Often race-deniers and cultural Marxists will bring up Haldane’s rule, arguing that since races can mix and create fertile offspring, the genetic distance is not too great. Haldane’s rule is “when in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterogametic [XY] sex.”
Haldane’s Rule: the Heterogametic Sex
Haldane’s simple observation, now known as Haldane’s rule, still interests and perplexes biologists studying the genetics of speciation. Why should hybrids of the heterogametic sex, or the one with different sex chromosomes (e.g., XY in mammals), be more afflicted than hybrids of the homogametic sex (e.g., XX in mammals)? One reason that biologists who study speciation focus on Haldane’s rule is that it is one of the few empirical generalizations about speciation that holds true in many different groups of animals. For instance, this pattern is followed in groups of organisms in which males are the heterogametic sex (such as flies and mammals), as well as in groups of organisms in which females are the heterogametic sex (e.g., birds and butterflies). The prevalence of this pattern suggests that it is generated by common mechanisms in diverse organisms.
Haldane’s original observations concerned just the fertility and viability of hybrids, but Haldane’s rule can also be extended to other traits. Indeed, in hybrids, the members of the heterogametic sex tend to suffer more adverse morphological effects than their homogametic counterparts. For instance, Wade and Johnson (1994) found that hybrid males produced by the mating of two species of flour beetle (Tribolium castaneum and Tribolium freemani) often have deformed legs and antennae. Such deformities are rare within either parent species of flour beetle. Interestingly, at the normal rearing temperature (29°C), hybrid females produced by this same mating have deformities only occasionally. However, at higher temperatures (e.g., 34°C), a considerable fraction of hybrid females are also deformed (Wade et al., 1999).
But what are the genetic and evolutionary causes of this pattern? Specifically, is Haldane’s rule due to properties of sex chromosomes or properties related to sex itself? Most evolutionary geneticists think Haldane’s rule is the result of multiple factors in both categories.