X Chromosome Inactivation in the Mothers of Gay Sons
A study reported in Human Genetics finds a relationship between X chromosome inactivation in mothers and the likelihood that their sons will be gay.
As I have mentioned previously, men have the classic XY chromosome combination. The single X chromosome, by necessity, performs all of this chromosome's genetically necessary functions.
In women, who have the XX chromosome combination, one of these X chromosomes is inactivated. Otherwise, a superfluous duplication of gene expression could have negative effects as it does in Down syndrome, where an extra chromosome 21 is present in the genome.
One of these X chromosomes came from the female egg, the other from the male sperm. Researchers found that in general, which one of the pair is left on, which is turned off, in the various tissue cells they sampled from the mothers in their study, is a more or less random 50/50 event. In some women, though, the same member of the pair is inactivated in every sampled cell in her body, an effect referred to as "extreme skewing."
When examining mothers who had no, one, or more than one gay son, they found the incidence of this effect in 4%, 13% and 25% of these samples respectively. Clearly there is a relation between how a woman's cells go about X chromosome inactivation and the likelihood that she will have a son who is gay.
The study provides no insights as to the existence of a "gay gene." If anything, it attests to the complexity of the genetic factors which contribute to sexual orientation determination. In fact, nothing in the study is in conflict with my previously cited reference.
People who still argue that a person becomes gay due to environmental factors would find it hard to explain how a child's upbringing could retroactively go back and change the mother's X chromosome inactivation propensity.
If we take a man who is gay and compare him with another unrelated man, we find that the chances that this other man is gay are the same 4%-5% we would expect for any man randomly chosen from the population. If we look at a gay man's fraternal brother, the likelihood that he too will be gay goes up to 11%. If the brother is an identical twin, the likelihood that he is also gay jumps to 22%. Hmm, 4%, 11%, 22% vs. 4%, 13%, 25% is this study. I see more than just a coincidence in these figures.
While this study leaves many questioned regarding the genetic factors that influence sexual orientation unanswered, I'm pleased to see progress being made in this important area.