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. The two lines can even be different sexes¹.

Nothing shows on the outside in most cases. Sometimes there are hints: two eye colours, uneven hair growth, or skin marks that show under UV light¹.

A much milder form is common. During pregnancy, cells cross the placenta both ways. Mothers keep small numbers of their children’s cells for decades. This is microchimerism³. A 2012 study found male DNA, probably from sons, in the brains of 37 of the 59 dead women examined⁶.

How common it is

The 100 figure is a count of found cases, not a rate. Fewer than 100 cases of true human chimerism have been reported since the first one in 1962⁵. Cases are often found only when samples are tested for another reason¹. So the count tracks how often people are tested, not how often chimeras are born. The true share of people is unknown. Microchimerism is a separate thing and is common. About 50% to 75% of women carry fetal immune cells after giving birth³. Wide DNA testing of ordinary people is what would change these numbers.

Is it a problem?

Usually not. Most chimeras go through life without knowing they are chimeras¹. When the two cell lines are of different sexes, the person can have intersex traits or changed fertility¹. The practical issue is that it can confuse blood typing, tissue matching, and DNA parentage tests, as it did for Lydia Fairchild². No doctor screens for it. It only comes up when a blood, transplant, or genetic test gives a result that does not add up.

History and culture

In 2002, Lydia Fairchild, an American mother, applied for child support enforcement². DNA tests showed she was not the mother of her two children². Prosecutors suspected fraud. A test taken at the birth of her third child, with a witness present, also failed to match². Her lawyer found the case of Karen Keegan, a Boston woman who was also a chimera². Tests for a kidney transplant had suggested Keegan was not her sons’ mother¹. As in Keegan’s case, DNA was taken from Fairchild’s wider family². Her cervical cells matched her children. Her skin and hair did not².

One route may start with a twin that is never born. Up to one in every eight multiple pregnancies ends with a twin absorbed early, and its cells can stay in the survivor⁴. That link is a plausible idea, not a counted number.