X-DNA Inheritance and Recombination

X-DNA has a unique inheritance pattern, and knowing about it can sometimes help you figure out the ancestors you share with a DNA match. Men inherit a Y chromosome from their father, and an X chromosome from their mother. It is easy to imagine that an X chromosome remains unchanged just like a Y chromosome does when it is passed from father to son, but this is not the case with X-DNA.

Here is a question from someone in the Research Like a Pro with DNA study group:

“This is how my X chromosome compares with my brother’s on 23andme. Since he and I both got our X chromosome from our mother, I would have expected they would be fully identical. What happened?”

The X chromosomes that each brother received from their mother were slightly different. Recombination – which is random and unpredictable – had occurred, and that explains the difference.

Here’s another example of a test taker and two cousins. One of the cousins is a first cousin (1C), and the other is a first cousin once removed (1C1R). Each of the DNA testers shown in different colored boxes in the chart received an X chromosome that was passed from their paternal grandmother down to their fathers, who were brothers, and then onto them. In the case of the paternal 1C1R, another X chromosome recombination occurred with his mother, and he received a little less of the same X-DNA than the paternal 1C did.

Use X-DNA inheritance charts to help you trace possible ancestors who passed segments of your X chromosome on to you. These charts were created by Blaine Bettinger and can be found in his blog The Genetic Genealogist.