TWO years ago Dean Hamer started his study of how sexual orientation can be influenced by heredity. The National Cancer Institute (NCI), in a suburb of Washington, was interested because it wanted to find out how far genetic factors were behind two forms of cancer, Kaposi's Sarcoma and lymphoma, which are frequently found in gay men with HIV, the virus that causes Aids.

Working with four colleagues, Dr Hamer studied the family backgrounds of 114 gay men and found that their brothers, maternal uncles and maternal male cousins were more likely to be homosexual than would be expected. In some families Dr Hamer's team was able to trace back gay relatives for three generations. Because in some families the homosexual uncles and male cousins of the gay subjects were raised separately in different homes, it seemed to Dr Hamer that heredity might be behind their sexual orientation. He and his colleagues then looked for and found direct evidence for a genetic connection by studying the X chromosome - the sex-linked chromosome that men inherit only from their mothers - of 40 pairs of gay brothers.

Thirty-three of the gay pairs had both inherited genetic markers in the same chromosome region, implying that 65 per cent of families studied were passing on a gene responsible for homosexual orientation. 'The statistical significance of the results was better than 99 per cent, which means that the possibility of obtaining our findings by chance is extremely unlikely,' Dr Hamer says.

As for the possibility that his discovery might lead to the 'screening out' of babies with the gene, he said this would be 'fundamentally unethical'.

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