When Cousins Marry

I was talking with someone about one of their 9x great grandparents.  The question came up-- how many ninth great grandparents do we have?  It turns out we typically have 2,048 ninth great grandparents.  Naturally, the number doubles with each succeeding generation.  It's simple math.  That is, until cousins get married.

There are several instances of cousins marrying each other in my tree.  One marriage, way back there, is in my direct line.  7th great grandmother Elizabeth Brashear married her first cousin Samuel Brashear in December of 1717 in Prince George's County, Maryland.  Even more egregious, they were double first cousins.  Their fathers were brothers and their mothers were sisters.  I understand cousins marrying was fairly common at the time--  well, maybe not double cousins.

The effect of this union changes the number of ancestors one can claim up and down the line.  Because they were double cousins, Elizabeth and Samuel shared both sets of grandparents.  Imagine sharing both sets of your grandparents with your spouse!  Their child, Benjamin has only 4 distinct great grandparents instead of the normal 8.  See in the graphic below how some of my 9th great grandparents show up twice in the same tree.  Each color represents an individual person.

The reduction in ancestors caused by this cousin union affects every descendant all the way down to me.  Instead of having 2,048 9th great grandparents, I have 4 less @ 2,044 (assuming there are no other cousin matches in my tree somewhere).  Consequently, that reduction doubles with each preceding generation.  I have 8 less 10th great grandparents and 16 less 11th great grandparents than typical.