The answer is, I think, much to do with how complicated our genes are, and how complex the relationship is between geontype and pheonotype (ie organism). The DNA is not a blueprint to the organism, it is a description of a number of processes that will, in the right environment, lead to the production of this organism. So the idea we should have some specific "gay gene" is, I think, quite unlikely. As others have pointed out, such a gene would be selected against in the wild.
It is worth noting that we receive genes equally from both our parents. Females carry all the genes for how to produce a male and vice versa. It is equally probable a man has inherited his genes for penis size and shape from his mother as from his father.
Also, our sexual attraction is to some degree a result of social processes. Our sex drive is obviously genetic, but what and who we are attracted to may well be the result mostly of social conditioning.
Finally, the physical differences between sexes are not that significant. The traits a heterosexual finds sexually attractive in the other sex, may well be prevalent also in his or her own. Such a person could, given the right social circumstances, end up being attracted mostly to his or her own sex.
In my view, homosexuality simply represents one side of the range of human sexuality. Heterosexual men, for example, are attracted to a range of female body types. I don't see a very distinct line to cross from being attracted to same sex. Obviously, in cultures such as ours, which frowns on homosexuality, people feel that way. But that is social conditioning. Some cultures consider homosexuality for what it actually is: one part of the range of normal human sexuality.
- Jan
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Faith, n. Belief without evidence in what is told by one who speaks without knowledge, of things without parallel. [Ambrose Bierce, The Devil´s Dictionary, 1911]