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
Wiki


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.