Haldane's law states that in interspecific hybrids, it is the heteroga
metic sex that is likely to be absent, rare, or sterile, In mammals, t
here is increasing evidence to suggest that this may be due to the hig
h mutation rate of male sex-determining genes on the Y chromosome. The
mule, humanity's first successful attempt at genetic engineering, pro
vides some support for this concept, Interspecific hybrids may also sh
ed new light on the importance of the maternal transmission of mitocho
ndrial DNA and the phenomenon of genomic imprinting.