Chimerism: two sets of DNA in one person

Imagine a DNA test, done for some unexpected reason, revealing that your body carries the DNA of two different people. It sounds like fiction, but such cases are real. The condition is called chimerism.

Lydia Fairchild

In 2002 in the United States, Lydia Fairchild had a DNA test during a child-support case. The result suggested she was not the mother of her own children, and a court came close to taking them away. During a later pregnancy, a test performed at the birth still did not match — until samples from different tissues of her body were tested. They revealed two distinct DNA profiles in one person.

Karen Keegan

In the same year in Boston, Karen Keegan’s sons were tested as possible kidney donors. Their DNA did not appear to match hers. Further testing confirmed that she, too, was a chimera.

How does it happen?

The most likely explanation is that each of these women began as non-identical twins — two fertilised eggs — that fused very early in the womb into one embryo. Each original cell line kept its own DNA, so different organs and tissues can carry different profiles: cells from saliva or blood may show one, while reproductive cells may carry the other.