One of the most influential studies on the genetics of homosexuality was done by Dean Hamer and his co-workers at the National Cancer Institute in Washington DC (1993). Hamer's research involved studying thirty-two pairs of brothers who were either "exclusively or mostly" homosexual. None of the sets of brothers were related. Of the thirty-two pairs, Hamer and his colleagues found that two-thirds of them (twenty-two of the sets of brothers) shared the same type of genetic material. This strongly supports the hypothesis that there is an existing gene that influences homosexuality. Hamer then looked closely at the DNA of these gay brothers to try and find the region of the X chromosome (since the earlier research suggested that the gene was passed down maternally) that most of the homosexual brothers shared. He discovered that homosexual brothers have a much higher likelihood of inheriting the same genetic sequence on the region of the X chromosome identified by Xq28, than heterosexual brothers of the same gay men.
The results of a new study by a team of British psychologists released today provide powerful new evidence that sexual orientation is 'hard-wired' in the human brain before birth.
Dr Qazi Rahman of the University of East London (UEL) has been working with Dr Veena Kumari and Dr Glenn Wilson of the Institute of Psychiatry to investigate sex differences in the startle response - our eye-blink reaction to sudden loud noises.
The team discovered significant differences in the response between male and female, and heterosexual and homosexual subjects. Because the startle response is known to be involuntary rather than learned, this strongly indicates that sexual orientation is largely determined before birth.
An estimated 4% of men and 3% of women are homosexual, but the hypothesis of a 'gay gene' remains controversial. Recent studies report links between foetal development, testosterone levels and adult sexuality, but this study offers the first independent evidence of a non-learned neurological basis for sexual orientation.
Dr Rahman said, "The startle response is pre-conscious and cannot be learned. It is mediated by an ancient region of the brain called the limbic system which also controls sexual behaviour. This is very strong evidence that female sexual orientation at least may be 'hard-wired' in this region."
Scientists at UCLA have discovered 54 genes in mice which suggest that homosexuality may not be a choice as sexual identity is hard-wired into the brain before birth.
Researchers at the University of California, Los Angeles, have reported that sexual identity and orientation is determined by genetics according to their findings released on Monday. The findings could mean that sexuality, including homosexuality and transgender sexuality, are not a matter of choice.
Dr Eric Vilain, a genetics, urology and pediatrics professor at the University of California, Los Angeles School of Medicine, said in a statement: "Sexual identity is rooted in every person's biology before birth and springs from a variation in our individual genome," Reuters reports.