What does DNA on chromosome X mean?

If you have done a genealogical DNA test you probably had a chance to work with chromosome X results and you know how useful they can be. In this article you will find out how to ‘squeeze’ the most information out of chromosome X 🙂 If you haven’t done the test yet, this article will allow you for a better understanding of the results when you receive them.

Our 23rd pair of chromosomes are sex chromosomes¹. A child always inherits chromosome X from the mother, however, a father can either pass on his chromosome X to the child – in this case, it will be a girl or chromosome Y and in this case, it will be a boy. Biology is here very straightforward.

Women have to chromosomes X, men one chromosome X and one Y

A chromosome X match can eliminate many hereditary lines, however, we must remember that a small match on chromosome X may result from dual kinship².

We will find information about chromosome X DNA in results of two companies: Family Tree DNA and 23andMe (infographics below). However, if you’ve done your test at Ancestry or My Heritage don’t worry, these companies also check chromosome X DNA, but to check your matches you will need to upload your result to f.ex. GEDMatch. Instructions, on how to download your data from Ancestry you will find here, from My Heritage here, and on how to upload your result to GEDMatch here.

Inheriting chromosome X in women

Women have two X chromosomes, one inherited from the mother and one from the father. The father passes on to his daughter the X chromosome, which he inherited from his mother. So the chromosome inherited from the father a daughter will have in whole from his mother (ie. her paternal grandma).

Because a woman inherits chromosome X from both parents, if she has a match on an X chromosome, this may mean kinship from both mother’s and father’s line. But we are still able to exclude some lines f.ex. kinship through grandfather on the father’s side or grandpa’s father on the mother’s side etc. Why? Because men pass on their chromosome X only to daughters, while to sons he passes on chromosome Y.

Let’s look at the diagram of inheriting chromosome X by women. Men are depicted in blue and women in pink. White fields mean ancestors, from whom the women did not inherit chromosome X.

Inheriting chromosome X by men

Men inherit their X chromosome from their mother, while from the father they receive chromosome Y. This means that if a man has a chromosome X match, the kinship is on his mother’s side.

The diagram below shows how chromosome X is inherited by men. It is clear at first sight, that there are fewer possible kinship lines than for women because a man gets his chromosome X only after his mother.

Because a man passes on his chromosome X, which he inherited from his mother to his daughter, this means that the daughter has one chromosome X identical to her father. Below you will find a graphical representation of my father; s X chromosome:

Here we can see two chromosomes. The one inherited from the father is identical for me and him:

If a father passes on his X chromosome to his daughters, this means that daughters from the same father will have one chromosome X identical. This is one of the points of DNA analysis in terms of kinship. F.ex. if two half sisters have the same father, then one of their X chromosomes will be identical. In the case of half-sisters having the same mother, this rule will not be as straightforward, as a mother passes on the recombination of her two X chromosomes in place of one X chromosome of the child.

If you have any questions, please feel free to ask them in the comment section.

Good luck in finding about your X chromosome!

1. apart from these, we have 22 pairs of autosomal chromosomes
2. f.ex. both from the mother’s and father’s sides.