Editing Selfish genetic element (section)

=== Segregation distorters ===
The mouse t-allele is a classic example of a segregation distorter system that has been modeled in great detail.<ref name=":14" /><ref>{{cite journal | vauthors = Lewontin RC, Dunn LC | title = The Evolutionary Dynamics of a Polymorphism in the House Mouse | journal = Genetics | volume = 45 | issue = 6 | pages = 705–22 | date = June 1960 | doi = 10.1093/genetics/45.6.705 | pmid = 17247957 | pmc = 1210083 }}</ref> Heterozygotes for a t-haplotype produce >90% of their gametes bearing the t (see [[#Segregation distorters|Segregation distorters]]), and homozygotes for a t-haplotype die as embryos. This can result in a stable polymorphism, with an equilibrium frequency that depends on the drive strength and direct fitness impacts of t-haplotypes. This is a common theme in the mathematics of segregation distorters:virtually every example we know entails a countervailing selective effect, without which the allele with biased transmission would go to fixation and the segregation distortion would no longer be manifested. Whenever sex chromosomes undergo segregation distortion, the population sex ratio is altered, making these systems particularly interesting. Two classic examples of segregation distortion involving sex chromosomes include the "Sex Ratio" X chromosomes of ''Drosophila pseudoobscura''<ref name=":11" /> and Y chromosome drive suppressors of ''Drosophila mediopunctata''.<ref>{{cite journal | vauthors = Carvalho AB, Vaz SC, Klaczko LB | title = Polymorphism for Y-linked suppressors of sex-ratio in two natural populations of Drosophila mediopunctata | journal = Genetics | volume = 146 | issue = 3 | pages = 891–902 | date = July 1997 | doi = 10.1093/genetics/146.3.891 | pmid = 9215895 | pmc = 1208059 }}</ref> A crucial point about the theory of segregation distorters is that just because there are fitness effects acting against the distorter, this does not guarantee that there will be a stable polymorphism. In fact, some sex chromosome drivers can produce frequency dynamics with wild oscillations and cycles.<ref>{{cite journal | vauthors = Clark AG | title = Natural selection and Y-linked polymorphism | journal = Genetics | volume = 115 | issue = 3 | pages = 569–77 | date = March 1987 | doi = 10.1093/genetics/115.3.569 | pmid = 3569883 | pmc = 1216358 }}</ref>