DNA Q&A: Counting Chromosomes

By Ann Turner

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Q: With all the talk about DNA in genealogy, I’m wondering how closely cousins are related genetically. What fraction of your genes do you share with first, second and third cousins?

A: Medical geneticists use the term degree of relationship. Your first-degree relatives are your parents, your children and your siblings. You share 50 percent of your genes with those relatives. This number comes from the fact that chromosomes (except the Y, which only men have) come in matched pairs, so you have two copies of every gene — one from your father and one from your mother.

For parents and children, the 50 percent number is exact: Your father passes on one of his copies of a gene, and you pass one of your copies to each of your children. But that figure is just an average for siblings. Whether your father deals you a gene he inherited from his father or his mother is a matter of chance. You might end up with more genes that came from your paternal grandfather, while your sibling might end up with more genes from your paternal grandmother.

Your second-degree relatives add another generation to the chain, halving the percentage again. You share about 25 percent of your genes with a grandparent, grandchild, aunt, uncle, niece or nephew. Third-degree relatives — first cousins, great-grandparents, great-grandchildren, great-uncles, great-aunts, great-nephews and great-nieces — share about 12.5 percent of your genes.

Second cousins add another generation to each line of descent from their common ancestor, making them fifth-degree relatives with 3.125 percent of their genes in common. Again, this is an average number — some second cousins might share more than that percentage, while others share less. Third cousins share about .78 percent of their genes.

To prevent statistics such as these from being misused, the House of Representatives passed the Genetic Information Non-discrimination Act in April 2007. It bans insurance and employment discrimination if even a fourth-degree relative tests positive for a medical condition. A first cousin once removed would be a fourth-degree relative. The Senate was considering the bill at press time.

From the January 2008 issue of Family Tree Magazine