When a woman is pregnant, some of her fetus's cells enter her bloodstream. It's called fetal microchimerism, and the resultant cells, which have the fetus's DNA, not the mom's, can stay in her body for over thirty years. Now scientists have found that the fetal cells don't just hang out in the blood — they can make it into women's brain tissue, and even affect their risk of Alzheimer's disease.
For a study recently published in online journal PLoS ONE, cancer researcher William F. Chan and his team looked at brain tissue from deceased women, 33 of whom had suffered from Alzheimer's and 26 of whom had not. They found male DNA in 63% of the brains they looked at (they didn't actually have records detailing whether each woman had sons, but that's the most likely source of the DNA). And even more intriguingly, they found that women with Alzheimer's were less likely to have male DNA in their brains than women without the disease. When it was present, male DNA was less concentrated in the areas of their brains most affected by Alzheimer's.
Chan and his co-authors note that their sample size is very small, and more research will be needed to see if their findings hold up. They're also not sure why tiny bits of male DNA would protect women's brains against Alzheimer's. However, they point out that men are less likely to get Alzheimer's than women — maybe the male DNA is somehow conferring a benefit men's brains already get. Also, there's evidence that fetal cells can also protect women from other diseases, like breast cancer (though they may raise the risk of others, like colon cancer).
So while we're very far from some sort of male-female mind-meld cure for Alzheimer's (complete with dumb jokes about women suddenly being unable to ask for directions or properly do laundry), what we do have is evidence that pregnancy, brains, and DNA are weirder than we ever thought.