A woman came to a doctor very upset one day claiming test results indicated that two of her three sons aren't hers even though she conceived them naturally - the test results indicated that technically she gave birth to someone else's children. This was a case presented to Doctor Margot Kruskall after a 52 year old woman (simply called Jane), in need of a kidney transplant, whose blood test results came back and indicated that her children weren't hers. After two years they discovered that she was a chimera.
A human chimera in a genetic sense is a person with a mixture of two or more genetically different cells e.g. they are two different people. In humans, when a chimera is formed they are refered to as tetragametic, they are made from a total of four gametes. This is thought to have occurred in Jane's case when her mother conceived two non-identical twins which in the very early stages of pregnancy fused into one body. In the case of Jane, her chimerism was from twin girls but there have been incidents where some chimeras are derived from one male embryo and one female embryo; when this happens the person can have both male and female genitalia or a rare mix of the two.
Chimerism is a double edged sword in that the rejection rate for chimeras is decreased as they are a mix of two genetically different cells and that allows for a greater range of donors to be used. However, it cast doubts on DNA test results, Dr Margot Kruskall was helping out a court case where a mother was suing the parents and claiming that he was father but ruled her out as the mother, chimerism could explain this case.
link: New Scientist vol 180 issue 2421 - 15 November 2003, page 34
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