Now What? Counting Chromosomes
1/1/2008
What fraction of your genes do you share with first, second and third cousins?

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.