Fig. 1.
Most sex chromosomes come in either of two primary types. a Animals such as birds, lepidopterans and plants such as wild strawberry (Spigler et al. 2008) show a pattern of female heterogamety, with a major Z chromosome and minor W chromosome. The W chromosome is limited to females, and therefore the effective population size of the W (NEW) is less than NEZ. b In male heterogamety, which is present in animals such and mammals and salmon (Woram et al. 2003), as well as plants including Silene (Guttman and Charlesworth 1998), the X chromosome is present in both sexes while the Y is restricted to males. NEY is less than NEX