Evolution of the Y Sex Chromosome in Animals

Sex chromosomes are part of the gender determination system of many organisms. They are common in animals and rare in plants. There are many forms of sex de­ termination. Gender may be deter­ mined genetically or in response to environmental cues such as tempera­ ture or social circumstance. Genetic sex determination is typically ex­ pressed in the developing zygote but can also be mediated maternally, with some females producing only daughters and others only sons. Zygotic sex determination has ~ many forms. In bees, ants, and wasps fertilized eggs develop into females and unfertilized eggs into males. In the guppy, and many other fish, in­ vertebrates, and plants, gender is determined by the presence or ab­ sence of a sex determining gene (G, or a cluster of tightly linked genes); Gg is male and gg female. The het­ erozygous (i.e., heterogametic) sex can be female or male, depending on the species. Gender can also be de­ termined by specialized sex chromo­ somes (X and Y). In mammals, fe­ males are XX and males are XY; in birds, females are XY (sometimes designated ZW) and males are XX (sometimes designated ZZ); and in nematodes, females are XX and males are XO, in which ° denotes the absence of a homologous chro­ mosome. More than two sex chro­ mosomes are found in some species,