Discovering that her son had an inherited disorder was painful enough, but then Pru Irvine had to track down the rest of her extended family and drop a genetic time-bomb in their midst ...
I knew there was something wrong with my new-born son. No one believed me. But after a year with Henry, who would not eat, sleep or smile, I knew I could not take the smallest risk of having another baby. That summer I was sterilised. My family thought it was just another example of the tidiness of my life. But though I had no knowledge to support it, I had a very strong intuition that not only I, but they, needed protection and in trying to find out what was wrong, I unwittingly opened a chapter in all our lives that we must now live through for ever.

Henry is our second child, but he is the first in the family to present the traits of Fragile X. Despite being one of the commonest forms of genetically inherited mental handicap, relatively little is known about it. It gets its name from a gene on the X chromosome, the tip of which looks like it's broken but not quite separated, hence Fragile X. Boys are particularly vulnerable to disorders caused by the X chromosome because they only have one copy of it. Girls have two.

It took two years to find out the truth about Henry. We knew the hospital had something to tell us because a very careless doctor mumbled something about a biological imbalance. But he didn't have Henry's notes so he'd have to call back. He didn't. Instead, I stood in reception in the psychiatric department of Edinburgh Sick Children's Hospital and was told the news over the telephone. Henry is mentally handicapped. "No, it can't be cured, and personally," said his psychiatrist, "I know very little about this condition. By the way, because it's inherited, there are quite grave consequences for your respective families [mine and my husband's]. We'll have to refer you to the genetics people. They'll have to find out where this faulty gene comes from - whether from your family or your husband's." What did this mean to us? Nothing. We had no experience of this world of mental handicap or of its own very private language.

That night, while the children were in the bath, Richard and I sat on our bed and cried. He took my hand and said very quietly: "Well, dear, we'll never be able to get divorced now." We laughed. At that moment, when I knew it was possible to laugh, I also knew the sadness would never go. Later that evening, I found Henry banging his head on the wall above the radiator in the hall. In the split-second between watching him and stopping him, I felt different about myself. I felt vindicated, and with that came a new purpose. For the first time in our embattled two years together, I understood that we had failed to make certain connections with each other because Henry couldn't make them naturally. He would have to be taught. For him, it would be a long and very slow learning process. For me, it was the beginning of an entirely new way of thinking. Here was a child who would be with me in all senses for the rest of my life. Unlike his brother, who would grow up and leave home, Henry's ability to manage in the world would be grounded in a never-changing, absolutely constant home life with his mother and father - for ever. But this was just the edge of what was to come for the family.

The blood test which can reveal the fragile site where the X chromosome loops out in an affected child has been available since 1977. But it took almost another 15 years to find the gene that causes the problem.

Richard and I were genetically counselled. Blood samples were taken and our family trees were mapped out with the most extraordinary attention to detail. We have to find the gene, they explained, because contact must be made with all members of the family who are of child-bearing age. The geneticists were not domineering but they were certain of the need to do this and of the implications. Once the gene was found, they explained, all these people would need to be counselled. Any of the women found to be carrying the gene would need to know that their future children had a 50 per cent chance of having Fragile X. It would be up to them to decide whether they wanted to be tested. What about our cousins and siblings who already have children, we asked. They too should be tested, they said, because although they and their children may not have it, they may be carrying it and will pass it on. Their children's children will have it.

Why, I wondered, would anyone not want to be tested? Who would really choose to be emotionally battered and deprived of sleep? Who would really choose to take responsibility for someone who could not make sense of their world? Someone who was furious and distressed because his principal carer couldn't read his mind and he couldn't explain himself? I am the first mother in the family to have a mentally handicapped child. Or am I? Telling the family about Henry was more about me coming to terms with their sadness and their wish to be empathetic than it was about Henry's actual condition. Warning them that the scientists were looking for the gene and what finding it would mean was like dropping a time-bomb. There was a sort of philosophical silence, a waiting tinged with fear and ignorance. In my quieter moments, I knew my family couldn't blame me. But I often dreaded the idea that Richard's parents might think to themselves secretly that if he hadn't married me, none of this would have happened.

As Richard's family were old Scots, they were easy to map out. We knew where they were and who was involved. Family relationships were also good, which would make it easier to talk. My family were harder to track. Being Jews and subject to the consequences of the Second World War, we could only really find immediate family - siblings, cousins, grandparents etc. Family relationships were not good in all areas, so the thought of making contact after many years of silence and for such an unhappy reason was very difficult. I felt angry and guilty. I was always the noisy and demanding child of the family. I was always centre stage through sheer grit and determination. And now I was hoist by my own petard. Secretly, I was also furious that some bastard somewhere in the family hadn't had the decency or maybe the ability to own up to having had a mentally handicapped child. Living in an enlightened age has both its merits and its drawbacks.

The gene came from my mother's family. In the end, I didn't want to manage that side of the family where relations had been strained. The geneticists did it for us and it meant a trawl as far as Jamaica to find a married but childless cousin. She chose not to be tested but her sister and nieces and nephew were all cleared. An uncle came out of the woodwork to empty his boiling pot of stories about how mad and bad my maternal grandfather had been. He failed to ask a single question about Henry. Like the others, no one felt sure that the words mental handicap did not really mean mad. For all that we know, for all of our knowledge, we are afraid of madness and see it everywhere, even where it is not. Mental handicap is not madness. Richard's family breathed the deepest sigh of relief I suspect they've ever breathed. My father's family sported a minimum of four single women, all in stable relationships and all of child-bearing age. They will never have to ask themselves whether the risk is worth taking. Those who knew what was involved were scared. And that left my brother. Despite wanting his own children, his partner can't have them and so they have an adopted daughter. It turns out that he is a carrier; it would be his children's children who would be afflicted. What a curious twist of fate.

My mother took it on the chin, as she takes most things. We still don't know whether it was her mother, who is still alive (but too old at 104 to be bothered by such things), or her father, who is not. I believe my mother disliked her father. I hope the Fragile X gene was not his because I wouldn't like to think of her saddled by anger about her dead parent. It's not good to live with that. Although I am Henry's mother and live with it and him on a daily basis, I sometimes wonder if my mother feels badly - caught between the guilt of disliking a parent and the handing down of that legacy to her loved child.

I talk about Henry all the time with my friends. If it's true that our fears represent our greatest desires then, I suppose, not talking about Henry would make me fear losing him. It takes a lot of talk to learn to live with a mentally handicapped person. Then, when you think that you've succeeded, somebody says something that drowns you in that very same sadness you felt at the start of your new way of living when your child was diagnosed. On Sunday, my husband said to our lunch guests that he had long resigned himself to the knowledge that he would never have grandchildren. It is not for us to decide whether our eldest child, who does not have Fragile X, should be tested. If he so chooses when he's older, he will have to face the consequences. He will make a choice which will affect all the generations of our family to come. Whatever he knows now about his brother will all have to be explained again. But it won't be about Henry then. It'll be about him. What will he say to his partner and how will he explain that love does not conquer all?n