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July 24, 2010

GENEALOGY: ‘Grandmother hypothesis’ the focus of several studies

Tamie Dehler Special to the Tribune-Star

TERRE HAUTE — Anthropologists have long looked at the extended family and suspected that children with a grandmother living nearby would have an increased chance of survival into adulthood because the grandmother is available to help the parents with nurturing and caring for the child. This “grandmother hypothesis” has been the focus of several studies by a number of researchers over time. The results of these scientific investigations have been a mixed bag: Some researchers indeed found that a grandmother living nearby had a positive effect on the survival of her grandchildren, but other studies did not show any effect. In these investigations, both types of grandmothers (maternal and paternal) were grouped together with all the grandchildren, whether they were male or female.

In October 2009, anthropologists from the University of Cambridge published a study in which they looked at the grandmother hypothesis in a new way. They wanted to investigate whether the positive effect of a grandmother living near her grandchild could be linked to the degree that the grandchild is related to the grandmother. It may be surprising, but not all grandchildren are related to their grandmothers in the same degree. The genetic degree of a grandchild’s relationship to his/her grandmother varies, depending on the gender of the grandchild and whether the grandmother is from the maternal or paternal line.

First, let’s review some genetic background. Each human being has 46 chromosomes arranged in pairs: 23 chromosomes (one in each pair) come from the father and 23 come from the mother. (The chromosomes, of course, carry the genes that cause us to inherit things from our parents.) One chromosome of the 46 is responsible for the gender of a child. This is the Y chromosome, which can only be inherited from the father. Women don’t have Y chromosomes; they have two X’s. Each man has an X and a Y. The X comes from his mother and the Y from his father. Eight percent of our genetic inheritance (1,529 genes) is located on the X chromosome. The Y chromosome is responsible for the male gender, but has very few other genes on it and accounts for very little genetic inheritance other than being a male.

Your maternal grandmother is your mom’s mother. She passes one of her two X chromosomes to your mother. (Your mother gets her other X from her father). When your mother, in turn, has a child, whether male or female, there is a 50-50 chance that the X from your grandmother (as opposed to the one from your grandfather) gets passed to you or your siblings. Each of the maternal grandchildren, whether a boy or girl, is therefore “X-related” to a maternal grandmother equally, by 25 percent.

The situation is different, however, for your paternal grandmother (your dad’s mother). She passes an X chromosome to her son (your dad) and he gets a Y chromosome from his father. When your dad, in turn, fathers a child, the degree of “X relatedness” of that child to the paternal grandmother depends upon whether that child is male or female. If your dad fathers a girl, there is a 100 percent chance that the X chromosome he contributes to his daughter came from his mother (because he has only one X, and it did come from his mother). If he fathers a boy, there is a 0 percent chance that his son gets an X chromosome from his paternal grandmother, because your dad passes his Y chromosome to his sons, and this Y came from his own father. Thus, the paternal grandmother is more closely “X related” to her granddaughters (50 percent) than is a maternal grandmother (25 percent), but is not at all “X related” to her grandsons (0 percent). Remember that this X-relatedness just covers one of the 23 pairs of chromosomes.

Tear out this column and keep it for next week, when we will cover the surprising findings of the anthropological study on maternal vs. paternal grandmothers.

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