THERE has been much debate over 鈥済ay genes鈥�. Now an intriguing study raises another possibility: in some cases, variations in the genetic program we inherit from our parents, rather than in the genes themselves, might determine sexual preference.
Our genome is 鈥減rogrammed鈥� by the addition of chemical markers called methyl groups to the DNA, which shut down genes. One of the most dramatic examples of methylation is the shutdown of one of the two X chromosomes (one from each parent) in every woman鈥檚 cells, a process called X-inactivation (快猫短视频, 10 May 2003, p 42).
Normally, this process is random; either of the X chromosomes can be inactivated. But when Sven Bocklandt of the University of California, Los Angeles, compared blood and saliva samples from 97 mothers of gay men with samples from 103 mothers without gay children he found this process was extremely skewed in the mothers with gay sons, with one X chromosome being far more likely to be inactivated than the other.
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Only 4 per cent of the mothers without gay sons showed this skewing, compared with 14 per cent of mothers with at least one gay son. Among mothers with two or more gay sons, the figure was 23 per cent.
Such skewing is generally associated with genetic disorders, but the mothers all appear to be healthy. Their daughters also seemed unaffected, with only 1 out of 24 showing skewing.
Bocklandt suspects that whatever is causing the skewed methylation of the X chromosome also affects the methylation of certain genes on the chromosomes the women pass on to their sons. Mothers might not be resetting their own 鈥淚 like males鈥� program, he told a meeting of the American Society of Human Genetics in Toronto last week.
鈥淚鈥檓 not absolutely persuaded, but it鈥檚 an interesting hypothesis,鈥� says Ian Craig of the Institute of Psychiatry in London. 鈥淯ntil you鈥檝e got some molecular way to test it, it鈥檚 just a nice idea.鈥�
