Figure 1.

Transmission of the sex chromosomes. Females are shown in red, males in blue. In male heterogamety (A), the Y chromosome is passed through the patriline and limited to males. The maternal X chromosome is passed from mother to both sons and daughters, but the paternal X can only be transmitted to daughters. Additionally, the X is present two-thirds of the time in females. In female heterogamety (B), the W chromosome is limited to females and passed solely from mother to daughter. The paternal Z chromosome can be passed from father to both daughters and sons; however, the maternal Z chromosome is only passed to sons. Converse to the X chromosome, the Z chromosome is resident in males two-thirds of the time.