There is an old wives' tale that babies bear, for their first year, a striking resemblance to their fathers - nature's way of cementing the paternal bond, of winning over a man, who may be tempted to desert his offspring. But in Delilah Seale's case the similarity has persisted. At 20, she has her father's forehead, his blond hair, his fine features. Her physical appearance is a constant reminder of a secret, just made public: Delilah is the biological daughter not of Patrick Seale, the man who raised her, but of Martin Amis, the novelist.

For two decades, both men kept their counsel. They saw each other across a room at parties from time to time, but never spoke of Delilah. Amis made no contact with his child, conceived during a brief but passionate affair with the Seventies author and beauty, Lamorna Heath. Her husband, the author and Middle East specialist, Patrick Seale, knew of the affair, which took place during a short separation. But he set it aside, the couple were reunited and he put his name on Delilah's birth certificate. Seale brought her up on his own, after his wife , a depressive, committed suicide when the child was two.

Now, the truth is out, following Seale's decision last year to tell Delilah about her natural father, whom she now meets regularly. "It was obviouslya rather shattering experience for Delilah," says Seale, "but it has been as untraumatic as could be hoped. It has helped that Delilah, myself and her brother have such a strong relationship and that my relationship with Martin has been amicable."

Shattering certainly. Imagine discovering that not only is your natural father the arrogant and grumpy Amis, but that grandad was the irascible and even grumpier Sir Kingsley. Amis the younger has a dark personality that dwells on the sinister. He may be a great writer, but he is often accused of failing ever to construct a sympathetic female character. Even now, at 46, he is unsettled: he divorced his wife, Antonia Phillips, a fortnight ago.

Seale is, however, generous in his admiration of the writer and appreciative of Amis staying silent for so long. "For many years I forgot the whole thing. She was my daughter and still is. There is just this rather interesting aspect to her life, which is rewarding and enriching. Knowing the truth has given her another dimension and helped her understand herself more profoundly. I felt I owed it to her that she should be told when she reached adulthood."

An affable, kindly man, Seale managed heroically, a little like Sylvia Plath's husband Ted Hughes, when he was left with two toddlers after their mother killed herself.

"I was present at Delilah's birth. She was absolutely adorable. It didn't seem a big deal to accept her as my child. In modern marriages today, lots and lots of people are bringing up other people's children. Perhaps it was easier for me because she was white and good looking. The fact that she looks so like her brother helped. Martin's absence also probably made it easier for me to feel that Delilah was my own. After Lamorna's death, it never crossed my mind to let her go."

All of this sounds like an extraordinary story. But it is less unusual than you might think. Literary and artistic figures are famously prolific in more than their craft. Byron fathered a child with a maid at Newstead Abbey, his ancestral home, and several others on the wrong side of the sheet. Robbie Burns is said to have had a rake of illegitimate children, the number probably exaggerated by his Scottish admirers. Augustus John remarked that he never walked down Maida Vale without patting a child's head, perchance he was its father.

Not surprisingly, the revelation of hidden parenthood has been a recurring theme in drama from Shakespearean comedy to the discovery ofKathy's daughter in EastEnders and Jack's realisation in The Importance of Being Earnest that Lady Bracknell is, in fact, his mother. Habeas Corpus, Alan Bennett's West End play, and Mike Leigh's recent film, Secrets and Lies, all revolve around finding one's true parents.

Drama reflects fantasy. Which child has not dreamed at some that its real parents were really not the dull, suburban types fate imposed? "Children often imagine that one day they will find parents who are more famous, richer, more loving than the parents they have," says Robin Skynner, the child psychiatrist and co-author with John Cleese of Families and How to Survive Them.

Martin Amis may not be Delilah's dream come true, but the arrival of Britain's most celebrate novelist into her life must still seem like something of a fantasy. Now a history student in Oxford, she has gone off adventuring in Latin America, no doubt to chew over the whole idea.

What hope then for the rest of us swapping parents and finding a rich and famous daddy? Less unlikely than you might think. Steve Jones, Professor of Genetics at University College, London claims in his book, The Language of Genes, that "in middle class society, about one birth in 20" is of a child that has not been sired by their mother's partner. British medical students are taught that the non-paternity rate is even higher: 10-15 per cent.

The lesson of all this is twofold. If a strange man pats you on the head as you are walking down the street, take a good look at him for any familiar features. And, clearly, Shakespeare's warning remains true today: "It is a wise father that knows his own child."