Figure 1.

The proportion of the overall mutation rate at different chromosome types attributed to the male mutation rate (μ_m) or the female mutation rate (μ_f). For both mammals (with male heterogamety) and birds (with female heterogamety), autosomes represent half μ_m and half μ_f. In mammals (XY males and XX females), the mutation rate of the heterogametic sex chromosome, the Y, is completely representative of the male mutation rate. Alternatively, in birds (ZZ males and ZW females), the heterogametic sex chromosome, the W, is entirely representative of the female mutation rate. For both mammals and birds, the homogametic sex chromosome (X and Z, respectively), represents two-thirds the mutation rate of the homogametic sex (females and males, respectively), and one-third the mutation rate of the heterogametic sex.