Scientist Sharon Moalem says we will soon be able to alter our children's lives with genetic manipulation – would you do it if you could?

Angelina Jolie and Brad Pitt … in spring 2013, Jolie had a double mastectomy after finding out that she carries the breast cancer gene. Photograph: Rex

Dr Sharon Moalem recently diagnosed a mother with a rare type of hereditary ataxia, a neurological disorder for which there is no cure. "It's a horrible, horrible, devastating condition similar to Huntington's. Once you start showing symptoms, usually within 10 years you're wheelchair-bound and you start losing memory function."

There is a 50% chance that the woman's children have inherited the condition. "When I counselled her, she said: 'I feel like a child abuser.' I told her: 'There's nothing you could have done to prevent this.'

"You can imagine what she's going through. She just found out. Her first thought, and this shows what kind of mother she is, was for her children. She saw what her father [from whom she inherited the condition] went through and said she wouldn't have had children if she'd known."

Then the woman asked for something that Moalem, a Toronto-based American physician and geneticist, couldn't give her. "She said to me: 'I want to test my children but I don't want to tell them the results.' I said: 'My hands are tied. In most countries, you can't test children like that – it's taking away their autonomy.'"

The case is even more poignant because several years ago the woman cancelled her disability and life insurance. "She thought: 'I'm young and healthy but I'm in financial difficulties, so I'll cut back on the insurance.' She said that was the worst decision she made in her life. She's going to be around for 10, 15 years and will need a lot of help, and she definitely won't be working."

Moalem tells this story to illustrate how we want to change our – and our children's – genetic inheritances. The point of his new book, Inheritance: How our Genes Change our Lives and our Lives Change our Genes, is that, increasingly, we can.

Actor Angelina Jolie, for example, underwent a series of procedures last year at Beverly Hills' Pink Lotus Breast Center including a double mastectomy designed not to remove a cancer, but to prevent it. Her decision to have the treatment followed screening of her BRCA1 gene, revealing she had an 87% chance of developing breast cancer and a 50% risk of ovarian cancer.

"Some of us carry genes that predispose us to cancer, like BRCA1. In the past there was nothing we could do about it. Now we can," says Moalem.

"For many of these women, watching sisters and mothers be diagnosed with breast cancer may make them go so far as to remove their breast tissue and ovaries because they say, 'I'm not going to wait around for that to happen – that 70% or whatever it is risk of developing breast cancer. I've done my child rearing.' It's the first time in history you can be a previvor – it's a new word."

But there are lots of unforeseen problems in this brave new genetic world. If you have a 50% chance of getting Huntington's, say, it's quite likely that you would like to live your life without knowing that, argues Moalem. Your loved ones, though, might feel otherwise. What is the right thing to do?

Moalem came across an example of this dilemma recently. The fiancee of a friend had recently been reunited with her biological father, who revealed that her biological mother had passed away after having symptoms that sounded like Huntington's disease.

Lisa declined to get tests to find out if she had inherited the condition, but David desperately wanted to know what, if anything, he was up against if they married. There was a 50% chance of Lisa having the disease and if so, as Moalem puts it in his book, David "wasn't going to grow old with her. Instead, he would have to watch her personality change as the disease remodelled her brain, slowly disassembling her mind. Would he have the emotional, mental and physical strength to properly care for her needs?"

When Moalem met his friend for a drink in New York, David suggested he get hold of Lisa's toothbrush or a strand of her hair for Moalem to test. But Moalem said he couldn't oblige – it would have been ethically and professionally improper. In 2012, the US Presidential Commission for the Study of Bioethical Issues called for such tests to be made illegal because they infringed on the right to privacy. "As the costs of genetic testing plummet and as it continues to get easier to do, we'll face more of these situations," says Moalem. "To hack or not to hack into the genome is the question we'll increasingly be faced with."

Sometimes knowing about one's genetic inheritance can save your life. One of Moalem's patients, a New York chef, Jeff, followed doctor's orders to cut back on red meat and eat more fruit and vegetables – but got cancer as a result. What his doctor didn't know was that Jeff had a rare genetic condition, hereditary fructose intolerance. "Had Jeff known his genetic inheritance, he would have carried on doing what he had been doing, which was avoiding fruit and vegetables. I don't think anybody would ever imagine that an apple a day would put you away, right?"

Fortunately, Jeff's cancer was detected early and now he leads a life happily devoid of fruit and veg.

Sharon Moalem suggests drawing up a family tree for your descendants and including medical facts, such as drug sensitivities, making it as detailed as possible.

Jeff's story is solace, Moalem argues, for all of us who have tried diets that don't work. "For me, there is no better way to tell people that there will never be a universal diet. You have daily allowances and you think, if I follow those I'll be healthy – but they don't work for you. Or you hear about a friend who tries a diet and you try it and fail miserably."

Jeff's story also helps to explain why, Moalem suggests, sitting down with your extended family and drawing up a genealogical history can be a great gift for your descendants. "Make it as detailed as possible. You never know how some seemingly inconsequential detail of one generation, like sensitivity to a specific drug, can lead to a useful bounty of family medical information."

That said, Moalem argues that we are increasingly likely to be able to change our genetic inheritances. Since the human genome was sequenced in 2000, we have witnessed the rise of tests for people waiting to do just that. Genetic screening can now cost US$2,000 (£1,200) – the price, he says, of a flat-screen TV.

"This is the first time we're really taking evolution into our own hands, completely changing the direction of humanity. If we decided as a society that the only children who are allowed to stay alive, for example, are those with 12 fingers because we want 12-fingered pianists, in one or two generations we'd completely shift genetically. We'd all have 12 digits."

Isn't this hideous genetic engineering? "It's positive eugenics – you can't really call it any other thing. We're making judgments about who should be born, essentially."

Such judgments are especially naked with couples contemplating IVF. "If you have 10 embryos you can decide which one to implant based on that screening information. You could say, 'Do you want an embryo that has a risk of dyslexia, alcoholism, diabetes, cancer, or do you just want to roll the dice?'"

Sharon Moalem: 'It’s positive eugenics – you can’t really call it any other thing. We’re making judgments about who should be born, essentially.' Photograph: Rex

But surely you can't guarantee that a child will not grow up to have these problems? "It's not a guarantee but screening can indicate the risks. A parent will say, 'OK, I'm selecting an embryo not to have Huntington's, why don't I select the best possible embryo that I can?' Maybe they want to have the embryo implanted that can have light complexion and blue eyes. Where does that end? These are questions society has to face."

Moalem fears that we are ethically unprepared to face them. "We have gone over that cliff with no parachute or safety net and no idea how to respond in a proper or a just way."

He saw a couple who wanted to terminate their pregnancy because their child had hereditary dyslexia. "The father said, 'You can't imagine how hard it was for me to get through life with this disability. I don't want my child to grow up to do the same.'"

What did Moalem tell him? "That that form of dyslexia is associated with creative thinking and thinking in a different way. Think of Jonathan Ive, the designer of many products for Apple. There's a very good chance that it's due to his dyslexia that he's come up with so many creative designs. That's the question we're not thinking about enough. We're yearning for inherited perfection but not knowing that that's going to come at a cost."

Soon, Moalem believes, those of us with money will be able to give our offspring genetic enhancements that will give them an advantage in learning, cognition and sports. Isn't that grotesquely unfair? "It is no different from 19th-century European society where the biggest leg up was proper nutrition," he says.

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More unfair no doubt is genetic discrimination, which is becoming a thorny legislative issue. In the US, the so-called anti-Gattaca law (after the 1997 film about a genetically tiered society), also known as the Genetic Information Nondiscrimination Act 2008, prohibits such discrimination in workplaces, but not in life and disability insurance. As a result, insurance companies are free to refuse cover for those who've disclosed genetic information online, for instance. "These days people are writing cancer blogs and posting on social media the results of genetic testing. Insurance companies can read that stuff. People don't realise they have made it more difficult for their family members to get insurance."

Would Moalem genetically screen his kids? "We don't have children yet, but my wife and I think about these issues constantly. Imagine if it turned out we were carriers for deafness and we could have had a child that was born deaf. What would we do? Is it child abuse to bring into the world a child who has that kind of condition?"

So knowledge isn't just power but a terrible responsibility?

"Right. Once you start looking you have no idea what you're going to uncover. And it's often difficult to know what you're going to do with what you find."