Chimerism

How it works

A tetragametic chimera starts as two fertilised eggs, which would normally become fraternal twins. In the first days of pregnancy the two embryos merge into one¹. The result is a single person with two cell lines. One line might make the blood and liver while the other makes the skin and ovaries. The two lines can even be different sexes¹.

Nothing shows on the outside in most cases. Sometimes there are hints: patches of skin in two different tones, two different eye colours, or a mix of blood types¹. Usually a chimera is found by accident, during a blood test, a transplant match, or a paternity test.

A much milder form is almost universal. During pregnancy, cells cross the placenta in both directions. Mothers keep small numbers of their children’s cells for decades. This is called microchimerism³. A 2012 study found male DNA, presumably from sons, in the brains of 63% of the women examined³.

How common it is

About 100 cases of tetragametic chimerism have been documented¹. The real number is unknown. Some estimates run as high as 10%, because nobody looks unless a test gives a strange result¹. About 30% of twin pregnancies end with one twin absorbed early, a vanishing twin, and this could produce a chimera⁴. That link is a hypothesis. The number of such cases is not known.

History and culture

In 2002, Lydia Fairchild, a mother in Washington State, applied for public assistance. DNA tests showed she was not the mother of her three children². Prosecutors suspected fraud. A test taken at the birth of her fourth child, with a witness present, also failed to match². Her lawyers found the case of Karen Keegan, a Boston woman whose blood DNA did not match two of her three sons. Keegan’s doctors had found a second cell line in her thyroid and other tissues¹. Fairchild was tested the same way. Her cervical cells matched her children. Her blood did not².