But, behind the advertisements which implore people to send off their DNA in the hope of finding something exciting in their family tree, there is a much more serious side.

It is something writer Dani Shapiro discovered — and with devastating psychological effects.

One day, she and her husband, Michael Maren, spat into two vials and sent them to a DNA testing company.

Weeks later an email arrived which contained their results.

Michael was almost 100 per cent eastern European Ashkenazi, but Dani’s results showed that she was only 52 per cent eastern European Ashkenazi — the rest was French, Irish, English and German.

It did not make sense until Dani decided to compare her results with those of her half-sister Susie, and discovered that the two of them were not related . . . at all.

It meant that either Dani’s father, Paul, was not her father, or that he was not Susie’s father.

And Dani knew that the former was likely to be true.

It led her on a tumultuous road and one which has resulted in her bestselling book Inheritance: A Memoir of Genealogy, Paternity and Love (Daunt Books, £9.99).

Dani was raised in an Orthodox Jewish, observant home in New Jersey.

But she knew, somewhere along the line, there was a secret about her that she just couldn’t place.

“I had a sense of not belonging,” Dani told me from her Connecticut home. “I remember being in shul feeling like I didn’t have a right to be there.

“It didn’t make sense to me because I came from all this yichus.”

It did not help that, growing up, blonde Dani was often questioned about her Jewishness.

“I was told every day that I didn’t look Jewish and that I looked like I came from another part of the world,” she recalled.

“My features are western European, but what upset me is that it was other Jews who used to say it to me.

“I found it strange — and still do — that one Jew can say that sort of thing to another Jew.

“I don’t think an African-American, for example, would say to another African-American, ‘hey, you don’t look black’.

“I think it is something quite specific to Jewish people, especially in America.”

When her DNA results came through, the mother-of-one asked her husband to phone the company as she thought there had been a mistake and that there had to be a reasonable explanation.

“The rug was pulled out from under me,” Dani said. “It was a huge thing to come to terms with.”

As far as she knows, her parents, who had had trouble conceiving, visited a fertility institute in Philadelphia.

Whom she thought was her father, Paul, had his sperm mixed with a man Dani refers to in her book as ‘Ben Walden’ and was used to inseminate her mother, Irene.

The idea was that nobody would know which sperm created the embryo, as clinics back then were unregulated and the practice was used to improve their results.

“My mother, years ago, let it slip in front of me that I had been conceived in Philadelphia, which seemed odd,” Dani said.

“There had been a practise of mixing sperm, so I asked her about it and, without pausing, she said ‘absolutely not — your father would never have agreed because it meant his daughter would not have been Jewish’.

“It was a strange thing for her to say because, of course, Judaism goes through the mother.”

The 57-year-old eventually tracked down her biological father, a physician living in Portland, Oregon.

He revealed that he had been a medical student at the University of Pennsylvania and donated sperm to the Farris Institute for Parenthood in 1961.

Dani finally met him in October, 2016.

“I was not looking for my father — I was looking for whom I came from, which is different,” Dani insisted.

“My dad was the one who raised me and to whom the book is dedicated. I was just looking at trying to put the pieces of myself together.

“We exchanged emails and he came round to wanting to meet me.

“I think, sometimes, we tend to want to think that nurture is all that matters, but, for me, there was always a sense of certain things not adding up.

“I had not understood why I was the way I was, but, when I met my biological dad, I saw there was a certain constitutional similarity between us.

“I look a lot like him and we share certain gestures and traits.”

Her mother, Irene, and Paul are no longer alive, but Dani, who has written nine other books, said she could not have penned Inheritance had they still been here.

“There is a line every writer has to find for themselves,” she explained.

“My mother and father clearly intended to go to the grave with this secret, but I do wonder how it would have felt to speak to them about what has happened.”

She has been asked if she now intends to celebrate Christmas or discover more about her Christian roots.

“I find that hilarious,” Dani said. “All my life I have been psychologically and emotionally a Jew to my core — that will not change.”

And, such is her devotion to her faith, she even organised a monthly Jewish get-together at her home in the Connecticut countryside as there are so few Jews in the area.

Dani also brought in rabbis Rachel Gurevitz and Suri Krieger for the meetings and arranged for her son Jacob’s barmitzvah to take place at a local Episcopal meeting house.

“We covered the cross and brought in a Torah and ark,” Dani recalled. “It was one of the most meaningful things I have done.

“Ironically, Jacob’s barmitzvah piece was about the Jews wandering in the desert and my batmitzvah piece was the Book of Ruth, which is about belonging.”

Inheritance has been so well received that a film based on it is now being developed by a production company.

A screenplay is currently being written, but not by Dani and Michael, who adapted Oscar Wilde’s The Happy Prince for HBO.

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