Editing Genetic linkage (section)

==Discovery==
[[Gregor Mendel]]'s [[Law of Independent Assortment]] states that every trait is inherited independently of every other trait. But shortly after [[Gregor Mendel#Rediscovery of Mendel's work|Mendel's work was rediscovered]], exceptions to this rule were found. In 1905, the [[United Kingdom|British]] geneticists [[William Bateson]], [[Edith Rebecca Saunders]] and [[Reginald Punnett]] cross-bred pea plants in experiments similar to Mendel's.<ref>{{Cite web |last1=Lobo |first1=Ingrid |last2=Shaw |first2=Kenna |title=Discovery and Types of Genetic Linkage |url=http://www.nature.com/scitable/topicpage/discovery-and-types-of-genetic-linkage-500 |access-date=21 January 2017 |website=Scitable |publisher=[[Nature Education]]}}</ref><ref>{{Cite book |last1=Bateson |first1=W |author-link1=William Bateson |url=https://archive.org/details/RoyalSociety.ReportsToTheEvolutionCommittee.ReportIi.Experimental |title=Reports to the Evolution committee of the Royal Society |last2=Saunders |first2=ER |author-link2=Edith Rebecca Saunders |last3=Punnett |first3=RC |author-link3=Reginald Punnett |date=18 May 1904 |publisher=Harrison and Sons, Printers |location=London |access-date=21 January 2017}}</ref> They were interested in trait inheritance in the sweet pea and were studying two genes—the gene for flower colour (''P'', purple, and ''p'', red) and the gene affecting the shape of pollen grains (''L'', long, and ''l'', round). They crossed the pure lines ''PPLL'' and ''ppll'' and then self-crossed the resulting ''PpLl'' lines.{{cn|date=December 2024}}

According to [[Mendelian inheritance|Mendelian genetics]], the expected [[phenotype]]s would occur in a 9:3:3:1 ratio of PL:Pl:pL:pl.  To their surprise, they observed an increased frequency of PL and pl and a decreased frequency of Pl and pL:{{cn|date=December 2024}}

{| class="wikitable"
|+'''Bateson, Saunders, and Punnett experiment'''
|-
!Phenotype and genotype||Observed||Expected from 9:3:3:1 ratio
|-
|Purple, long (''P_L_'')||284||216
|-
|Purple, round (''P_ll'')||21||72
|-
|Red, long (''ppL_'')||21||72
|-
|Red, round (''ppll'')||55||24
|}

Their experiment revealed linkage between the ''P'' and ''L'' alleles and the ''p'' and ''l'' alleles. The frequency of ''P'' occurring together with ''L'' and ''p'' occurring together with ''l'' is greater than that of the recombinant ''Pl'' and ''pL''. The [[Genetic recombination|recombination]] frequency is more difficult to compute in an F2 cross than a backcross,<ref name="Fisher">{{Cite journal |last1=Fisher |first1=RA |author-link1=Ronald Fisher |last2=Balmukand |first2=B |date=July 1928 |title=The estimation of linkage from the offspring of selfed heterozygotes |journal=[[Journal of Genetics]] |volume=20 |issue=1 |pages=79–92 |doi=10.1007/BF02983317 |s2cid=27688031}}</ref> but the lack of fit between observed and expected numbers of progeny in the above table indicate it is less than 50%. This indicated that two factors interacted in some way to create this difference by masking the appearance of the other two phenotypes. This led to the conclusion that some traits are related to each other because of their near proximity to each other on a chromosome.{{cn|date=December 2024}}

The understanding of linkage was expanded by the work of [[Thomas Hunt Morgan]]. Morgan's observation that the amount of [[Chromosomal crossover|crossing over]] between linked genes differs led to the idea that crossover frequency might indicate the distance separating genes on the [[chromosome]]. The [[centimorgan]], which expresses the frequency of crossing over, is named in his honour.{{cn|date=December 2024}}