http://www.geocities.com/southbeach/.../biobasis.html
So we find that the stronger the genetic relationship between twins, the higher the likelihood, that if one is gay, the other will be as well.
If the twins are not related (adoption) the presence of a gay twin means nothing above society's baseline incidence.
This proves that there is a genetic basis. And it also proves that it is not "environmental" as the non-genetic twins would have an equal incidence of homosexuality as the genetic twins.
But if the twins are identical, why aren't they
always of the same sexual orientation.
I think that this can be explained by saying that they have the genetic potential to become gay, but a some time during gestation this potential needs to be activated. This, I think boils down to percentages/random chance.
There are many conditions which exhibit this phenomenon. It is called "penetrance". Huntingtons chorea is a neurological disorder with 100% penetrance. That means if you have the gene you WILL ALWAYS get the disease. A disease with less penetrance is Neurofibromatosis. If you have the gene, sometimes you get the disease, sometimes you don't.
This is offered just to clearly state that a genetic basis for homosexuality is there. The data is right there. They have figured out that you get it from your Mother on her "x" chromosome. The exact mechanism can be worked out by beaker boys who care about that kind of stuff.
Whether people want to call it a disease or not is up to them.
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