Abstract
"Haldane's rule” states that, if species hybrids of one sex only are inviable or sterile, the afflicted
sex is much more likely to be heterogametic (XU) than homogametic ( XX). We show that most or all
of the phenomena associated with HALDANE’S rule can be explained by the simple hypothesis that alleles
decreasing hybrid fitness are partially recessive. Under this hypothesis, the XY sex suffers more than
the XX because X-linked alleles causing postzygotic isolation tend to have greater cumulative effects
when hemizygous than when heterozygous, even though the XXsex carries twice as many such alleles.
The dominance hypothesis can also account for the “large Xeffect,” the disproportionate effect of the
Xchromosome on hybrid inviability/sterility. In addition, the dominance theory is consistent with: the
long temporal lag between the evolution of heterogametic and homogametic postzygotic isolation, the
frequency of exceptions to HALDANE’S rule, puzzling Drosophila experiments in which “unbalanced”
hybrid females, who carry two X chromosomes from the same species, remain fertile whereas F, hybrid
males are sterile, and the absence of cases of HALDANE’S rule for hybrid inviability in mammals. We
discuss several novel predictions that could lead to rejection of the dominance theory
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