Segregation distorsion of the sex chromosomes (unequal transmission in the gametes of the heterogametic sex) is a phenomenon which has beenknown for more than three-quarters of a century and which has been described in plants, insects and mammals. It results in imbalance in the sex ratio in descendants and populations. However, the nature of the genes or genetic elements responsible and the related molecular mechanisms remain unknown.

The segregation distorsion discovered in D. simulans consists of preferential transmission of the chromosome X in the male gametes. It results from meitotic abnormalities which involve specifically the Y chromosome. The trait is linked to the X chromosome and is multigenic. Fine mapping of this chromosome has shown that the region concerned includes at least two distortion elements which, in a permissive context, interact in male meiosis to prevent transmission of the Y chromosome to spermatozoids (Drosophila males are cytogenetically XY). As a consequence of this, the majority of the descendants of these males are female, or even exclusively female.