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Homosexuality is a normal, healthy variant of human sexuality. It isn't a disease, doesn't mean that you have faulty genes or a brain that's wired wrong, and it isn't in the least harmful to the individual organism. It also isn't harmful on a species-wide level unless it reaches the prevelance that makes it so that the population cannot be maintained at a healthy sustainable level.
There is evidence that the likelihood of homosexuality is increased if one has a homosexual sibling (anectotally, in my family there we have one lesbian, one bisexual woman, one transsexual woman out of nine children; in Grace's family two lesbians out of seven children). However, this does not indicate an exclusively genetic cause, as siblings also tend to share, to some extent, both a natal environment and a social environment, both of which can have an effect on sexuality.
Twin studies have found roughly a 50% correlation among identical twin boys. If one boy is homosexual, the other has a 50% chance of being homosexual. This indicates that there are at least some environmental factors at play here, as the boys in these cases share all of their genes, and all of them are active. There's a slight difference in female identical twins, in that thought they'll have all the same genes, a small number will differ in which is active. The human body needs only one active X chromosome. Since women have two, one from the father and one from the mother, one of these gets switched off in each cell. This happens when the zygote is at about 100-120 cells, half the maternal and half the paternal genes are deactivated and then reproduced true for the remainder of the development, resulting in small genetic differences in terms of which genes are active in which systems.
The most common explanation for the difference in identical twin boys is that of gestational hormones, which can vary even between identical twins, same womb, different hormonal environment. Because these hormones have a profound effect on brain development moreso in males than in females due to the fact that female is the default format and male requires active changes, variant levels can have life-long effects on later behavior.
Gilda
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