Perhaps this?
Several studies also have shown that homosexuality tends to run in families. The probability that the brother of a gay man is gay is about four times higher than normal. Similarly, the odds that the sister of a lesbian is also a lesbian is significantly higher than normal. However, male homosexuality and lesbianism tend to run in different families: sisters with gay brothers are not more likely than normal to be lesbian. A 1993 study that traced the pedigree of pairs of gay brothers found that homosexuality tends to run on the maternal side of the family tree: the brothers had a higher than average number of maternal nephews and uncles who are gay.
Identical twin studies shed additional light on the genetic underpinnings of sexual preference. If there are differences in preference between identical twins, who share the same genes, then that difference cannot be genetic. Here, the research indicates that in cases where one identical male twin is gay, about half the time the other twin is gay as well. "This is way above 4 percent, so it's got to be genetic, but it is nowhere near 100 percent," Goldstein said. The percentages for lesbian twins are slightly lower, but generally comparable.
Differences between gay and straight sexual orientation appear at a very early age. In a study, a group of openly homosexual men were asked when they first became aware of their attraction to men and boys, when they realized that they were gay and when they "came out" to others. The group reported becoming aware of their attraction at a very young age, between 5 years old and puberty. Self-awareness of their sexual orientation took place around puberty, and coming out of the closet took place much later.