From a blog called A Survivor's Guide To Huntington's Disease?, there is a stern note from its author to an anticipated future self.

"Angela, however you feel, whatever is happening, you must listen. If you can't listen to anyone else, listen to me. You can trust me because I am you.

1) The world owes you nothing. You have been desperately unlucky, that is all.

2) Sign those power of attorney forms. Your attorneys are to be your sister and Mr F. You will allow them to act jointly and severally, except in big financial matters where they must act jointly.

3) Any research opportunities offered to you, you must take them. Grab every chance you have.

4) Any pills prescribed to you for HD, you must take them and follow the instructions to the letter.

5) Please honour me and who I am - do not push away those that love me, do not intentionally hurt or deceive them. Use the blog to let off steam not your loved ones."

That's only half of Angela's list but enough to demonstrate the provision she's made for losing her mind. Angela F - her online name - knows there is every chance this will happen because it's happening to her mother, as it happened to her mother's mother too. Huntington's is an illness of habit: if you inherit the disordered gene that gives it life, it has a way of being the death of you, beginning with the you that you once were. Angela learned after being tested last year that the gene in question had been handed down to her, as it is to 50% of the children of those with Huntington's. Early symptoms include clumsiness, mood swings and concentration failures. They typically start showing between the ages of 30 and 50. Angela is 28.

"I've already started looking out for them," she tells me. "I didn't do that before I took the test. The other day, I was filling the kettle and accidentally left the water running so it overflowed. I had to laugh: you know, was this the start?"

You'll have spotted straight away that Angela has the heart of a lion. Further evidence is her intention to leap from an airplane to raise funds for research into a cure for Huntington's, a disease that eats away at body and mind alike. Its varied manifestations may go unidentified or lie dormant until those afflicted become parents. To the damage done by the disease is added the dismay of discovering what you may have bequeathed.

This is what happened to Angela's demanding, deteriorating mother. Her daughter, who provides all the support she can, has decided to spare her the ultimate bad news about her medical legacy. "She will be the only person I won't tell," says Angela. "I can't see any benefit for either of us." She knows, though, that the enemy is going to come for her. In some respects, her prior knowledge has simply brought forward her confrontation with dilemmas that assail those with grave illnesses of any kind: which friends and family members do you tell about your diagnosis? When do you tell them, and how?

Angela's fiance, "Mr F", was with her to hear the outcome of her test: "He's a top bloke," Angela confirms. When and how, though, would Mr F break the news to his family? The next time he and she saw them would be Christmas. "Great timing!" wrote Angela, wryly.

But the repercussions of inherited diseases tend to be of particular kinds. Increasing knowledge of the genetic basis of some illnesses generates clues about the future, and NHS provision of genetic counselling has increased accordingly. Frances Flinter is a consultant clinical geneticist at Guy's hospital in London. She says that roughly half of those using its counselling service have concerns about cancer: "They'll want to know if they have a predisposition and if they do what the best course of action is. Should they enter a screening programme, for example?" And whatever the disease in question, there are huge concerns about passing it on to children, existing and prospective ones alike.

In the case of Huntington's, the numbers crunch unambiguously. With others, both mother and father need to supply the chromosome containing the flawed gene for the illness to have any chance of developing. The child of a pair of "carriers" - people with the gene but not the symptoms - has a one-in-two chance of being a carrier too and a one-in-four chance of being born with the condition.

The sickle-cell group of disorders and cystic fibrosis are two of the best known of this latter category. The percentages governing their transmission mean that they may not be conspicuous in family histories, and carriers are therefore often unaware of being so. Carol Nwosu didn't know she was a carrier of sickle cell until she had a blood test when pregnant with her first child, Mark. After a test on her husband, Ranti, showed that he was a carrier too, they knew that their baby had a 25% risk of having to live with the consequences of a malfunction in the haemoglobin that can cause anaemia, severe pain and fatally damage internal organs. Although ruling out a termination, the Nwosus opted for a foetal test, which showed that Mark would be born a carrier, like his parents, but would not develop the disease. "It was such a relief," Carol recalls.

Her memory of her second pregnancy, though, is pure anguish. By then, Carol knew much more about sickle cell and its effects. This time the foetus was revealed to have sickle-cell disease. Despite severe misgivings rooted in her Christianity, Carol felt she could not bring such a child into the world, and so she had a termination - a decision she later found she was unable to repeat. When she became pregnant once more, she declined all tests: "I decided, whatever God gives me, that's what I'll take."

She got Danny, now 10, who has sickle-cell disease and has already survived some of the worst it can do: as well as routinely enduring pain, Danny has had his tonsils and adenoids removed and, when he was two, his spleen. At six, he had a stroke. "If you met him there's no way you wouldn't like him," says Carol. "He's just so different, so courageous. When I'm low, he says, 'Come on, Mummy, snap out of it!' It should really be the other way round." She's since had a third child - six-year-old Ruth is a carrier like Mark. As with him, this was discovered during pregnancy. "I was so happy when I heard, I jumped up and down on my bed," Carol recalls. But what if Ruth had been as unlucky as Danny? "I think we'd have gone ahead anyway," says Carol, "believing that God will not give us what we cannot bear."

Rebecca and Adrian Saunders are two more parents with no regrets, even though they too could have done things differently. When they learned from scans during Rebecca's pregnancy of the possibility of their first child, Seren, having cystic fibrosis, Rebecca says she'd "never even heard of it". But within weeks, she and the disease were intimate. Seren, now four, has received antibiotics "every 12 hours since she was nine days old". Her daily routine is unrelenting: "She has medication with every meal and we have to pound her chest twice every day to loosen the mucus."

The Saunders' decision to try for a second child may strike some as inexplicable. To rush to that judgment, though, would be to dismiss the year of agonising over the rights and wrongs. "We'd always wanted a large family," Rebecca says, "but at first, because of her having CF, we decided Seren would be our only child. Yet by the time she was eight months I literally ached for another baby. And Seren was doing so well." Seren's symptoms, Rebecca explains, have always been quite mild. "Her coughs and colds hang around too long, but she's never had a chest infection and that's unusual with CF."

This scenario unfolded without the new technique of pre-implantation genetic diagnosis (PGD) being an option for the Saunderses. PGD can be carried out on embryos conceived by means of IVF and so helps those who know or suspect that there is a problem to eliminate it by having only those embryos found to be free of the faulty gene in question implanted in the mother's womb. It has the great advantage of preventing any decision about "selective termination". Rebecca and Adrian, though, had a plainer set of choices. They proceeded on the basis that even if any second child they had fell, like Seren, into the unlucky 25%, at least the consequences were likely to be as manageable as Seren's were proving to be.

Sadly, their two-year-old son, Dylan, not only has the disease but has it more severely. The initial blow of this discovery was particularly heavy. Having already ruled out a termination, the Saunderses had declined amniocentesis, which would have revealed the condition well before term. But when all Dylan's antenatal scans showed no cause for concern, they felt they had every reason for optimism. "When I found out he had it, I thought I would explode with grief," Rebecca says.

She and Adrian immediately ruled out having a third child: "If we'd had another with CF, I don't know how we would have coped. And if it hadn't, it would probably have felt left out because the other two need so much attention." And Dylan? "If we could turn back time to just before we conceived and I'd known he was going to get it, I don't know if we'd have done it. But as soon as I was pregnant with him, he was here to stay."

A harsh view might be that the Saunderses played the odds, lost, and have only themselves to blame for what has happened to them and, most importantly, to their son. But those affected by inherited diseases know best of all how difficult such decisions are to make. For Angela, living in the shadow of Huntington's, the conundrums have long been part of her daily life. And now she knows the worst she is obliged to consider - the demands this will, at some stage, make on others. This requires no great leap of her imagination: "I only have to look at my own mother," she explains. Again, she's done her out-loud thinking on her blog. These lines appeared on December 20 last year:

"When I first realised that HD ran in the family, I was completely against having children. I thought it would be unfair of me to inflict my potential illness on them. I thought it would be selfish to put my desire for children above their need for a stable family home. And maybe it would be. But as time has passed, I have started to think about children more and more ... "

Her thinking has produced one firm conclusion: she and "Mr F" are to marry and hope to start a family by way of PGD. But what of the other side of the quandary?

"Maybe life will not be easy for my children, but who can guarantee an easy life? I have so much inside me that I can share, and will be able to share. After all, whatever problems my mum had and will have in the future, her and my dad brought [their children up] well. We all have happy memories of our childhoods, we all went to uni, we all have good jobs, we are in long-term relationships. We are all content with what we have. All this, even though we all had a 50/50 chance of having the HD gene ourselves. This, at least, is something I will be able to protect my children from."

Angela F's blog: survivinghuntingtons.blogspot.com

Huntington's Disease Association: hda.org.uk, 020-7022 1950

Carol Nwosu runs a sickle cell and young stroke survivors support group in London SE15, call 020-7635 9810.

Sickle Cell Society, sicklecellsociety.org, 020-8961 7795

Cystic Fibrosis Trust, cftrust.org.uk, 020-8290 7912