By Andy Coghlan. A genetic analysis of pairs of gay brothers, including sets of twins, has provided the strongest evidence yet that gay people are born gay. The study clearly links sexual orientation in men with two regions of the human genome that have been implicated before, one on the X chromosome and one on chromosome 8. The finding is an important contribution to mounting evidence that being gay is biologically determined rather than a lifestyle choice. The region on the X chromosome picked out by the study, called Xq28, was originally identified in by Dean Hamer of the US National Institutes of Health in Bethesda, Maryland, but attempts to validate the finding since have been mixed.
How Older Brothers Influence Homosexuality
Largest study of gay brothers homes in on 'gay genes' | New Scientist
A new study of twins provides the strongest evidence yet that homosexuality has a genetic basis, researchers say, though they say other factors like social conditioning may be important. The study, published in the December issue of The Archives of General Psychiatry, adds to evidence that sexual orientation does not result from a maladjustment or moral defect, one author said. Michael Bailey, an assistant professor of psychology at Northwestern University in Evanston, "which is exactly the kind of pattern you would want to see if something genetic were going on. Bailey was referring to brothers by adoption. The study examined 56 identical twins, 54 fraternal twins and 57 adoptive brothers recruited through advertisements in gay-interest publications. Identical twins are genetic clones, having developed in the womb from a single egg that split after being fertilized by a single sperm. Fraternal twins develop simultaneously from two separate eggs fertilized by two separate sperm cells, making them only as similar as non-twin siblings.
Fraternal birth order and male sexual orientation
Volunteers from over families responded. Researchers asked them questions about their sexuality, the size and structure of their families, and took DNA samples. The region also goes by another name: GAY-1, a genetic marker linked to male homosexuality. Conservative attitudes remain unchanged, however. For instance, identical twin brothers siblings derived from the same fertilised egg are more likely to both be gay than fraternal twins twins that develop from separate eggs.
By Hannah Frishberg. Researchers analyzed 10 separate studies on sexual orientation, involving a total of 5, men and information regarding their birth order. The findings also showed that t he more older brothers a man had, the higher the likelihood he would be gay. The study also found a correlation between mothers giving birth to homosexual males and having more children, as compared to moms of straight males having fewer children.