The Y chromosome as a battle ground for sexual selection.

The Y chromosome was once thought to be devoid of genetic information. However, recent work shows that it contains numerous genes related to sperm production and dimorphic traits (such as body size and tooth development). Among mammals, these traits influence a male's competitive ability in male-male contests and in sperm competition. Therefore, sexual selection could have favoured genes on the Y chromosome that enhance male fertilization success because they spread unaltered through the male line. In contrast, female heterogamety among birds makes it possible for genes that benefit females to spread through the female line, a mechanism that could explain the prevalence of female choice.