More Adventures in DNA: The X Chromosome

The sources of X chromosome DNA for women, from
The Genetic Genealogist

Thanks to Gedmatch.com, every once and a while I will come across someone who I share a lot of DNA with, but only or mostly in the X chromosome. What does that mean? It means we can rule out several branches of our tree as the source of our shared ancestry. For men, it means ruling out their entire paternal side because men, who have an X and a Y chromosome, get their X chromosome from their mother and their Y chromosome from their father. Women, on the other hand, get one X chromosome from their mother and one from their father, but we can still rule out several branches because it does not come through two males in a row. In other words, men can pass on DNA from their X chromosome but it's from their mother's side, not their father's. So as a woman, the shared ancestry could be on my father's side but it would have to come from my father's mother's side, which conveniently is entirely Italian so if the person I share X chromosome DNA with has no Italian in their tree, I can probably rule out my dad's whole side of my tree; my mother's side will be the source.

If that doesn't make any sense, this article from The Genetic Genealogist, complete with charts, probably explains it a lot better. Being a very visual person myself, the charts made it much easier for me to understand the sources of DNA in the X chromosome.