Genes are involved in determining sexual orientation, a new scientific study has found. Michael Bailey, a psychologist at Northwestern University in Illinois, and his colleagues discovered that at least two chromosomes have an effect on whether a man was gay or straight. Bailey disclosed the results of tests of 409 gay brothers and their family members at the annual American Association for the Advancement of Science conference in Chicago. Genetic factors, the researchers claimed, account for between 30 and 40 percent of a person's homosexuality. Bailey told The London Times, “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play and we found evidence for two sets that affect whether a man is gay or straight.” However, Bailey's research does not support the existence of a sole "gay gene." He said sexuality was also formed, to a significant degree, by environmental factors. "Don't confuse 'environmental' with 'socially acquired'," he said. "Environment means anything that is not in our DNA at birth, and that includes a lot of stuff that is not social." Dr. Alan Sanders, associate professor of psychiatry at Northwestern University, who led the study, told The Telegraph, “We don’t think genetics is the whole story. It’s not. We have a gene that contributes to homosexuality, but you could say it is linked to heterosexuality. It is the variation.” Bailey and his colleagues' study echoed the findings of a 1993 study by the geneticist Dean Hamer, which suggest that the genes influencing homosexuality in men may be associated with the Xq28 marker on the X chromosome. Bailey's study separately identified a second genetic region, on Chromosome 8, which also appeared to predict whether a man would be homosexual. To date, no similar genes have been discovered which influence female homosexuality. The latest study also bears out the results of a study of almost 4,000 same-gender twin pairs in 2008 by researchers from Queen Mary's School of Biological and Chemical Sciences, and the Karolinska Institutet in Stockholm, which found a combination of genetics and environmental factors, "which are specific to an individual, and may include biological processes such as different hormone exposure in the womb," were key factors in determining homosexuality. At the time, Dr. Qazi Rahman, the study's co-author, said the study "puts cold water on any concerns that we are looking for a single 'gay gene' or a single environmental variable which could be used to 'select out' homosexuality--the factors which influence sexual orientation are complex. And we are not simply talking about homosexuality here--heterosexual behavior is also influenced by a mixture of genetic and environmental factors."