“The problem with the gene pool is that there's no lifeguard,” the science-fiction writer David Gerrold once remarked. For Masha Gessen the lack of a lifeguard in her genetic make-up was no joke. Eleven years after her mother died at the age of 49 from breast cancer in 1992, Gessen took a test to see if she had inherited one of the two known mutations that would put her at a high risk of developing the disease. When she tested positive for the BRCA1 mutation, or, as many geneticists call it, “misprint”, she was faced with the staggering odds of an 85% chance of contracting breast cancer and a 50% chance of ovarian cancer.

Oncologists know that there are, roughly speaking, two types of cancer patients: the ones who don't want to know any more than they have to, and the ones who spend hours on the internet researching their disease. Gessen, an American writer existing in the strange limbo land of a “previvor”, was so decidedly in the latter camp that she set out to find everything she possibly could about genetics. Blood Matters is both a personal account of the decisions she made regarding her own potentially fatal genetic predisposition and a study of how far our genes rule not only our medical make-up but also our psyches.

Genetic inheritance accounts for approximately 7% of all breast cancers. The BRCA1 and BRCA2 misprints are particularly prevalent in the descendants of Ashkenazi Jews. Gessen starts her book with a hunt through history to find the beginnings of the Ashkenazim. While this is interesting genealogical research, she doesn't begin to tackle the huge questions arising from the recent boom in genetic information until she describes her own indecision about the choices she has to make.

Having the BRCA2 misprint myself, I know that genetic knowledge is one thing, acting on it another. All women in our position are faced with tough decisions. Do you opt for a preventive double mastectemy and an oophorectomy (the operation to remove your ovaries) or rely on constant testing and watchfulness; hoping, if cancer strikes, to catch the disease early and thus have a higher chance of defeating it? These are harrowing choices, involving infertility, the early onset of the menopause, and one's own feeling of femininity, not to mention long and potentially tricky operations. Gessen describes wrestling with these decisions with admirable lucidity and frankness.

Moving on from her own rogue gene to others, Gessen delves into Huntingdon's disease and the little-known but deadly maple syrup urine disease (MSUD) - so called because the urine of childen who have it smells of maple syrup - which affects 1 in every 200 of old-order Memnonites in Pennsylvania. She spends time with the families of sufferers as well as with doctors who are devoted to helping these conditions, and portrays the pain, frustration and heartache raining down on people whose DNA has betrayed them.

After throwing herself so wholeheartedly into the deep end of the genetic pool, Gessen may be forgiven for becoming a little overzealous - especially when she moves from medical to psychological research. The chapter in which she has her own genes analysed for character traits comes across as more gypsy fortune-telling than science. Informed that she has a “short-short serotonin receptor”, which points to a tendency to neuroticism, she immediately decides that “the entire personality I had constructed was a function of my hypercompensation”. All her seeming self-confidence and adventurous spirit had been, she realises, with the zeal of a convert, a cover-up. “Now my true personality had been exposed by a genetic test.”

And yet her writing shows no signs of tending to the neurotic as she takes the reader along on her remarkably well-researched journey to the genetic frontier. In what feels like a Holy Grail quest to find answers to questions at the heart of life's meaning, she travels across the world: to Israel, Austria, Estonia and Siberia, talking to doctors, psychologists, religious leaders and historians, as well as to men and women doing genetically based behavioural research with rats, mice and foxes. The desire to cover every single aspect of genetics makes Blood Matters into something of a mishmash. This is a brave, passionate and well-written book, but for the reader struggling to take in all the medical, psychological and ethical issues raised, it can be overwhelming.

Disconcertingly, Gessen crosses into dangerous territory in her final chapter, What We Fear Most. When talking to a doctor about genetic screening for embryos with BRCA1 and BRCA2 misprints, she asks a question that is really a statement: “Seriously, why would I consider saddling a daughter of mine with the night-time fears, the obsessive medical examinations, or, worse, surgeries, or worse, yet, the torturous treatment for breast or ovarian cancer?”

My two sisters died of breast cancer: one at 48, the other at 61. Five years ago, I was diagnosed with it, too. In that one sentence Gessen has effectively wiped out all three of our lives. We were all scared, we all had operations, we all went through torturous treatment, but at no point did either of my sisters say she wished she'd never been born, and neither would I.

I didn't have the gene test until after I had been diagnosed, when I was 51. Still, if my 25-year-old daughter or 27-year-old son have inherited the misprint from me, I could never wish retrospectively for the ability to screen - and terminate - my embryos. Even the thought of it is offensive. This endorsement of genetic selection and search for engineered perfection is exactly what we should fear most.

Blood Matters begins with the hope that the plethora of new genetic information can help avoid early death. Its conclusion with a future in which this same information is used to cancel out life is sadly ironic. If her mother had had our genetic knowledge and made use of it in the way proposed by the author, this book wouldn't exist. Masha Gessen wouldn't be alive to write it.

Blood Matters by Masha Gessen
Granta Books £18.99 pp321 Buy the book from Books First £17.09 including free delivery

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