Perspective XY and ZW: Is Meiotic Sex Chromosome Inactivation the

The sex chromosomes are among the most rapidly evolving and most diverse genetic systems in all of biology. Students of model organisms may, however, have the false impression that there is only one chromosomal mechanism of specifying sex. Among the best-studied metazoans, the XY system is indeed the rule, with inheritance of two X’s determining the female sex (XX), and inheritance of an X and a Y specifying the male sex (XY) [1]. In this system, females produce only one type of oocyte (X), whereas males produce two types of sperm (X and Y). However, sex is not always determined this way