Editing Cytogenetics (section)

== Applications of cytogenetics ==

=== McClintock's work on maize ===
[[Barbara McClintock]] began her career as a [[maize]] cytogeneticist. In 1931, McClintock and [[Harriet Creighton]] demonstrated that cytological recombination of marked [[chromosomes]] correlated with recombination of genetic [[Trait (biological)|trait]]s ([[gene]]s). McClintock, while at the [[Carnegie Institution]], continued previous studies on the mechanisms of chromosome breakage and fusion flare in maize. She identified a particular chromosome breakage event that always occurred at the same locus on maize chromosome 9, which she named the "''Ds"'' or "dissociation" locus.<ref>{{cite journal |last1=Ravindran |first1=Sandeep |title=Barbara McClintock and the discovery of jumping genes |journal=Proceedings of the National Academy of Sciences of the United States of America |date=11 December 2012 |volume=109 |issue=50 |pages=20198–20199 |doi=10.1073/pnas.1219372109 |pmid=23236127 |pmc=3528533 |doi-access=free }}</ref> McClintock continued her career in cytogenetics studying the mechanics and inheritance of broken and ring (circular) chromosomes of maize. During her cytogenetic work, McClintock discovered [[transposon]]s, a find which eventually led to her [[Nobel Prize]] in 1983.

=== Natural populations of ''Drosophila'' ===
In the 1930s, [[Dobzhansky]] and his coworkers collected ''[[Drosophila pseudoobscura]]'' and ''[[Drosophila persimilis|D.&nbsp;persimilis]]'' from wild populations in [[California]] and neighboring states. Using Painter's technique<ref>{{cite journal |last1=Painter |first1=T. S. |title=A new method for the study of chromosome rearrangements and the plotting of chromosome maps |journal=Science |date=22 December 1933 |volume=78 |issue=2034 |pages=585–586 |doi=10.1126/science.78.2034.585 |pmid=17801695 |bibcode=1933Sci....78..585P }}</ref> they studied the [[polytene|polytene chromosomes]] and discovered that the wild populations were polymorphic for [[chromosomal inversions]]. All the flies look alike whatever inversions they carry: this is an example of a cryptic polymorphism.{{cn|date=February 2024}}

Evidence rapidly accumulated to show that [[natural selection]] was responsible. Using a method invented by L'Héritier and Teissier, Dobzhansky bred populations in ''population cages'', which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating [[insect migration|migration]] as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised. By the time Dobzhansky published the third edition of his book in 1951<ref>Dobzhansky T. 1951. ''Genetics and the origin of species''. 3rd&nbsp;ed, Columbia University Press, New York.</ref> he was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most [[polymorphism (biology)|polymorphisms]].<ref>Dobzhansky T. 1970. ''Genetics of the evolutionary process''. Columbia University Press N.Y.</ref><ref>[Dobzhansky T.] 1981. ''Dobzhansky's genetics of natural populations''. eds Lewontin RC, Moore JA, Provine WB and Wallace B. Columbia University Press N.Y.</ref>

=== Lily and mouse ===

The lily is a favored organism for the cytological examination of meiosis since the chromosomes are large and each morphological stage of meiosis can be easily identified microscopically. Hotta, [[Ann Chester Chandley|Chandley]] et al.<ref name="pmid593319">{{cite journal |last1=Hotta |first1=Yasuo |last2=Chandley |first2=Ann C. |last3=Stern |first3=Herbert |title=Meiotic crossing-over in lily and mouse |journal=Nature |date=September 1977 |volume=269 |issue=5625 |pages=240–242 |doi=10.1038/269240a0 |pmid=593319 |bibcode=1977Natur.269..240H |s2cid=4268089 }}</ref> presented the evidence for a common pattern of DNA nicking and repair synthesis in male meiotic cells of lilies and rodents during the zygotene–pachytene stages of meiosis when crossing over was presumed to occur.  The presence of a common pattern between organisms as phylogenetically distant as lily and mouse led the authors to conclude that the organization for meiotic crossing-over in at least higher eukaryotes is probably universal in distribution.{{cn|date=February 2024}}