Main contributor: Roberta Estes

X-DNA is a sex chromosome that has an inheritance pattern somewhat similar to autosomal DNA. X-DNA is sometimes confused with mitochondrial DNA (mtDNA) because males inherit an X chromosome only from their mother, but X-DNA is not mitochondrial DNA and is entirely different. X-DNA has a specialized purpose and a unique inheritance path. X-DNA, along with the Y chromosome makes up the 23rd pair of chromosomes. A child that inherits an X chromosome from each parent will be a biological female, and a child that inherits an X chromosome from their mother, but a Y chromosome from their father will be a biological male.

Chromosome 23 Father Contributes Mother Contributes
Male child Y chromosome X chromosome
Female child X chromosome X chromosome

Together, the X and Y sex chromosomes comprise the 23rd chromosome pair.

Inheritance of X-DNA

The X chromosome is inherited in the same manner as chromosomes 1-22, meaning that children inherit some portion of their parent’s and their grandparents X chromosome, but they don’t inherit the same proportion of X DNA from each ancestor. A child inherits either an X or a Y chromosome from each parent. The father cannot contribute an X chromosome to his sons because he contributes a Y chromosome to them.A mother does not have a Y chromosome to contribute, so the mother always contributes an X chromosome to her offsping of both sexes. This means that the mother’s two X chromosomes, one from each of her parents, combine in the same way as DNA from chromosomes 1-22. A mother can give her child all of her mother’s X chromosome, all of her father’s X chromosome, or a combination of both. The X chromosome is similarly recombined in each generation.

In the following image, females in your tree are represented by red stars indicating the inheritance path of their X chromosome, and males by blue stars. Men who are followed by sons are designated by a black X, because their X chromosome is not passed on to their sons.

A female child inherits an X chromosome from each parent. Her mother received an X chromosome from both of her parents, while the father only received his mother’s X chromosome, so the female child always inherits her paternal grandmother’s X chromosome intact.

The father has only his mother’s X chromosome to contribute, not one from each parent. He gives no X chromosome to his sons, but he gives the entire X chromosome that he inherited from his mother to his daughters. The father has no X chromosome from his father, so there is no question about where his X chromosome originated. A man’s single X chromosome is always inherited from his mother. [1]

Genealogy research for X-DNA

Because of this unique inheritance path, anyone who matches a male’s X chromosome must be related on his mother’s side. Additionally, the man’s match cannot be related to any ancestor whose location in the tree is marked with a black X, because that ancestor did not contribute their X chromosome to their male child. This unique X-matching inheritance path is quite useful for identifying common ancestors, but conversely, the reduced number of X-DNA inheritance events means that the expected relationship cannot be calculated accurately using the X chromosome because the number of recombination events varies with the inheritance path. Most vendors exclude X matching for that reason, or provide X matching as a separate feature, not combined with the DNA of chromosomes 1-22.[2]

Explore more about X-DNA

References

  1. Except for rare medical conditions.
  2. X Chromosome Master Class. DNAeXplained – Genetic Genealogy
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Contributors

Main contributor: Roberta Estes
Additional contributor: Maor Malul