Human chimera: What the rare condition is, and why there might be many more people with it than we realised

Doctors are finding more and more instances of human chimeras — a condition that might affect far more people than we realise.

Human chimerism hit the headlines this week after it was revealed that a man had failed a paternity test because he had the condition — which meant that the genetic father of his child was his unborn twin.

The condition occurs when one unborn child absorbs the cells of another miscarried sibling in the womb, leading to the surviving person taking some of the genes of their “ghost” twin. That can lead to strange results in the DNA of people that have the condition — but it can often go completely unnoticed.

The condition is relatively common in some animals — but it may happen more often than we realise in humans, and go unnoticed. And it may become yet more common, as more people use fertility treatments to conceive their children.

In previous cases, people have been found to have two different blood types, or to develop different sexual organs from those that would be associated with the rest of their body.

Another example involved Karen Keegan, a woman from Boston in the US who had a different set of genes in her blood cells and her ovaries. Her children were not genetically hers — and the real genetic mother was a twin sister that was never born.

In the case of the Washington man reported this week, different genes were found in his cheek — where the saliva in the test was taken from — and his sperm. That meant that he was excluded from a paternity test, but a DNA-based ancestry check showed that he was the child’s uncle.

But many people who have the condition will never take a paternity test for their children, or find out through other means. That is likely to mean that many people could have the condition, and never know.

Studies have suggested that the condition might be present in 21 per cent of triplets, and eight per cent of twins. But many of those people would never know — because testing is likely to show up the result, unless it is being searched for.

Chimerism might already be far more common than we realise, but it could become even more prevalent, Barry Starr, a geneticist at Stanford. Fertility treatments are much more likely to lead to multiple births, and so are in turn far more likely to produce more people with chimerism.

A version of the condition called “artificial chimerism” can also occur through transfused blood stem cells — though that is less common because transfused blood now tends to be hit with radiation to help the host absorb it — or through bone marrow transplantation.