Figure 3.

Fertility varies with age in PWKB6 hybrids. A fertility curve was constructed from combining two breeding experiments in which males had continuous access to fertile females. Most males tested were infertile. However, some mice were fertile at ages 15–35 weeks. One experiment was designed to determine the age of fertility onset and tested mice in the age range of 5–21 weeks. Of the 26 fertile males tested, 3 sired first litters, at 15, 17, and 21 weeks. The other experiment tested fertility after age 20 weeks; 36% of males were fertile in this window, but this proportion decreased sharply with age. No males sired litters after age 35 weeks.