funny how you try to lump the genetics of sex determination into the same category as genetic disease--two very, very different mechanisms. (and yes, Michael Jackson very much had a not-uncommon skin disease found in blacks.
Vitiglio)
So,
XY= male
XX = female?
hands down?
end of story?
Evolutionarily speaking, sex determination involves interplay and information from genes found on the autosomes (non sex chromosomes) as well as a handful of gender (male)-specific genes that exist on the Y-chromosome.
Consider that the current iteration of the Y is essentially a degenerate, ancient X chromosome. In some species, a shrimpy Y is fused to an arm of the X chromosome.
Now, because the Y chromosome is indeed disappearing due to the fact that it doesn't recombine (one of the more important mechanisms of driving novel genetic information in sexual reproduction), it stands to reason, then, that sex determination is and most be completely valid through other mechanisms--the very real possibility that gender specific genes can and will migrate to other regions, or simply lose their necessity.
Plenty of species exist today that have completely tossed the Y--males that are characterized as XO, and of course systems in many, many animals (all birds, and many snakes, where females are heterogametic and males are homogametic). Then there is the noble platypus, with 10 sex chromosomes).
So, while sex chromosomes are, indeed, essential for determining sex in species, it doesn't tell the whole story.