[CORed]

Dibitter has the right answer as to how it could be hereditary: recessive gene or genes, and possibly the genes confer some sort of evolutinary advantage to monozygotic individuals. I think at this point the scientific answer is "insufficient evidence". I believe that there have been some twin studies done: If being gay is totally genetic, both identical twins would always be gay.

IIRC, there was a fairly high conordence rate, but it wasn't 100%. Also, if both twins were raised by the same parents, it's hard to determine whether the fact that both twins are gay is due to heredity or upbringing. It is possible to look at both fraternal and identical twins. If the concordence rate is higher in identical twins, that's an indication that genetics are a factor. As with many things in human behavior, it's likely that it's a combination of genetics and environment: There may be a gene or genes that crate a predisposition to be gay, but social factors may trigger it. A lot of gay people say they knew they were different from a very early age. However, it's hard to say whether this is real, or a rationalization for a choice which carries qutie a bit of social stigma.

Okay, here's a question for the christian right types. If it can be determined through genetic testing that there is a 80% probability that an unborn child will be gay, should that child be aborted?