William, 24, has been homosexual ever since he was aware of his sexuality. 'It is so deeply engrained, so far back, that you can't separate 'nature' or 'nurture',' he said.
The possiblity that genes could be reponsible for homosexuality is of little personal concern for William, who came out when he went to university and professes not to have any curiosity about why he is gay.
But for his mother, Marion, there is the beginning of an explanation of why William is gay but his two brothers are not.
'I would not be surprised. From my knowledge and from what William has told me, he was aware that he was gay at a very young age. That suggests to me that there was something in his physical or chemical make up that was the cause.
'I don't take responsibility in terms that I dominated him or browbeat him, which is often the reason given,' she said.
'When he was 16 he went through an androgynous phase when he would wear a lot of make-up and jewellery. I found him borrowing my clothes but I don't think he became gay because it was fashionable.
'When he told me I was sad. Life is hard enough without having to face bigotry and hostility. But if he is happy about what he is then it's less of a problem for me,' she added.
The pink cardiganed uncle has died and Mrs Stewart, 59, a social worker in Cambridge, could not think of any other gay relatives.
Dixi, who is 27 and co-presenter of Gay and Lesbian London on the BBC's GLR radio station, said family discussions on sexuality are often heated but there is no aggression or bitterness.
She is bisexual and maintains that the debate over genetic influences masks the central issue that people have a free choice over their own sexuality.
She does not worry about the possibility that she will pass on William's gay genes to the next generation. 'I want to have children very much. I am very grateful that William is there as there will always be a strong gay role model.
'My great fear is if the children are straight. A straight daughter I could deal with but a straight son I would find difficult,' she said.Reuse content