shape of a [[double helix]]. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.<ref name=griffiths2000sect1523>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1523 |chapter=Mechanism of DNA Replication}}</ref>

[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, showing 22 [[homologous chromosome]] pairs, both the female (XX) and male (XY) versions of the [[sex chromosome]] (bottom right), as well as the [[human mitochondrial genetics|mitochondrial genome]] (at bottom left) {{further|Karyotype}}]]
Genes are arranged linearly along long chains of DNA base-pair sequences. In [[bacteria]], each cell usually contains a single circular [[Nucleoid|genophore]], while [[Eukaryote|eukaryotic]] organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million [[base pair]]s in length.<ref>{{cite journal | vauthors = Gregory SG, Barlow KF, McLay KE, Kaul R, Swarbreck D, Dunham A, Scott CE, Howe KL, Woodfine K, Spencer CC, Jones MC, Gillson C, Searle S, Zhou Y, Kokocinski F, McDonald L, Evans R, Phillips K, Atkinson A, Cooper R, Jones C, Hall RE, Andrews TD, Lloyd C, Ainscough R, Almeida JP, Ambrose KD, Anderson F, Andrew RW, Ashwell RI, Aubin K, Babbage AK, Bagguley CL, Bailey J, Beasley H, Bethel G, Bird CP, Bray-Allen S, Brown JY, Brown AJ, Buckley D, Burton J, Bye J, Carder C, Chapman JC, Clark SY, Clarke G, Clee C, Cobley V, Collier RE, Corby N, Coville GJ, Davies J, Deadman R, Dunn M, Earthrowl M, Ellington AG, Errington H, Frankish A, Frankland J, French L, Garner P, Garnett J, Gay L, Ghori MR, Gibson R, Gilby LM, Gillett W, Glithero RJ, Grafham DV, Griffiths C, Griffiths-Jones S, Grocock R, Hammond S, Harrison ES, Hart E, Haugen E, Heath PD, Holmes S, Holt K, Howden PJ, Hunt AR, Hunt SE, Hunter G, Isherwood J, James R, Johnson C, Johnson D, Joy A, Kay M, Kershaw JK, Kibukawa M, Kimberley AM, King A, Knights AJ, Lad H, Laird G, Lawlor S, Leongamornlert DA, Lloyd DM, Loveland J, Lovell J, Lush MJ, Lyne R, Martin S, Mashreghi-Mohammadi M, Matthews L, Matthews NS, McLaren S, Milne S, Mistry S, Moore MJ, Nickerson T, O'Dell CN, Oliver K, Palmeiri A, Palmer SA, Parker A, Patel D, Pearce AV, Peck AI, Pelan S, Phelps K, Phillimore BJ, Plumb R, Rajan J, Raymond C, Rouse G, Saenphimmachak C, Sehra HK, Sheridan E, Shownkeen R, Sims S, Skuce CD, Smith M, Steward C, Subramanian S, Sycamore N, Tracey A, Tromans A, Van Helmond Z, Wall M, Wallis JM, White S, Whitehead SL, Wilkinson JE, Willey DL, Williams H, Wilming L, Wray PW, Wu Z, Coulson A, Vaudin M, Sulston JE, Durbin R, Hubbard T, Wooster R, Dunham I, Carter NP, McVean G, Ross MT, Harrow J, Olson MV, Beck S, Rogers J, Bentley DR, Banerjee R, Bryant SP, Burford DC, Burrill WD, Clegg SM, Dhami P, Dovey O, Faulkner LM, Gribble SM, Langford CF, Pandian RD, Porter KM, Prigmore E | title = The DNA sequence and biological annotation of human chromosome 1 | journal = Nature | volume = 441 | issue = 7091 | pages = 315–321 | date = May 2006 | pmid = 16710414 | doi = 10.1038/nature04727 | doi-access = free | bibcode = 2006Natur.441..315G }}</ref> The DNA of a [[chromosome]] is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called [[chromatin]]; in eukaryotes, chromatin is usually composed of [[nucleosome]]s, segments of DNA wound around cores of [[histone]] proteins.<ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/NBK26834/ II.4. DNA and chromosomes: Chromosomal DNA and Its Packaging in the Chromatin Fiber] </ref> The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the [[genome]].

DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus.<ref name="ruth">{{cite web |title=Ruth Sager |url=https://www.britannica.com/biography/Ruth-Sager |website=Encyclopaedia Britannica |access-date=8 June 2020}}</ref> In plants, these are often found in the chloroplasts and in other organisms, in the mitochondria.<ref name="ruth" /> These nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations.<ref name="ruth"/>

While [[haploid]] organisms have only one copy of each chromosome, most animals and many plants are [[diploid]], containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical [[Locus (genetics)|loci]] of the two [[homologous chromosomes]], each allele inherited from a different parent.<ref name=griffiths2000sect484>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.484 |chapter=Mendelian genetics in eukaryotic life cycles}}</ref>

Many species have so-called [[sex chromosome]]s that determine the sex of each organism.<ref name="griffiths2000sect222">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.222 |chapter=Sex chromosomes and sex-linked inheritance}}</ref> In humans and many other animals, the [[Y chromosome]] contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the [[X chromosome]] is similar to the other chromosomes and contains many genes. This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring.<ref name="lyon">{{cite journal | vauthors = Rastan S | title = Mary F. Lyon (1925-2014) | journal = Nature | volume = 518 | issue = 7537 | page = 36 | date = February 2015 | pmid = 25652989 | doi = 10.1038/518036a | publisher = Springer Nature Limited | s2cid = 4405984 | bibcode = 2015Natur.518...36R | doi-access = free }}</ref> Lyon's discovery led to the discovery of X-linked diseases.<ref name="lyon" />

=== Reproduction ===
{{Main|Asexual reproduction|Sexual reproduction}}

[[File:Zellsubstanz-Kern-Kerntheilung.jpg|thumb|left|[[Walther Flemming]]'s 1882 diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome copies separate into the daughter cells.]]

When cells divide, their full genome is copied and each [[Cell division|daughter cell]] inherits one copy. This process, called [[mitosis]], is the simplest form of reproduction and is the basis for asexual reproduction. Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called [[Cloning|clones]].<ref>{{cite web |url=https://www.merriam-webster.com/dictionary/clone |title= clone|author=<!--Not stated--> |date= |website=Merriam-Webster Dictionary |publisher= |access-date=13 November 2023 |quote=}}</ref>

[[Eukaryote|Eukaryotic]] organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome ([[haploid]]) and double copies ([[diploid]]).<ref name=griffiths2000sect484 /> Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell [[gamete]]s such as [[sperm]] or [[Ovum|eggs]].<ref>{{Cite web |title=Haploid |url=https://www.genome.gov/genetics-glossary/haploid |access-date=2024-02-10 |website=www.genome.gov |language=en}}</ref>

Although they do not use the haploid/diploid method of sexual reproduction, [[bacteria]] have many methods of acquiring new genetic information. Some bacteria can undergo [[Bacterial conjugation|conjugation]], transferring a small circular piece of DNA to another bacterium.<ref name="griffiths2000sect1304">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1304 |chapter=Bacterial conjugation}}</ref> Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as [[Transformation (genetics)|transformation]].<ref name="griffiths2000sect1343">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1343 |chapter=Bacterial transformation}}</ref> These processes result in [[horizontal gene transfer]], transmitting fragments of genetic information between organisms that would be otherwise unrelated. [[Transformation (genetics)|Natural bacterial transformation]] occurs in many [[bacteria]]l species, and can be regarded as a [[sexual reproduction|sexual process]] for transferring DNA from one cell to another cell (usually of the same species).<ref name=Bernstein2018>{{cite journal | vauthors = Bernstein H, Bernstein C, Michod RE | title = Sex in microbial pathogens | journal = Infection, Genetics and Evolution | volume = 57 | pages = 8–25 | date = January 2018 | pmid = 29111273 | doi = 10.1016/j.meegid.2017.10.024 | doi-access = free | bibcode = 2018InfGE..57....8B }}</ref> Transformation requires the action of numerous bacterial [[gene product]]s, and its primary adaptive function appears to be [[DNA repair|repair]] of [[DNA damage (naturally occurring)|DNA damages]] in the recipient cell.<ref name=Bernstein2018 />

=== Recombination and genetic linkage ===
{{Main|Chromosomal crossover|Genetic linkage}}

[[File:Morgan crossover 2 cropped.png|thumb|right|[[Thomas Hunt Morgan]]'s 1916 illustration of a double crossover between chromosomes]]

The diploid nature of chromosomes allows for genes on different chromosomes to [[independent assortment|assort independently]] or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of [[chromosomal crossover]]. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes.<ref name="griffiths2000sect929">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbar |title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.929 |chapter=Nature of crossing-over}}</ref> This process of chromosomal crossover generally occurs during [[meiosis]], a series of cell divisions that creates haploid cells. [[Origin and function of meiosis|Meiotic recombination]], particularly in microbial [[eukaryote]]s, appears to serve the adaptive function of repair of DNA damages.<ref name=Bernstein2018/>

The first cytological demonstration of crossing over was performed by Harriet Creighton and [[Barbara McClintock]] in 1931. Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other.<ref>{{cite journal | vauthors = Creighton HB, McClintock B | title = A Correlation of Cytological and Genetical Crossing-Over in Zea Mays | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 17 | issue = 8 | pages = 492–497 | date = August 1931 | pmid = 16587654 | pmc = 1076098 | doi = 10.1073/pnas.17.8.492 | doi-access = free | bibcode = 1931PNAS...17..492C }}</ref>

The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated.<ref name="Staub1994">{{cite book | vauthors = Staub JE |title=Crossover: Concepts and Applications in Genetics, Evolution, and Breeding |url=https://books.google.com/books?id=R43qWg5A-GsC&pg=PA55 |year=1994 |publisher=University of Wisconsin Press |isbn=978-0-299-13564-5 |page=55}}</ref> For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear [[Genetic linkage#Linkage map|linkage map]] that roughly describes the arrangement of the genes along the chromosome.<ref name="griffiths2000sect899">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbar |title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.899 |chapter=Linkage maps}}</ref>

== Gene expression ==

=== Genetic code ===
{{Main|Genetic code}}

[[File:Genetic code.svg|class=skin-invert-image|thumb|left|upright=1.3|The [[genetic code]]: Using a [[Genetic code#Discovery|triplet code]], DNA, through a [[messenger RNA]] intermediary, specifies a protein.]]

Genes [[Gene expression|express]] their functional effect through the production of proteins, which are molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each composed of a sequence of [[amino acid]]s. The DNA sequence of a gene is used to produce a specific [[Protein primary structure|amino acid sequence]]. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called [[Transcription (genetics)|transcription]].

This [[messenger RNA]] molecule then serves to produce a corresponding amino acid sequence through a process called [[translation (biology)|translation]]. Each group of three nucleotides in the sequence, called a [[codon]], corresponds either to one of the twenty possible amino acids in a protein or an [[stop codon|instruction to end the amino acid sequence]]; this correspondence is called the [[genetic code]].<ref>{{cite book |title=Biochemistry |vauthors=Berg JM, Tymoczko JL, Stryer L, Clarke ND |edition=5th |year=2002 |publisher=W.H. Freeman and Company |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.685 |chapter=I. 5. DNA, RNA, and the Flow of Genetic Information: Amino Acids Are Encoded by Groups of Three Bases Starting from a Fixed Point |url-status=live |archive-url=https://web.archive.org/web/20060411095303/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.685 |archive-date=11 April 2006}}</ref> The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNA—a phenomenon [[Francis Crick]] called the [[central dogma of molecular biology]].<ref name="crick1970">{{cite journal | vauthors = Crick F | title = Central dogma of molecular biology | journal = Nature | volume = 227 | issue = 5258 | pages = 561–563 | date = August 1970 | pmid = 4913914 | doi = 10.1038/227561a0 | url = http://www.nature.com/nature/focus/crick/pdf/crick227.pdf | url-status = live | s2cid = 4164029 | bibcode = 1970Natur.227..561C | archive-url = https://web.archive.org/web/20060215024341/http://www.nature.com/nature/focus/crick/pdf/crick227.pdf | archive-date = 15 February 2006 }}</ref>

The specific sequence of amino acids [[protein folding|results]] in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions.<ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/NBK26830/ I.3. Proteins: The Shape and Structure of Proteins] </ref><ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/NBK26911/ I.3. Proteins: Protein Function] </ref> Some are simple structural molecules, like the fibers formed by the protein [[collagen]]. Proteins can bind to other proteins and simple molecules, sometimes acting as [[enzyme]]s by facilitating [[chemical reaction]]s within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein [[hemoglobin]] bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood.<ref>{{cite web |last1=Dutta |first1=Shuchismita |last2=Goodsell |first2=David |title=PDB101: Molecule of the Month: Hemoglobin |url=https://pdb101.rcsb.org/motm/41 |website=RCSB: PDB-101 |access-date=31 July 2025}}</ref>

A [[Single-nucleotide polymorphism|single nucleotide difference]] within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example, [[sickle-cell anemia]] is a human [[Genetic disorder|genetic disease]] that results from a single base difference within the [[coding region]] for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties.<ref>{{cite web |title=How Does Sickle Cell Cause Disease? |url=http://sickle.bwh.harvard.edu/scd_background.html |date=11 April 2002 |access-date=23 July 2007 |publisher=Brigham and Women's Hospital: Information Center for Sickle Cell and Thalassemic Disorders |url-status=live |archive-url=https://web.archive.org/web/20100923165921/http://sickle.bwh.harvard.edu/scd_background.html |archive-date=23 September 2010}}</ref>
Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of [[red blood cell]]s carrying the protein. These sickle-shaped cells no longer flow smoothly through [[blood vessel]]s, having a tendency to clog or degrade, causing the medical problems associated with this disease.<ref>{{Cite journal |last1=Elendu |first1=Chukwuka |last2=Amaechi |first2=Dependable C. |last3=Alakwe-Ojimba |first3=Chisom E. |last4=Elendu |first4=Tochi C. |last5=Elendu |first5=Rhoda C. |last6=Ayabazu |first6=Chiagozie P. |last7=Aina |first7=Titilayo O. |last8=Aborisade |first8=Ooreofe |last9=Adenikinju |first9=Joseph S. |date=2023-09-22 |title=Understanding Sickle cell disease: Causes, symptoms, and treatment options |journal=Medicine |language=en |volume=102 |issue=38 |article-number=e35237 |doi=10.1097/MD.0000000000035237 |issn=0025-7974 |pmc=10519513 |pmid=37746969}}</ref>

Some DNA sequences are transcribed into RNA but are not translated into protein products—such RNA molecules are called [[non-coding RNA]]. In some cases, these products fold into structures which are involved in critical cell functions (e.g. [[ribosomal RNA]] and [[transfer RNA]]). RNA can also have regulatory effects through hybridization interactions with other RNA molecules (such as [[microRNA]]).<ref>{{Cite journal |last1=Marques |first1=Tânia Monteiro |last2=Gama-Carvalho |first2=Margarida |date=2022-02-19 |title=Network Approaches to Study Endogenous RNA Competition and Its Impact on Tissue-Specific microRNA Functions |journal=Biomolecules |language=en |volume=12 |issue=2 |page=332 |doi=10.3390/biom12020332 |doi-access=free |pmid=35204832 |pmc=8868585 |issn=2218-273X }}</ref>

=== Nature and nurture ===
{{Main|Nature and nurture}}

[[File:Niobe050905-Siamese Cat.jpeg|thumb|upright|[[Siamese (cat)|Siamese cats]] have a temperature-sensitive pigment-production mutation.]]

Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. The phrase "[[nature and nurture]]" refers to this complementary relationship. The phenotype of an organism depends on the interaction of genes and the environment. An interesting example is the coat coloration of the [[Siamese (cat)|Siamese cat]]. In this case, the body temperature of the cat plays the role of the environment. The cat's genes code for dark hair, thus the hair-producing cells in the cat make cellular proteins resulting in dark hair. But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and [[Denaturation (biochemistry)|denature]] in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. The protein remains functional in areas of skin that are colder—such as its legs, ears, tail, and face{{emdash}}so the cat has dark hair at its extremities.<ref>{{cite journal | vauthors = Imes DL, Geary LA, Grahn RA, Lyons LA | title = Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation | journal = Animal Genetics | volume = 37 | issue = 2 | pages = 175–178 | date = April 2006 | pmid = 16573534 | pmc = 1464423 | doi = 10.1111/j.1365-2052.2005.01409.x }}</ref>

Environment plays a major role in effects of the human genetic disease [[phenylketonuria]]. The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid [[phenylalanine]], causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive intellectual disability and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy.<ref>{{cite web |url=https://www.medlineplus.gov/phenylketonuria.html |title=MedlinePlus: Phenylketonuria |access-date=15 March 2008 |publisher=NIH: National Library of Medicine |url-status=live |archive-url=https://web.archive.org/web/20080725183720/http://www.nlm.nih.gov/medlineplus/phenylketonuria.html |archive-date=25 July 2008}}</ref>

A common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves [[twin study|studying identical and fraternal twins]], or other siblings of [[multiple birth]]s.<ref>For example, {{cite book |title=Nature via Nurture: Genes, Experience and What Makes Us Human |vauthors=Ridley M |publisher=Fourth Estate |year=2003|isbn= 978-1-84115-745-0 |page=73}}</ref> Identical siblings are genetically the same since they come from the same zygote. Meanwhile, fraternal twins are as genetically different from one another as normal siblings. By comparing how often a certain disorder occurs in a pair of identical twins to how often it occurs in a pair of fraternal twins, scientists can determine whether that disorder is caused by genetic or postnatal environmental factors. One famous example involved the study of the [[Genain quadruplets]], who were [[Multiple birth|identical quadruplets]] all diagnosed with [[schizophrenia]].<ref name="Genain">{{Cite journal |title=The Genain Quadruplets: A Case Study and Theoretical Analysis of Heredity and Environment in Schizophrenia |journal=Behavioral Science |volume=9 |issue=4 | vauthors = Rosenthal D |year=1964 |page=371 |doi=10.1002/bs.3830090407 }}</ref>

=== Gene regulation ===
{{Main|Regulation of gene expression}}

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. [[Transcription factor]]s are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene.<ref>{{cite journal | vauthors = Brivanlou AH, Darnell JE | title = Signal transduction and the control of gene expression | journal = Science | volume = 295 | issue = 5556 | pages = 813–818 | date = February 2002 | pmid = 11823631 | doi = 10.1126/science.1066355 | s2cid = 14954195 | citeseerx = 10.1.1.485.6042 | bibcode = 2002Sci...295..813B }}</ref> Within the genome of ''[[Escherichia coli]]'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid [[tryptophan]]. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the [[Trp repressor|tryptophan repressor]] (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating [[negative feedback]] regulation of the tryptophan synthesis process.<ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/NBK26872/ II.3. Control of Gene Expression – The Tryptophan Repressor is a Simple Switch That Turns Genes On and Off in Bacteria] </ref>

[[File:Zinc finger DNA complex.png|thumb|upright|left|Transcription factors bind to DNA, influencing the transcription of associated genes.]]

Differences in gene expression are especially clear within [[multicellular organism]]s, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and [[Cell signaling|intercellular signals]] and gradually establishing different patterns of gene expression to create different behaviors.

Within [[eukaryote]]s, there exist structural features of [[chromatin]] that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells.<ref>{{cite journal | vauthors = Jaenisch R, Bird A | title = Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals | journal = Nature Genetics | volume = 33 | issue = Suppl | pages = 245–254 | date = March 2003 | pmid = 12610534 | doi = 10.1038/ng1089 | s2cid = 17270515 }}</ref> These features are called "[[epigenetic]]" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types [[cell culture|grown]] within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of [[paramutation]], have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.<ref>{{cite journal | vauthors = Chandler VL | title = Paramutation: from maize to mice | journal = Cell | volume = 128 | issue = 4 | pages = 641–645 | date = February 2007 | pmid = 17320501 | doi = 10.1016/j.cell.2007.02.007 | s2cid = 6928707 | doi-access = free }}</ref>
{{clear}}

== Genetic change ==

=== Mutations ===
{{Main|Mutation}}

[[File:Gene-duplication.png|thumb|upright|Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism.]]

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100&nbsp;million bases—due to the "proofreading" ability of [[DNA polymerase]]s.<ref name="griffiths2000sect2706">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2706 |chapter=Spontaneous mutations}}</ref><ref name="Kunkel">{{cite journal | vauthors = Freisinger E, Grollman AP, Miller H, Kisker C | title = Lesion (in)tolerance reveals insights into DNA replication fidelity | journal = The EMBO Journal | volume = 23 | issue = 7 | pages = 1494–1505 | date = April 2004 | pmid = 15057282 | pmc = 391067 | doi = 10.1038/sj.emboj.7600158 }}</ref> Processes that increase the rate of changes in DNA are called [[mutagenic]]: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while [[UV radiation]] induces mutations by causing damage to the DNA structure.<ref name="griffiths2000sect2727">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2727 |chapter=Induced mutations}}</ref> Chemical damage to DNA occurs naturally as well and cells use [[DNA repair]] mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence. A particularly important source of DNA damages appears to be [[reactive oxygen species]]<ref name="pmid23378590">{{cite journal | vauthors = Cadet J, Wagner JR | title = DNA base damage by reactive oxygen species, oxidizing agents, and UV radiation | journal = Cold Spring Harbor Perspectives in Biology | volume = 5 | issue = 2 | article-number = a012559 | date = February 2013 | pmid = 23378590 | pmc = 3552502 | doi = 10.1101/cshperspect.a012559 }}</ref> produced by [[cellular respiration|cellular aerobic respiration]], and these can lead to mutations.<ref name="pmid22750987">{{cite journal | vauthors = Jena NR | title = DNA damage by reactive species: Mechanisms, mutation and repair | journal = Journal of Biosciences | volume = 37 | issue = 3 | pages = 503–517 | date = July 2012 | pmid = 22750987 | doi = 10.1007/s12038-012-9218-2 | s2cid = 14837181 }}</ref>

In organisms that use [[chromosomal crossover]] to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence—[[Gene duplication|duplications]], [[Chromosomal inversion|inversions]], [[Gene deletion|deletions]] of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes, [[chromosomal translocation]].<ref name="griffiths2000sect2844">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2844 |chapter=Chromosome Mutation I: Changes in Chromosome Structure: Introduction}}</ref>[[File:Mutations.svg|thumb|This is a diagram showing mutations in an RNA sequence. Figure (1) is a normal RNA sequence, consisting of 4 codons. Figure (2) shows a missense, single point, non silent mutation. Figures (3 and 4) both show [[frameshift mutation]]s, which is why they are grouped together. Figure 3 shows a deletion of the second base pair in the second codon. Figure 4 shows an insertion in the third base pair of the second codon. Figure (5) shows a repeat expansion, where an entire codon is duplicated.]]

=== Natural selection and evolution ===

{{Main|Evolution}}
{{further|Natural selection}}

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive [[fitness (biology)|fitness]].<ref>{{cite book | vauthors = Schaechter M |title=Encyclopedia of Microbiology |url=https://books.google.com/books?id=rLhdW5YzuO4C&pg=RA1-PA551 |year=2009 |publisher=Academic Press |isbn=978-0-12-373944-5 |page=551}}</ref> Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial.<ref name="CalverLymbery2009">{{cite book | vauthors = Calver M, Lymbery A, McComb J, Bamford M |title=Environmental Biology |url=https://books.google.com/books?id=HemnRxzdiFQC&pg=PA118 |year=2009 |publisher=Cambridge University Press |isbn=978-0-521-67982-4 |page=118}}</ref> Studies in the fly ''[[Drosophila melanogaster]]'' suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations are harmful with the remainder being either neutral or weakly beneficial.<ref>{{cite journal | vauthors = Sawyer SA, Parsch J, Zhang Z, Hartl DL | title = Prevalence of positive selection among nearly neutral amino acid replacements in Drosophila | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 16 | pages = 6504–6510 | date = April 2007 | pmid = 17409186 | pmc = 1871816 | doi = 10.1073/pnas.0701572104 | doi-access = free | bibcode = 2007PNAS..104.6504S }}</ref>

[[File:Eukaryote tree.svg|thumb|left|An [[evolutionary tree]] of [[Eukaryote|eukaryotic]] organisms, constructed by the comparison of several [[orthologous gene]] sequences]]

[[Population genetics]] studies the distribution of genetic differences within populations and how these distributions change over time.<ref name="griffiths2000sect3842">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3842 |chapter=Variation and its modulation}}</ref> Changes in the [[Allele frequency|frequency of an allele]] in a population are mainly influenced by [[natural selection]], where a given allele provides a selective or reproductive advantage to the organism,<ref name="griffiths2000sect3886">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3886 |chapter=Selection}}</ref> as well as other factors such as [[mutation]], [[genetic drift]], [[genetic hitchhiking]],<ref>{{cite journal | vauthors = Gillespie JH | title = Is the population size of a species relevant to its evolution? | journal = Evolution; International Journal of Organic Evolution | volume = 55 | issue = 11 | pages = 2161–2169 | date = November 2001 | pmid = 11794777 | doi = 10.1111/j.0014-3820.2001.tb00732.x | s2cid = 221735887 | doi-access = free }}</ref> [[artificial selection]] and [[Gene flow|migration]].<ref name="griffiths2000sect3906">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3906 |chapter=Random events}}</ref>

Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called [[adaptation]], selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.<ref name="Darwin">{{cite book | vauthors = Darwin C |author-link=Charles Darwin |year=1859 |title=On the Origin of Species |place=London |publisher=John Murray |edition= |page=1 |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 |isbn=978-0-8014-1319-3 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20061212020054/http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 |archive-date=12 December 2006 }}<br />Earlier related ideas were acknowledged in {{cite book | vauthors = Darwin C |author-link=Charles Darwin |year=1861 |title=On the Origin of Species |place=London |publisher=John Murray |edition=3rd |page=xiii |url=http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 |no-pp=true |isbn=978-0-8014-1319-3 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20110223145332/http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 |archive-date=23 February 2011 }}</ref> New species are formed through the process of [[speciation]], often caused by geographical separations that prevent populations from exchanging genes with each other.<ref name="Gavrilets">{{cite journal | vauthors = Gavrilets S | title = Perspective: models of speciation: what have we learned in 40 years? | journal = Evolution; International Journal of Organic Evolution | volume = 57 | issue = 10 | pages = 2197–2215 | date = October 2003 | pmid = 14628909 | doi = 10.1554/02-727 | s2cid = 198158082 }}</ref>

By comparing the [[Sequence homology|homology]] between different species' genomes, it is possible to calculate the evolutionary distance between them and [[Molecular clock|when they may have diverged]]. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form [[evolutionary tree]]s; these trees represent the [[common descent]] and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as [[horizontal gene transfer]] and most common in bacteria).<ref>{{cite journal | vauthors = Wolf YI, Rogozin IB, Grishin NV, Koonin EV | title = Genome trees and the tree of life | journal = Trends in Genetics | volume = 18 | issue = 9 | pages = 472–479 | date = September 2002 | pmid = 12175808 | doi = 10.1016/S0168-9525(02)02744-0 }}</ref>

== Research and technology ==
=== Model organisms ===

[[File:Drosophila melanogaster - side (aka).jpg|thumb|right|The [[common fruit fly]] (''Drosophila melanogaster'') is a popular [[model organism]] in genetics research.]]

Although geneticists originally studied inheritance in a wide variety of organisms, the range of species studied has narrowed. One reason is that when significant research already exists for a given organism, new researchers are more likely to choose it for further study, and so eventually a few [[model organism]]s became the basis for most genetics research. Common research topics in model organism genetics include the study of [[gene regulation]] and the involvement of genes in [[morphogenesis|development]] and [[cancer]]. Organisms were chosen, in part, for convenience—short generation times and easy [[genetic engineering|genetic manipulation]] made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium ''[[Escherichia coli]]'', the plant ''[[Arabidopsis thaliana]]'', baker's yeast (''[[Saccharomyces cerevisiae]]''), the nematode ''[[Caenorhabditis elegans]]'', the common fruit fly (''[[Drosophila melanogaster]]''), the zebrafish (''[[Danio rerio]]''), and the common house mouse (''[[Mus musculus]]'').<ref>{{cite web |url=http://www.loci.wisc.edu/outreach/text/model.html |title=The Use of Model Organisms in Instruction |access-date=15 March 2008 |publisher=University of Wisconsin: Wisconsin Outreach Research Modules |archive-url=https://web.archive.org/web/20080313023531/http://www.loci.wisc.edu/outreach/text/model.html |archive-date=13 March 2008}}</ref>

=== Medicine ===

[[File:Biochemistry, genetics and molecular biology.svg|alt=|thumb|Schematic relationship between [[biochemistry]], genetics and [[molecular biology]]]]

[[Medical genetics]] seeks to understand how genetic variation relates to human health and disease.<ref>{{cite web |url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=gnd |title=NCBI: Genes and Disease |publisher=NIH: National Center for Biotechnology Information |access-date=15 March 2008 |archive-url=https://web.archive.org/web/20070220074727/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=gnd&ref=sidebar |archive-date=20 February 2007}}</ref> When searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic [[pedigree chart]]s to find the location on the genome associated with the disease. At the population level, researchers take advantage of genome wide association studies (GWAS) to look for locations in the genome that are associated with diseases, a method especially useful for [[Quantitative trait locus|multigenic traits]] not clearly defined by a single gene.<ref>{{Cite journal |last=Uffelmann |first=Emil |last2=Huang |first2=Qin Qin |last3=Munung |first3=Nchangwi Syntia |last4=de Vries |first4=Jantina |last5=Okada |first5=Yukinori |last6=Martin |first6=Alicia R. |last7=Martin |first7=Hilary C. |last8=Lappalainen |first8=Tuuli |last9=Posthuma |first9=Danielle |date=2021-08-26 |title=Genome-wide association studies |url=https://www.nature.com/articles/s43586-021-00056-9 |journal=[[Nature Reviews Methods Primers]] |language=en |volume=1 |issue=1 |pages=1–21 |doi=10.1038/s43586-021-00056-9 |issn=2662-8449}}</ref> Once a candidate gene is found, further research is often done on the corresponding (or [[Homology (biology)|homologous]]) genes of model [https://www.nature.com/scitable/topicpage/the-use-of-animal-models-in-studying-855/ organisms]. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of [[pharmacogenetics]]: the study of how genotype can affect drug responses.<ref>{{cite web|url=http://www.nigms.nih.gov/Initiatives/PGRN/Background/FactSheet.htm |title=Pharmacogenetics Fact Sheet |access-date=15 March 2008 |publisher=NIH: National Institute of General Medical Sciences |archive-url=https://web.archive.org/web/20080512012316/http://www.nigms.nih.gov/Initiatives/PGRN/Background/FactSheet.htm |archive-date=12 May 2008}}</ref>

Individuals differ in their inherited tendency to develop [[cancer]], and cancer is a genetic disease. The process of cancer development in the body is a combination of events. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger [[Apoptosis|cell death]], but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of [[natural selection]] occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous [[Tumour heterogeneity|tumor]] that grows and invades various tissues of the body. Normally, a cell divides only in response to signals called [[growth factor]]s and [[Contact inhibition|stops growing once in contact with surrounding cells]] and in response to growth-inhibitory signals. It usually then divides a limited number of times and dies, staying within the [[epithelium]] where it is unable to migrate to other organs. To become a cancer cell, a cell has to accumulate mutations in a number of genes (three to seven). A cancer cell can divide without growth factor and ignores inhibitory signals. Also, it is immortal and can grow indefinitely, even after it makes contact with neighboring cells. It may escape from the epithelium and ultimately from the [[primary tumor]]. Then, the escaped cell can cross the endothelium of a blood vessel and get transported by the bloodstream to colonize a new organ, forming deadly [[metastasis]]. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny ([[somatic mutation]]s). The most frequent mutations are a loss of function of [[p53 protein]], a [[tumor suppressor]], or in the p53 pathway, and gain of function mutations in the [[Ras proteins]], or in other [[oncogene]]s.<ref>{{cite journal | vauthors = Frank SA | title = Genetic predisposition to cancer - insights from population genetics | journal = Nature Reviews. Genetics | volume = 5 | issue = 10 | pages = 764–772 | date = October 2004 | pmid = 15510167 | doi = 10.1038/nrg1450 | s2cid = 6049662 }}</ref><ref>{{cite book |vauthors=Strachan T, Read AP |title=Human Molecular Genetics 2 |url=https://archive.org/details/humanmolecularge0002stra |url-access=registration |year=1999 |publisher=John Wiley & Sons Inc. |edition=second}} [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hmg.chapter.2342 Chapter 18: Cancer Genetics] {{webarchive|url=https://web.archive.org/web/20050926163641/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hmg.chapter.2342 |date=26 September 2005 }}</ref>

=== Research methods ===

[[File:Ecoli colonies.png|thumb|right|upright=0.8|[[Colony (biology)|Colonies]] of ''[[Escherichia coli|E. coli]]'' produced by [[Cloning#Unicellular organisms|cellular cloning]]. A similar methodology is often used in [[molecular cloning]].]]

DNA can be manipulated in the laboratory. [[Restriction enzymes]] are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA.<ref>Lodish et al. (2000), [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.1582 Chapter 7: 7.1. DNA Cloning with Plasmid Vectors] {{webarchive|url=https://web.archive.org/web/20090527183555/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.1582 |date=27 May 2009 }}</ref> DNA fragments can be visualized through use of [[gel electrophoresis]], which separates fragments according to their length.<ref>{{Cite journal |last1=Timms |first1=John F. |last2=Cramer |first2=Rainer |date=December 2008 |title=Difference gel electrophoresis |url=https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/pmic.200800298 |journal=Proteomics |language=en |volume=8 |issue=23–24 |pages=4886–4897 |doi=10.1002/pmic.200800298 |pmid=19003860 |issn=1615-9853|url-access=subscription }}</ref>

The use of [[DNA ligase|ligation enzymes]] allows DNA fragments to be connected. By binding ("ligating") fragments of DNA together from different sources, researchers can create [[recombinant DNA]], the DNA often associated with [[genetically modified organism]]s. Recombinant DNA is commonly used in the context of [[plasmid]]s: short circular DNA molecules with a few genes on them. In the process known as [[molecular cloning]], researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate [[Cloning#Unicellular organisms|clones of bacteria cells]]). "Cloning" can also refer to the various means of creating cloned ("clonal") organisms.<ref>{{cite journal | vauthors = Keefer CL | title = Artificial cloning of domestic animals | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 29 | pages = 8874–8878 | date = July 2015 | pmid = 26195770 | pmc = 4517265 | doi = 10.1073/pnas.1501718112 | bibcode = 2015PNAS..112.8874K | doi-access = free }}</ref>

DNA can also be amplified using a procedure called the [[polymerase chain reaction]] (PCR).<ref>Lodish et al. (2000), [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=PCR&rid=mcb.section.1718 Chapter 7: 7.7. Polymerase Chain Reaction: An Alternative to Cloning]{{dead link|date=July 2025|bot=medic}}{{cbignore|bot=medic}}</ref> By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.<ref name="Chang_2017">{{cite journal |last1=Chang |first1=Dingran |last2=Tram |first2=Kha |last3=Li |first3=Ben |last4=Feng |first4=Qian |last5=Shen |first5=Zhifa |last6=Lee |first6=Christine H. |last7=Salena |first7=Bruno J. |last8=Li |first8=Yingfu  |date=2017-06-08 |title=Detection of DNA Amplicons of Polymerase Chain Reaction Using Litmus Test |journal=Scientific Reports |volume=7 |issue=3110 |page=3110 |doi=10.1038/s41598-017-03009-z |pmid=28596600 |pmc=5465217 |bibcode=2017NatSR...7.3110C }}</ref><ref name="Garibyan_2013">{{cite journal |last1=Garibyan |first1=Lilit |last2=Nidhi |date=March 2013 |title=Polymerase Chain Reaction |url=https://www.jidonline.org/article/S0022-202X(15)36139-X/fulltext |journal=Journal of Investigative Dermatology |volume=133 |issue=3 |pages=1–4 |doi=10.1038/jid.2013.1 |pmid=23399825 |access-date=2024-02-27|pmc=4102308 }}</ref>

=== DNA sequencing and genomics ===
{{Main|DNA sequencing}}
DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of [[Sanger sequencing|chain-termination sequencing]], developed in 1977 by a team led by [[Frederick Sanger]], is still routinely used to sequence DNA fragments. Using this technology, researchers have been able to study the molecular sequences associated with many human diseases.<ref>{{cite book |vauthors=Brown TA |title=Genomes 2 |edition=2nd |year=2002 |isbn=978-1-85996-228-2 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK21117/ |chapter=Section 2, Chapter 6: 6.1. The Methodology for DNA Sequencing |publisher=Bios |location=Oxford}}</ref>

As sequencing has become less expensive, researchers have [[Genome project|sequenced the genomes]] of many organisms using a process called [[genome assembly]], which uses computational tools to stitch together sequences from many different fragments.<ref>Brown (2002), [https://www.ncbi.nlm.nih.gov/books/NBK21117/ Section 2, Chapter 6: 6.2. Assembly of a Contiguous DNA Sequence] </ref> These technologies were used to sequence the human genome in the Human Genome Project completed in 2003.<ref name=human_genome_project>{{cite web |url=http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml |title=Human Genome Project Information |access-date=15 March 2008 |publisher=Human Genome Project |archive-url=https://web.archive.org/web/20080315062131/http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml |archive-date=15 March 2008}}</ref> New [[DNA sequencing#New sequencing methods|high-throughput sequencing]] technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars.<ref>{{cite journal | vauthors = Service RF | title = Gene sequencing. The race for the $1000 genome | journal = Science | volume = 311 | issue = 5767 | pages = 1544–1546 | date = March 2006 | pmid = 16543431 | doi = 10.1126/science.311.5767.1544 | s2cid = 23411598 }}</ref>

[[Next-generation sequencing]] (or high-throughput sequencing) came about due to the ever-increasing demand for low-cost sequencing. These sequencing technologies allow the production of potentially millions of sequences concurrently.<ref name=hall2007>{{cite journal | vauthors = Hall N | title = Advanced sequencing technologies and their wider impact in microbiology | journal = The Journal of Experimental Biology | volume = 210 | issue = Pt 9 | pages = 1518–1525 | date = May 2007 | pmid = 17449817 | doi = 10.1242/jeb.001370 | doi-access = free | bibcode = 2007JExpB.210.1518H }}</ref><ref name=church2006>{{cite journal | vauthors = Church GM | title = Genomes for all | journal = Scientific American | volume = 294 | issue = 1 | pages = 46–54 | date = January 2006 | pmid = 16468433 | doi = 10.1038/scientificamerican0106-46 | s2cid = 28769137 | bibcode = 2006SciAm.294a..46C | author-link1 = George M. Church }}{{subscription required}}</ref> The large amount of sequence data available has created the subfield of [[genomics]], research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of [[bioinformatics]], which uses computational approaches to analyze large sets of [[biological data]].

== Society and culture ==
{{See also|Genetics in fiction}}

On 19 March 2015, a group of leading biologists urged a worldwide ban on clinical use of methods, particularly the use of [[CRISPR]] and [[zinc finger]], to edit the human genome in a way that can be inherited.<ref name="NYT-20150319">{{cite news | vauthors = Wade N |title=Scientists Seek Ban on Method of Editing the Human Genome |url=https://www.nytimes.com/2015/03/20/science/biologists-call-for-halt-to-gene-editing-technique-in-humans.html |date=19 March 2015 |work=[[The New York Times]] |access-date=20 March 2015 |url-status=live |archive-url=https://web.archive.org/web/20150319230002/http://www.nytimes.com/2015/03/20/science/biologists-call-for-halt-to-gene-editing-technique-in-humans.html |archive-date=19 March 2015}}</ref><ref name="NYT-20150303-AP">{{cite news | vauthors = Pollack A |title=A Powerful New Way to Edit DNA |url=https://www.nytimes.com/2014/03/04/health/a-powerful-new-way-to-edit-dna.html |date=3 March 2015 |work=[[The New York Times]] |access-date=20 March 2015 |url-status=live |archive-url=https://web.archive.org/web/20150326051509/http://www.nytimes.com/2014/03/04/health/a-powerful-new-way-to-edit-dna.html |archive-date=26 March 2015}}</ref><ref name="SCI-20150319">{{cite journal | vauthors = Baltimore D, Berg P, Botchan M, Carroll D, Charo RA, Church G, Corn JE, Daley GQ, Doudna JA, Fenner M, Greely HT, Jinek M, Martin GS, Penhoet E, Puck J, Sternberg SH, Weissman JS, Yamamoto KR | title = Biotechnology. A prudent path forward for genomic engineering and germline gene modification | journal = Science | volume = 348 | issue = 6230 | pages = 36–38 | date = April 2015 | pmid = 25791083 | pmc = 4394183 | doi = 10.1126/science.aab1028 | bibcode = 2015Sci...348...36B }}</ref><ref name="NAT-20150312">{{cite journal | vauthors = Lanphier E, Urnov F, Haecker SE, Werner M, Smolenski J | title = Don't edit the human germ line | journal = Nature | volume = 519 | issue = 7544 | pages = 410–411 | date = March 2015 | pmid = 25810189 | doi = 10.1038/519410a | doi-access = free | bibcode = 2015Natur.519..410L }}</ref> In April 2015, Chinese researchers [[CRISPR#Society and culture|reported]] results of [[basic research]] to edit the DNA of non-viable [[human embryos]] using CRISPR.<ref name="NYT-20150423">{{cite news | vauthors = Kolata G |title=Chinese Scientists Edit Genes of Human Embryos, Raising Concerns |url=https://www.nytimes.com/2015/04/24/health/chinese-scientists-edit-genes-of-human-embryos-raising-concerns.html |date=23 April 2015 |work=[[The New York Times]] |access-date=24 April 2015 |url-status=live |archive-url=https://web.archive.org/web/20150424050616/http://www.nytimes.com/2015/04/24/health/chinese-scientists-edit-genes-of-human-embryos-raising-concerns.html |archive-date=24 April 2015}}</ref><ref name="PC-20150418">{{cite journal | vauthors = Liang P, Xu Y, Zhang X, Ding C, Huang R, Zhang Z, Lv J, Xie X, Chen Y, Li Y, Sun Y, Bai Y, Songyang Z, Ma W, Zhou C, Huang J | title = CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes | journal = Protein & Cell | volume = 6 | issue = 5 | pages = 363–372 | date = May 2015 | pmid = 25894090 | pmc = 4417674 | doi = 10.1007/s13238-015-0153-5 }}</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Bacterial genome size]]
* [[Cryoconservation of animal genetic resources]]
* [[Eugenics]]
* [[Embryology]]
* [[Genetic disorder]]
* [[Genetic diversity]]
* [[Genetic engineering]]
* [[Genetic enhancement]]
* [[Glossary of genetics (M−Z)]]
* [[Index of genetics articles]]
* [[Medical genetics]]
* [[Molecular tools for gene study]]
* [[Neuroepigenetics]]
* [[Outline of genetics]]
* [[Timeline of the history of genetics]]
* [[Plant genetic resources]]
{{div col end}}

== References ==
{{Reflist}}

== Further reading ==
{{See also|Bibliography of biology#Genetics}}
{{refbegin}}
* {{cite book | vauthors = Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P |title=Essential Cell Biology, 4th Edition|url=https://books.google.com/books?id=Cg4WAgAAQBAJ&pg=PP1|year=2013|publisher=Garland Science|isbn=978-1-317-80627-1 |ref=none}}
* {{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |url=https://archive.org/details/introductiontoge0000unse_v1d3 |ref=none}}
* {{cite book | vauthors=Hartl D, Jones E |title=Genetics: Analysis of Genes and Genomes |edition=6th |publisher=Jones & Bartlett |year=2005 |isbn=978-0-7637-1511-3 |url=https://archive.org/details/genetics00dani |ref=none}}
* {{cite book | vauthors = King RC, Mulligan PK, Stansfield WD |title=A Dictionary of Genetics |edition=8th |year=2013 |isbn=978-0-19-976644-4 |publisher=Oxford University Press |location=New York |ref=none}}
* {{cite book |vauthors=Lodish H, Berk A, Zipursky LS, Matsudaira P, Baltimore D, Darnell J |title=Molecular Cell Biology |edition=4th |year=2000 |isbn=978-0-7167-3136-8 |publisher=Scientific American Books |location=New York |url=https://archive.org/details/molecularcellbio00lodi |ref=none}}
{{refend}}

== External links ==
{{WVD}}
{{Commonscat|Genetics}}
* {{Wikiquote-inline}}
* {{Wikibooks-inline|Genetics}}
* {{Library resources about}}
* {{In Our Time|Genetics|p00547md|Genetics}}
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