Saturday, December 22, 2012

Close Cousin: Percentages

Two people who are related closer than third cousins will share a percentage of identical DNA within a predictable range.  My new match at 23andMe, my maternal first cousin once removed, fits this predicted percentage.

International Society of Genetic Genealogy

My mother and her brother both share 11.9% identical DNA with their first cousin.  As the next generation from these three, my share of the identical DNA should be cut in half.  My mother's cousin and I match 5%.  This percentage is on the lower end of the predictable amount shared.  We should be grateful for this because the lower percentage bumped us into a second cousin prediction and allowed him to be revealed in my account as a match, enabling me to reach out to him.

For illustration purposes, my father and his first cousin once removed match at 6.71%.  I match this first cousin twice removed at 4.44%, which is just short of the 5% I share with my first cousin once removed.  That is how random DNA inheritance works.  You end up carrying more DNA from some ancestral lines than others.