Siddhartha Mukherjee's new book - The Gene: An Intimate History - begins with a startling confession: "Madness has been among the Mukherjees for generations."

Here, he is referring to his late uncles, Rajesh and Jagu, his father's siblings who had both been stricken with mental illness, and his first cousin, Moni, now 52, who "is confined to an institution for the mentally ill (a 'lunatic home', as my father calls it), with a diagnosis of schizophrenia".

Poor Moni, whom Mukherjee finds aged almost beyond recognition during a trip to Calcutta in 2012, "is kept awash in antipsychotics and sedatives, and an attendant watches, bathes, and feeds him through the day".

Mukherjee is accompanied by his father, Sibeswar, a former executive with Mitsubishi, who is "a sullen and brooding presence, lost in a private anguish... At least part of my father's reluctance to accept Moni's diagnosis lies in a grim suspicion that something of the illness may be buried, like toxic waste, in himself."

Aware that he, too, might have inherited the "madness" gene, Mukherjee comes clean when he finds love. "When I met Sarah, now my wife, for the fourth or fifth time, I told her about the splintered minds of my cousin and two uncles. It was only fair to a future partner that I should come with a letter of warning."

Like most Bengalis, his parents had "elevated repression and denial to a high art form". But even so, questions about this particular history were unavoidable, he states. "Moni; Rajesh; Jagu; three lives consumed by variants of mental illness. It was hard not to imagine that a hereditary component lurked behind this family history. Had Moni inherited a gene, or set of genes, that made him susceptible - the same genes that had affected our uncles?"

This is why he had subtitled his book "An Intimate History". He is due to undertake a tour of India, with a trip to Calcutta slotted for July 7.

Mukherjee, who was born in Delhi in 1970 and left for the United States at 18, remains an oncologist in New York. He and I had first talked in 2009. That was a year before the publication of The Emperor of All Maladies, his "biography of cancer", which won him a Pulitzer and turned him into an international celebrity.

I ask him whether Rajesh, Jagu and Moni could have led less traumatic lives had we known then as much about mental illness - and genes - as we do now.

"We still don't know what goes wrong and what goes right (with the genetic structure) - we are still trying to find out," Mukherjee tells me. "Stories like these will be part of our lives."

Most common diseases have multiple genes and they are related to the environment and chance for them to be fully revealed, he adds.

"The book tries to remind us that all illnesses lie in the complex intersection between genes, environment and chance - and that when we try to understand ourselves and we try to understand illnesses, we should try to remember these intersections are extremely important as we move forward," he explains.

Put another way, "this book is the story of the birth, growth and future of one of the most powerful and dangerous ideas in the history of science: the 'gene', the fundamental unit of heredity, and the basic unit of all biological information".

I catch up with Mukherjee again in Wales at the annual Hay on Wye literary festival, which is sponsored by Tata and attracts big name authors from around the world. He engages in conversation before a large audience with Claire Armitstead, books editor of The Guardian and Observer.

We learn that family tensions reached breaking point after Jagu came to live in Delhi. This happened one day when Mukherjee's father launched a physical assault on Jagu who was borrowing money he could not repay from strangers.

But Jagu's mother, who also lived with the family, fiercely protected him, threatening to "wash" her womb off Sibeswar's traces if he hit his cowering brother.

On the critical question of whether mental illness is inherited, he states that it is incorrect to say that schizophrenia is not genetic, or that it is completely genetic. "There is strong, strong evidence from studies on identical twins, from studies in families, from studies across inheritance that there is a strong component of inheritability in schizophrenia. It means there are multiple genes for schizophrenia, probably tens, dozens, and there is still a strong role for the environment, whether it is triggered by chance to unveil that gene."

In Bengal, it has been suggested that the trigger in many families, such as his, "could be the stress of Partition. It could be things that we don't understand. We now know that severe forms of human stress can unveil various forms of mental illness."

He raises the question of interfering with genes to remove perceived defects.

"The dream of geneticists is to improve genes," and "breed better human beings", he observes. "The question is, what is our dream? In our dream do we wash it? Do we clean it? Do we erase it? When things that exist within ourselves are not desirable, how do we change it and what are the moral or human costs of that change?"

In the 1910s and 1920s, the idea that genes could be manipulated to make the human race better was thought to be progressive, he points out. "The idea metastasises in the United States where it moves from 'let's breed better human beings to let's sterilise those who carry illness'."

His book is part dedicated to Carrie Buck (1906-1983), "the first woman to be sterilised because she carried mental illness in her genes - (this was) probably nonsensical. She was sterilised by a Supreme Court order: 'Three generations of imbeciles is enough!'... Then it metastasised yet again to Nazi Germany where it moves from selective breeding to selective sterilisation to selective extermination."

There are echoes almost of Sanjay Gandhi's sterilisation campaign during the Emergency in the tale he has to tell.

There is a grim warning, for example, on the misuse of genetics: "Readers from India and China might note, with some shame and sobriety, that the largest 'negative eugenics' project in human history was not the systematic extermination of Jews in Nazi Germany and Austria in the late 30s and early 40s. That ghastly distinction falls on India and China, where more than 10 million female children are missing because of infanticide, abortion and neglect of female children."

He recalls his shock at visiting a children's cancer ward in a New Delhi government hospital where the patients were all boys. "So I dug a little bit deeper and found out that typically when boys in certain communities have illnesses they are rushed to hospital, but the girls are just left to themselves. In certain parts of the country there are 850 girls to 1,000 boys."

It is a good thing that a "fellow Bengali" clinician at the book fest injects a lighter note by asking: "Have your children inherited the Bengali love of food?"

Everyone laughs.

"The Bengali love of food, what a lovely idea!" Mukherjee responds. "Yes, my daughters have acquired the Bengali love of food!"