One human chimera came to light when a 52-year-old woman demanded an explanation from doctors after tests showed that two of her three grown-up sons were biologically unrelated to her.

Although the woman, "Jane", conceived them naturally with her husband, tests to see if she could donate a kidney suggested that somehow she had given birth to somebody else's children.

A study in the New England Journal of Medicine by Dr Margot Kruskall, of the Beth Israel Deaconess Medical Centre in Boston, Massachusetts, showed that Jane is a chimera, a mixture of two individuals - non-identical twin sisters - whose cells intermingled in the womb and grew into a single body.

Dr Kruskall believes the most likely explanation is that Jane's mother conceived non-identical twin girls, who fused at an early stage of the pregnancy to form a single embryo, according to a report published today in New Scientist.

For some reason, cells from only one twin dominate in Jane's blood - used for tissue-typing. In her other tissues, however, including her ovaries, cells of both twins live amicably alongside each other, hence the apparently impossible genetics of her three sons.

One son came from an egg derived from the twin whose cells dominate Jane's blood, while his brothers came from eggs derived from the other twin's cells.

Around 30 similar instances of chimerism have been reported, and there are probably many more who will never discover their unusual origins. Most chimeras probably go through life unaware of their unusual constitution.