ManOnFire:
I think we’re talking past each other. I hope I can clear some things up:
Behaviorial genetics almost always involves more than one gene, simply because behavior is too complex to be reduced to a single gene, which more often than not codes for a single protein. Furthermore, in all genetic cases, simply having a gene doesn’t mean a person will express that gene. To begin, a gene has to be copied into an RNA molecule (which is very similar to DNA), and then that RNA copy has to be translated into a protein by an organelle which itself is constituted by RNA (and thus has its own genes to code for the structure!). For any gene to be transcribed into RNA takes a host of transcription factors, which are proteins which themselves are created by separate genes. You might have a particular gene for a particular trait, but if the transcription factors necessary to produce the effects of that gene cannot be made (due to the effects of a different gene) then you won’t express the trait indicative of that gene.
Once you have a functional RNA copy, a tRNA molecule has to facilitate its translation into protein (and, yes, that tRNA has its own genes to code for itself). As you could see, the problem becomes compounded in that the simplistic genetics we’re taught in introductory biology isn’t true. For example, we’re taught (correctly) that sickle cell anemia is due to a single gene, namely the HgbS gene on Human Chromosome 11. If you have one copy of this gene, you have partial sickle cell, if you have two copies you have full blown sickle cell. The odd thing is that there exists individuals who have one copy of this gene yet exhibit nothing resembling sickle cell phenotype (i.e. sickled shaped red blood cells). Why is this? Its due to another gene mutation on chromosome 15 (rSwns)which corrects the initial sickle cell condition. In this case we’ve found a genetic cause for a condition (sickle cell) which can be alleviated by another genetic cause. In both cases, the cause is undoubtedly genetic.
There are other cases of obvious genetic diseases which can be corrected by changes in cell biochemistry, resulting in the lack of expression of a gene an individual nonetheless carries. This is what I meant by the complexities of genetics.
We have very strong evidence of sexual orientation being biological, and more than that, genetic (see the references I cited earlier, particularly the first three). You will never find a 100% co-incidence rate among monozygotic twins for anything simply because the genetic expression of our DNA depends on more than just the mere DNA sequence.