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One theory that fits the available observations is that the penetrance of the "gay gene(s)" is approximately 67%. That causes about half of the males with the gene(s) become gay. "There could be hundreds of millions of straight men walking around with this gay allele but who are straight simply because it didn't penetrate" In the case of the "gay gene(s)" perhaps 10% or more of all males have the allele that causes homosexuality, but in many cases is the allele not "triggered."

Assuming that the penetrance of the "gay gene" or "gay genes" is 67%, then one would expect that if one fraternal twin was gay that the other would also be gay about 22% of the time. This number also agrees with studies of families with fraternal twins.