An Evolutionary Edge
Begley, Sharon, Newsweek
Byline: Sharon Begley
How grandmas may play favorites.
The question is asked in every language, in every era: "So, dear, when will you give me grandchildren?" Darwin would approve.
At least he would if the "grandma hypothesis" is right. According to this idea, the reason women--uniquely among primates--outlive their child-bearing years is that a female who survives past menopause can contribute to the care of her children's children, improving their chances of reaching adulthood. Natural selection favors behavior that increases an individual's genetic contribution to future generations; surviving long enough to help grandkids is thus an evolutionary adaptation.
Too bad data don't support this intriguing notion. In some studies, a grandmother living nearby was indeed associated with better survival of grandchildren, as the hypothesis predicts. But other studies found no such benefit. Leslie Knapp, a biological anthropologist at the University of Cambridge, and her graduate student Molly Fox wondered if the inconsistency reflected a basic fact of genetics--namely, that because of how the X chromosome is passed down from parents to children, grandmothers are more closely related to some grandkids than to others.
Here's why. A paternal grandmother, like all women, has two X chromosomes. She passes one to her son (who gets his Y chromosome from Dad, which is why he's a he). He then passes grandma's X--the one and only X he has--to his daughter. But Dad passes his Y chromosome to his son, who therefore does not carry his paternal grandma's X. A maternal grandmother, too, passes one of her X's to her daughter; there is a 50-50 chance that that X will be transmitted to the daughter's child, of either sex. A maternal grandmother, therefore, has only a 50-50 chance that her X will be transmitted to a grandchild. A little math shows that maternal grandmothers are related to granddaughters and grandsons equally, for an "X-relatedness" of 25 percent. But paternal grandmothers are twice as close to granddaughters (50 percent) and not at all to grandsons (zero percent), explains Knapp. It may seem arbitrary to focus on X, one of 23 chromosomes, but it has 8 percent (1,529) of all our genes, including some for fertility and intelligence, which affect reproductive success. …