Chimerism is a genetic condition where a single organism is composed of cells with more than one distinct genotype). In humans, chimerism can occur when a pregnant woman is carrying twins and one embryo dies, and the other embryo absorbs the twins cells. This results in the surviving fetus having two sets of cells with two distinct types of DNA. Another way that chimerism can occur in humans is through organ transplantation, where the recipients body can absorb the donors cells and DNA).

There are different types of chimerism, including microchimerism, tetragametic chimerism, and twin chimerism. Microchimerism occurs when a pregnant woman absorbs a few cells from her fetus, or vice versa. Tetragametic chimerism occurs when a pair of twins is conceived and one embryo dies in the womb, and the surviving fetus absorbs some of the cells of its deceased twin. Twin chimerism occurs when a person absorbs some of their twins cells while in their mothers womb.

Chimerism doesnt usually have many signs or symptoms, but if people with the condition arent diagnosed, it may have ramifications for individuals and their families, including losing parental rights or financial support. However, most often, a persons health is not negatively impacted by chimerism. If someone suspects they may have chimerism, they can contact a genetic specialist such as a genetic counselor, who can evaluate their medical history, interpret their standard parental DNA test results, design a chimerism-appropriate testing strategy, and interpret the new results.