Haldane's rule states that species hybrids of the XY sex are preferentially sterile or inviable. In all taxa known to obey this rule, the Y is inert and X-Linked genes show full expression in XY individuals. Until recently, all theories of Haldane's rule depended on this hemizygosity. A test of Haldane's rule in animals Lacking a hemizygous sex-mosquitoes having two functional sex chromosomes in both sexes-reveals that these species show Haldane's rule for sterility but not inviability, A related group having a "normal" hemizygous X obeys Haldane's rule for both sterility and inviability. These results support the faster male and dominance theories of Haldane's rule.