In 1979, Stanford University researchers first discovered that women have some male Y-chromosomes in their blood. Women have only X-chromosomes, so the Y-chromosomes had to come from someone else. In this case, the women were pregnant, so it was inferred that the chromosomes came from the male babies in their wombs. It was interesting enough that not only does the mother’s genetic material enter the baby, but the genetic material from the baby (and thus from the baby’s father) enters the mother.
This cell-swapping phenomenon is known as microchimerism. Since those early studies, we have realized that women who have never had sons may also have these foreign Y-chromosomes in their bodies. These cells also may affect a woman’s immune system, and a study in 2012 showed that the foreign cells are found in the brain.
