Segregation distortion

Segregation distortion is a genetic phenomenon that has been observed in the fruit fly. Normally when sperm forms, a cell splits by meiosis, producing four haploid cells, all of which become viable sperm cells. When the fly has the segregation distorter gene, the two cells that do not have the gene malfunction; thus all the sperm produced by that fly carries the SD+ gene.

Segregation distortion is controlled by two genes, SD and R (responder), which are on the same chromosome. The wild state is SD- R+, with a sizable fraction of SD- R-. A mutant chromosome, which has lots of transpositions to discourage crossovers, has the genes SD+ R-. The gene combination SD+ R+ does not occur because it is suicidal.