Competition among genes within an individual male fruit fly can cause its sperm to produce a high proportion of female offspring. Now, scientists have identified a gene responsible for this well-known phenomenon as well as the gene that later evolved to restore gender balance.
In essence, the two fruit fly genes engage in a tug-of-war in which each succeeds evolutionarily if it can spread widely among future generations.
The imbalance favoring females happens because the sex-skewing gene, called Distorter on the X (Dox), is located on the X chromosome. Females each have two X chromosomes, and pass on only Xs to their offspring. Males each have an X and a Y. They pass an X chromosome to their female offspring and a Y to their male offspring.