Dean Hamer, a geneticist at the National Cancer Institute in Washington DC, said he could be months away from finding the gene that appears to be linked with sexual orientation in some gay men.
Once the gene is found it would be possible to develop a test that could help detect carriers of the genetic trait. But Dr Hamer warned that if such a test were devised, it would be no more than 50 per cent accurate in identifying homosexuality; little better than relying on a toss of a coin.
Speaking at the American Association for the Advancement of Science in San Francisco, Dr Hamer said: 'People are worried that eventually someone - scientists, perhaps the military or insurance companies - might try to develop blood tests for sexual orientation, or a pre-natal test so that expectant mothers could abort a foetus at risk of being gay.'
He added: 'I think that would be wrong, unethical and a terrible abuse of the research. It's wrong to discriminate on the basis of genes.'
Dr Hamer said he would oppose the development of a blood test at every opportunity and would try to block companies that attempted to invent a test based on his work. 'We'll have the intellectual property rights for that work and that means we will be able to not give those rights to people who are likely to commercialise them.
'I've been informed that it is possible to take out a special type of legal thing, which is not a patent, saying basically I discovered this, certain applications are obvious, I claim the right to all such applications. If I could get such a document then of course I would not allow it to be used commercially.
'I would try to prevent any commercially available test of this work . . . I am going to do everything within my ability (to stop a test being developed).'
However, he admitted there was no guarantee he could stop companies from circumventing his opposition: 'I would hope it would slow it down. Ultimately it will be up to the expectant mothers.'
Dr Hamer said last July that he had found evidence of a genetic trait on the X chromosome - which men inherit from their mothers - linked to the sexuality of gay men. Further research has since supported the 'gay gene' theory.
Finding the gene was anywhere from two months to 20 years away. 'When we come closer to actually finding genes that are involved, the ethical quandaries are made much more real. This is not an issue that just affects some people. This is going to affect everybody. There are going to be genes for IQ, musical ability and the like.'
Dr Hamer said research into sexuality was important despite the ethical risks involved. 'We'd like to understand how the human mind works and how genes and the environment contribute to the wiring. Sexuality is a very important part of human behaviour . . .'
If the 'gay gene' was found it would be possible to try to work out what it did. One possibility was that it produced a protein or enzyme in the brain which led to differences in brain development between gay and straight men. Alternatively 'it could be there's a gene that makes (some) men self-reliant and not dependent upon the views of others'.