Haldane's rule
= Haldane's rule
{c}
When one sex of first-generation <hybrids> is absent, rare or sterile, it tends to be the <heterogametic sex>. This predicts a sex bias within crosses, rather than a universal difference in isolation between birds and mammals.<Sex ratio and unisexual sterility in hybrid animals by J. B. S. Haldane>[Journal of Genetics 12, pp. 101–109]{ref}