Friday, March 13, 2009


A few years back, a woman in Massachusetts was suffering from kidney failure and she and her sons underwent tests to see if either of them would match as a donor. Her doctors informed her that not only did they not match, but neither of them could be her child. A double case of baby swapping? The actual situation was even less probable.

After extensive tests of various body tissues and fluids, it became apparent that the woman was a chimera – that is, she had two separate genetic identities that were the equivalent of sisters. Some organs, including her ovaries related to one of the genetic identities, while others, like her blood and kidneys, were related to the other. So her sons inherited a different genetic makeup than the one that determined her kidney compatibility needs.

Chimera are created when two fertilized eggs merge at a very early stage of development and different parts of the body develop from the stem cells from the two genetic sources. While rarely detected, chimera may occur more often than we think. Scientists have identified over 30 cases so far, including one of a man who was the product of the merger of one male and one female zygote. When operating to free what they thought was an undescended testicle, doctors discovered a uterus and fallopian tubes.

Let’s consider the implications of chimera for non-paternity events. If it is possible to have chimerical functioning sets of both male and female organs, it is also possible to end up with chimerical sets of male testes. The latter situation could arise when zygotes fertilized by two different men merge and one genetic makeup is expressed in one testicle and the other in the second. If this man then has children, his sons could be descended from either ‘grandfather’.

Another scenario might produce an apparent NPE when none exists. Consider a woman whose mother has blue eyes (two blue-eyed genes) and whose father has brown eyes, but who has one blue and one brown-eyed gene. Some of their children will have blue eyes and some brown. However, a chimerical child could develop blue eyes but have ovaries that produce ova with a brown-eyed gene. If she then marries a blue-eyed man, their children could have brown eyes. Just how do you explain that event to a skeptical husband?

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