In round numbers, if one identical twin is gay, odds are about 50% that the other twin is gay. Sharing less genetic material, if one fraternal twin is gay, there is about a 25% probability that the other twin is gay. These percentages are more than 10 times and five times (respectively) the incidence of homosexuality in the general population.
I have argued that twin tests prove that homosexuality has a genetic link, at least in part. According to medical science, sexual orientation involves genetic and environmental factors. Environmental to scientists means anything that is not genetic. Some speculate, for example, that a birth mother’s hormones are an environmental factor contributing to sexual orientation.
Religious conservatives argue that identical twins have the same DNA. Therefore, if sexual orientation is even partly genetic, identical twins should have identical sexual orientation. I have countered that the difference is attributable to the subgenome.
An article in the New York Times seems to put the conservative argument to rest:
In recent years, scientists have gained a clearer picture of the early development of the embryos of identical twins. Originating from a single fertilized egg, they later acquire unique genetic mutations. New advances in DNA sequencing are making it possible to pinpoint those mutations — and to tell identical twins apart.
The article in question is about telling identical twins apart for criminal forensic purposes. The article confirms that there are genetic differences in identical twins even though they have identical DNA.
A mutation is a change in a genetic sequence. Mutations include changes as small as the substitution of a single DNA building block, or nucleotide base, with another nucleotide base. Meanwhile, larger mutations can affect many genes on a chromosome.