Beatrix writes:
Hmm…Even assuming homosexuality is on the X chromosome….
What’s the benefit of producing offspring that don’t/can’t reproduce? Seems like a waste of a lot of potential reproductive years & resources to bear & raise offspring that are genetic ‘dead ends’. Long term this would still lead to homosexuals being bred out of existence. Now if you could get some sort of ‘bisexual’ behavior resulting in viable offspring out of a certain percentage of mom’s male offspring that might tip the scales a bit.
No it would not. Lots of X stuff sticks around because the males don’t pass it on. All of the male children will get the gene, and in some it will express and it others it will not. Some of her sons will be gay, but they will not pass on the gene even if they bred so there is no way for the deleterious gene to go out. The straight sons will not pass it on either.
The only way of passing on the gene would be via her daughters, who would get the gene on their X and then possibly pass it on. The gene poses no fitness disadvantage to the females who have it so they can have as many kids as they want and nothing happens. All the girls will get the gene to pass on but it will not hurt them. All the males will get the gene to do whatever, but they can’t pass it on anyway, so the gene sticks around.
Lots of lousy things are on the X. They tend to stick around because they are only passed on by females who are not effected. Actually some very serious diseases are on the X and are passed on from mothers to sons, who are effected. Stuff on the X simply does not go out. There is no reason for it to go out.
There is no selection pressure on sex-linked traits as long as the females are only carriers and it doesn’t hurt them. Sex-linked traits are not selected for or against really. They don’t effect the female so there is no selection pressure on her and the males can’t pass it on anyway, so selection pressure on them is irrelevant.
C0lorblindness is on the X. Hemophilia is on the X. There’s no benefit to either of those, but as long as it doesn’t hurt the female carrier, it is going to stick around as long as women have daughters, which is like forever.
