"Cancer was in my genes. I had to stop it any way I could"

But she also visited Amish and Mennonite communities vulnerable to over 80 different genetic mutations and met families who decided to undergo, en masse, radical preventative surgery.

She travelled to the site of the world's longest-running genetic experiment in Siberia, in which animals are genetically engineered to produce specific personality traits, and spoke to new companies offering to map out clients' genetic make-up.

"I quickly realised that there was a whole group of people living by rules that the rest of the world will be living by in a generation - trailblazing by coincidence," she says.

Chief among these are Ashkenazi Jews who, like Gessen, have high rates of genetic conditions such as Tay-Sach's, Niemann-Pick, Canavan, Gaucher's and, of course, some forms of breast cancer.

But Gessen, who was born in Russia, raised in the US, and has recently been based in Moscow as correspondent for American publications, emphasises that Ashkenazim should not feel they have a bad deal genetically.

Rather, they are better studied than other groups, because of their tendency to marry each other, and live together near medical research centres.

"In fact," she says, "Ashkenazi women should feel very lucky because their breast cancer mutations were found first."

Partially as a result, they are very proactive about genetic medicine. In Israel, for example, testing for genetic mutations before embryos are implanted is positively encouraged, while it is banned outright in Germany, Austria, Italy and Switzerland.

"In the aftermath of World War II, Jews are least afraid of the spectre of eugenics," says Gessen.

She, too, dismisses the ethical concern that pre-implantation genetics could lead to some forms of life being valued more than others.

"There is a difference between state policy and individual behaviour," she says.

"Antisemitism is the best analogy. It's a bad thing, but while you have the right to be an antisemite as an individual, you don't have the right to be an antisemitic state - that's fascism.

"As an individual you have the right to make stupid or amoral decisions about the way you conceive, select or raise your children, as long as it is not raised to be state policy."

In the US she visited Dor Yesharim, an Orthodox Jewish group which offers genetic screening to youngsters trying to find a match to reduce the incidence of fatal genetic disorders in the community.

Members are never given their results, but phone the organisation whenever they date someone new to be told whether they are genetically compatible.

As a result, the number of children born with in the Orthodox community in North America with Tay-Sachs, for example, has gone from an average of 50-60 a year in the 1980s to between four and six a year over the past decade. "It works because matchmaking is so systematised," Gessen says. "It already functions like a database. They just insert a new field."

Could this almost fool-proof system catch on in the wider world? "It will always sound quite bizarre to say I was in love with a guy but broke up because we were both Tay-Sachs carriers, but it will become increasingly reasonable to say I married him, then decided to have children through IVF," says Gessen.

"At the moment people have IVF and test as a result. That process will become turned around, with people having IVF because they want the testing."

In the future, says Gessen, this might give rise to two classes of people - those who can afford to have healthy children through testing and IVF, and those who cannot.

It might also create a situation whereby only the poor - who cannot afford IVF - need to have abortions, as they will be the main ones carrying genetically impaired embryos.

"These are important issues to discuss now," she says, "because the practice is getting so far ahead of the conversation."

She is convinced, however, that genetic information will eventually change the way we think of ourselves - we will talk "biobabble" rather than "psychobabble".

Already, she writes: "We have started speaking of genes that determine behaviour, personality traits, and even lifestyle preferences before we have had a chance to learn what these genes might be."

Many of the people she interviewed resisted genetic testing as they do not want to saddle themselves with difficult decisions, or with disturbing information they would be unable to act upon.

But eventually, it will become much more mainstream for adults to be tested for genetic mutations.
"Studies seem to show that women who are aware of family history but decide not to take the test suffer from anxiety more than others," says Gessen.

"When I was tested there was a lack of accessible information and support networks, but even in four years this has changed massively."

She does not regret being tested, despite saying she was unprepared for the results.
And Gessen, who has one adopted son and a biological daughter, says she is "absolutely happy" with her eventual decision - after months of tormented research - to have her breasts removed, but to keep her ovaries.

She writes: "All I could do was reduce the amount of knowledge that drove me crazy. In my mangled internal math, the 40 per cent risk of ovarian cancer was knowledge I could live with, and the 87 per cent risk of breast cancer was not."

Blood Matters is published by Granta at £18.99