When, towards the end of 2004, Sarah Gabriel decided to undergo testing to see if she had an inherited predisposition for breast and ovarian cancer, she thought little about the result, assuming it would be clear. It seems amazing now, Gabriel reflects, given that her own mother died at 42 from ovarian cancer and a cousin of her own generation had already been diagnosed with breast cancer. But despite this, 12 weeks later, when phoning for the results, she pulled rank on a nurse who suggested she make an appointment to attend the unit.

"It's my blood," Gabriel told her, wanting to put the whole thing behind her: "I want the result now." "I'm very sorry, Mrs Gabriel," the nurse said, "you have tested positive for a mutation on the BRCA1 gene. We are not sure of its significance... But it is a mutation."

Within two years of this phone call, Gabriel, then aged 45, had lost her breasts, her ovaries, her hair and was fighting for her life; fighting for a way to prevent her two daughters, aged five and three, from growing up motherless, just as she had grown up herself (though she had been a teenager when her own mother died).

Gabriel's devastation was not only about cancer, and the obvious threat of death. (She had several grade three tumours in her breast, the cancer was invasive, and with mastectomy and chemotherapy she was given a 75% chance of surviving five years.) It was also the realisation that she had spent more than 20 years blaming her mother for dying and leaving her alone in the world. But BRCA gene mutations were only identified in 1994. Now Gabriel understood fully that her mother had been utterly powerless in the face of her genetic make-up. The very illness that had robbed her of her mother was now coming to get her, too.

Sarah Gabriel is sitting on her sofa in a terraced house in Oxford. She is a quiet, reflective woman, with the considered, intellectual manner of a bluestocking (she is married to an Oxford academic and taught English herself at one point). In physical contrast, she is glamorous, dressed in white jeans, flats and a tasteful, flowing shirt. The parts of her identity initially lost to cancer - her breasts and her hair - now look fabulous. Despite being told that her flowing blondeness could well grow back grey and bushy after chemotherapy - one of the many staggering insensitivities on the part of all sorts of people, from shrinks to mums on the school run - it is shiny and expensively highlighted. To an outside eye, her breast reconstruction is the best any woman could hope for in such dreadful circumstances.

Gabriel acknowledges that a diagnosis of cancer affects women in many different ways: "Some women never contemplate death," she says quietly. "For me, I'd watched it, it was part of my life experience. You always worry about repeating the patterns of your same-sex parent, and I had watched my mother die."

That moment was 1980, when, at the age of 19, she had been called to her mother's bedside in Scotland from her room at Oxford, where she was reading English. She got home just in time to watch her mother slip away. Until then, she had been clueless that her mother even had cancer, although she had been very ill. It is hardly surprising that Gabriel could never get beyond her rage at her mother for suddenly leaving the world with apparently no warning. As if this were not tragic enough, her father, left alone save for grief and the responsibility of five dependent children (Gabriel is the eldest) shut down. Ten weeks later he began a new relationship with another woman and tried to inoculate himself to the despair surrounding him by never talking about his deceased wife.

For Gabriel, there came the unbearable recognition that where once she had been the abandoned, raging teenager silently screaming at her dying mother, "I hate you!", her daughters were now destined for the same fate. It was crippling to contemplate for her beautiful, happy children, quite apart from the obvious implications of a genetic disease. (Inherited mutations on the BRCA genes are not a given, which is to say that some relatives are luckier than others in escaping them).

It was this emotional complexity that drove her to write down her feelings following the diagnosis in March 2006. The result is Eating Pomegranates, A Memoir of Mothers, Daughters and Genes, which is published next week. It is a beautiful, heartrending book (Gabriel is also a graduate of the prestigious UEA creative writing course), written in real time, with a tone that moves from anger and bitterness - at her illness, at her mother and her father - through to peaceful acceptance. Primarily, it is a story of mothering: "Despair for me was a very energetic state," she explains. "I had such extremes of mood and feeling that if I hadn't had the focus of writing sentences in a row, one after the other, I don't know what would have happened to me. I just didn't know where to put all the feeling I had."

Shortly after her mother died, Gabriel married the man who is now her husband - not because she was ready for marriage but because he was a port in the storm (they have weathered the storm well). When they first met, she was still sleeping with pills under her pillow, and in the years to come she would see pretty much every type of counsellor and psychiatrist to help her overcome her despair. Only after more than a decade of marriage did she feel ready for motherhood herself. The moments of true happiness were the birth of her daughters, and the process of bringing them up.

"What am I to say to them?" she writes of her beloved girls after she finds out about the breast cancer. "That I am going to die, as my mother did before me. That I am going to grow thin, thin, thin. That my eyes are going to hollow out until they are so intensely blue they will follow them for the rest of their lives, waking and sleeping. That I will be sick in a bowl because of the agony I am in. That the sight of their golden faces is not possible for me any more because I know I have to leave them. That they will have to watch death claim me, their beloved mother, and be helpless to do a thing about it."

In another passage, she records how she felt one bath time, watching her little daughters, at that point oblivious to her illness, stuff damp towels down their pyjama tops and prance around with their chests puffed out, bumping into each other, helpless with laughter and shouting, "I've got boobies and they're bigger!" "Look, mine are bigger!" "And for them?" she writes, contemplating the genetic predisposition, "will this be their fate?... However miraculous a recombination you are, however much nothing your like has ever been seen before in nature, nor ever will again, you may be doomed to disaster, to amputations, or to an early and painful death... Can it be that for the people one loves most, one is this helpless?"

The sections of the book which tell the story of how she coped with her girls, how their home deteriorated as she and her saintly-sounding husband tried to come to terms with what was happening, are the most painful and the most powerful. "The biological bond is broken," Gabriel says, at one point, of her daughters. For a time, Gabriel withdrew, unable to mother them at all. She could not get them ready for school, or read them a story, or deal with them in any meaningful way; as her children demanded pudding or a story, or refused to brush their teeth, she was at her most ill. The youngest starts peeing herself at school. The eldest says to her in a fit of rage at her lifelessness: "I hate you! I completely completely hate you!" "How I remember it, that hatred," Gabriel writes, "my loathing for my mother knew no bounds if she exhibited the least sign of weakness."

Gabriel simply wanted tranquillisers and to stay in hospital. At another point during treatment, she went as far as researching Dignitas, the assisted suicide organisation, and talking to her GP about travelling to Switzerland. I tell her I was shocked by this, unable to go the distance required to imagine having such extreme feelings of withdrawal from my children: "Yes," she says, "it was a real and awful thing. It was like shattering pieces falling round a self that no longer existed. Straight after diagnosis, I couldn't deal with the children any more than I could deal with a neighbour looking at me, a mum looking at me sympathetically at the school gate, someone asking me how I was. I still can't think about that time without feeling upset. But the break with the children probably lasted several weeks, a couple of months, in and out. And then continued with the trauma of each surgery, when I had to be reinserted too quickly into family life and didn't have the strength for it."

Much of Gabriel's life, until the birth of her children, had been shaped by a sense of worthlessness, medicated by low-dosage antidepressants. In the midst of motherlessness herself, she assumed herself unworthy of staying around for. Her mother's death went to the heart of her self-esteem: "When everybody else has a mother, it is very difficult not to think that the reason you haven't got one is because you're not worth that care and support. What it can feel like is that your mother has gone away because..." she breaks off. Was her terror that her children would feel this internalised shame too? Is that why she withdrew from them? "I think it was more simple than that. You have to make a safe narrative for children, that is your job as a mother and I wasn't in a place to do that. I was in too much shock, too terrified. I didn't know what to say to them because I couldn't make it safe for myself. I think being put in a similar position to my mother probably triggered another whole deep grief cycle, too. I felt devastated that this thing I had had killed her, that we hadn't been able to save her."

Slowly, with the help of various charities and counsellors, Gabriel and her family began to face the disease together. Gabriel learned how to talk to her children about it: "It sounds very simple, but children can think being left is their fault," she explains. "It is a matter of saying to them, 'I am trying my best to beat this. I do not want to leave you and I am doing everything I can.'" Eventually, together, they moved on. Gabriel is a different person now, she says. She is in remission, with a 90% chance of survival. "It's been three years since diagnosis and as you move through treatment and recovery, it gets easier." She continues, "When your life is not immediately under threat, it does wonders for your temper. The volatility of mood that can come with cancer is not really talked about. I don't think I've done a very good job at being honest about that."

I tell her I think the truth is quite to the contrary. I stop short of saying that her caustic rage described in the book led to me dreading our meeting, and unsure about whether to meet her illness with hesitant delicacy or matter-of-fact acceptance. Both approaches anger her at various points on the page. Sometimes it is a look from another mother on the school run - either real or imagined. Sometimes it is an attempt to chivvy her into optimism: "Be positive!" "Everything will be OK!" There are some shocking comments; for instance, from the woman who says that Gabriel's cancer might have been caused by the stress of deadlines (Gabriel is a freelance journalist) or the Jungian who talks about the "cancer personality". All in all, save for the kindly band of mothers who cooked for her and looked after her children, society as a whole comes across as pretty lacking: "Until going through it myself, I would have belonged to that wounding tribe of people I describe in my book who would have tried to be kind but have actually been in flight," she says. "Why is cancer still so scary, compared with heart disease or a broken leg? Why can we still not ask straightforward questions about it, and handle it, like other illnesses?"

In the UK, one in 12 women will develop breast cancer in their lifetime. In the US, the figure is one in eight. The majority of cases occur after the age of 50. Sarah Gabriel is an assumed name, not because Gabriel wants to protect her own identity, but because she has made a pact with her family that she will protect theirs. Because of this, one obvious question hangs in the air between us: have her sisters tested positive for the mutation on the BRCA1 gene? "I'm sorry," she says, in a tone of rehearsed emotional flatness, "I cannot say anything about my family."

When Gabriel's mutation - called M18T - was identified in February 2005, she faced medical uncertainty about what might happen next. The head of the cancer genetics division at the Royal Marsden told her that if the mutation proved to be "deleterious", she would have an 85% lifetime risk of developing breast cancer. However, at that time it was not officially established that M18T was deleterious, as it was extremely rare. The head of the division at that point had a clinical hunch that it was, but nothing had been proven. The policy is still not to urge women with mutations of "uncertain significance" to have bilateral mastectomies (the removal of all of both breasts). The answer was to screen Gabriel with regular mammograms: "I felt horror at the idea of a bilateral mastectomy," she explains, "like a horse that refuses a jump, mostly because I hadn't watched anybody die of breast cancer and because I thought mammograms were a safe form of screening. I avoided cancer books like the plague," she says, "and I was terrified to talk to anybody with cancer. I even shrank from going to a BRCA support group that had women with cancer in it."

However, the decision behind having her ovaries removed was more clear cut. Although the odds were less drastic - a 60% chance of developing ovarian cancer over her lifetime - doctors warned that screening is currently so imprecise that there is no guarantee scans will pick up a tumour until it is too late (four out of five women with ovarian cancer die). This, combined with her mother's fate, led to her decision to have her ovaries removed as a precaution. In August 2005, she went into crash menopause. Despite clear mammograms, in March 2006, she found a lump. It was confirmed: "I'm so sorry. I'm so very sorry," the doctor told her. Gabriel had six tumours in her left breast, four malignant, requiring immediate amputation. (Gabriel later learned that mammograms alone detect only 23 per cent of tumours in women with "deleterious" mutations. MRI scans for breast cancer screening in women in her situation were then not available on the NHS. Now those between 30 and 49 years old with known "deleterious" mutations can have free annual MRI scans on the NHS in conjunction with mammograms, as can women with sufficiently high breast cancer risk.) Even with the removal of her right breast and chemotherapy, there was still a 25% risk she might die within five years. Does she wish she'd had preventative surgery? "We were taking in so much," she replies, "I thought the scans would be enough. In the event, we should all have been a lot more terrified than we were about that."

Given that Gabriel did go on to develop breast cancer, one might now hope that her "statistic" will provide more evidence of the danger of the M18T mutation and therefore protect her other family members, who will at least understand the importance of MRI scanning.

The threat of death is one of two life-changing episodes she cites as having reconciled her with her father. The other is the birth of her children. The picture she paints of her relationship with him is as complex as that of her relationship with her mother's death. As well as examining the effects of cancer, Eating Pomegranates is an eloquent comment on how the parent/child relationship is constantly evolving. For Gabriel, the death of her mother and her father's inability to cope arrested time, freezing her for 20 years in that moment of adolescence, and her memory of their rocky marriage.

"For me," Gabriel explains, "I never got to know my mother from the perspective of an adult. She'd been to Oxford, was the cleverest of her family and yet I saw her as this domestic drudge, serving my father and looking after five children." Her parents' relationship angered her: "Any love and affection in their relationship were invisible to me at that time and I came to reject my father. I could scarcely bear his perfunctory kiss each night, but suffered it in silent fury." And then her mother died. Roughly one year later, her father married the woman he had begun seeing shortly after her mother's death. Somewhere in between, they all moved in together and the house where Gabriel's mother died was sold, as were many of her parents' joint belongings (some remained). For Gabriel, these twin acts felt like a rejection of her parents' life together, and therefore, as a product of that, a rejection of the children's lives as they had known them.

Gabriel, as the oldest, was expected to cope the best: "And I couldn't cope," she says. "I think for all of us, at least I am sure in speaking for myself, it was clear that this was not a new mother, but a partner for my father. My stepmother was marrying him, not us."

Gabriel spent years on the couch trying to get beyond feeling belittled in her father's presence, and the feeling, too, that she was not given the space to grieve. In the book, Gabriel writes about the moment during her own illness when she discovered an organisation called Seesaw, set up by a former Macmillan nurse to help children with cancer and children in the process of losing a parent or sibling: "The retrospective vision of our own isolation as a family when my mother died, the complete lack of support, the silence around her death that has endured to the present day, breaks my heart," she writes. "That such an organisation should exist now is almost unbearable. Except that it may be of value to my children."

Although Gabriel feels that her father failed to meet her emotional needs, by the end of the book they are reconciled. After years of not talking about the death of Gabriel's mother, the threat of another early death nudges both father and daughter to a place of understanding and empathy, so that Gabriel feels deep love and forgiveness for him. Gabriel finally finds the strength to tell him she wishes she'd known her mother was dying. Her father, in his round-toed comfortable shoes and cosy Aran sweater, says, finally, with a sad sigh: "In plea of mitigation, it was all so very fast at the end."

Gabriel will not talk to me about her father, except to say: "the relationship has gone through its changes and is in a different place now. I love him very much. He is very much part of our lives." The book is dedicated to "my beloved parents". She says sometimes she can go whole nights without waking up her husband in a blind panic. She is taking antidepressants because she knows she needs them - not because of the fear of death, but because she has always needed them, "much as a diabetic takes insulin". "I feel I lost quite a number of years of my life to undiagnosed depression," she explains.

The Gabriels plan holidays now, and are about to move to a bigger home. The girls are great, she says, resilient and strong. Children are never tested for BRCA gene mutations. That will come in adulthood and only if they want it. Screening, she says, will have advanced considerably by then and she takes great strength in this fact: "Knowledge offers protection," she says, "and I do so many things with a sense of my children's protection in mind."