Editing Genetics (section)

=== 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>