Hybrids and Evolution

Female ligers and tigons are fertile. Apparently, the homogametic sex in hybrids (XX females in the case of mammals) are more likely to be fertile than the heterogametic sex. I don't know why this is, but the following explanations are offered by wikipedia:Homogametes are protected from any deleterious recessive genes on the sex chromosones, whereas the heterogametes aren't.In mammals, sexual selection drives faster evolution in male development patterns, so the Y chromosone is more divergent in the different species and less able to make a viable animal (though you'd think this would have the opposite effect in birds, where males still tend to be more heavily influenced by secual selection but where they are the homogametic sex).Plus another couple that I don't think I've quite grasped from their brief overview. It's here in the article on Haldane's Rule.ABE: I just had a quick read, and apparently female mules can produce offspring with either horse or donkey males.Edited by caffeine, : No reason given.

Thinking about it, isn't it essential that at least some combinations of different numbers of chromosones are capable of producing viable offspring? Otherwise, it wouldn't be possible for secually reproducing organisms to evolve different numbers of chromosones in the first place. The first mutant with a duplicated or deleted chromosone would be necessarily infertile except in the extremely unlikely case of bumping into a member of the opposite sex with exactly the same mutation.