I've noticed that in comparing multiple autosomal kits, I often get a bunch of matches in chromosome 23 for various triangulation compares I've performed.

It seems matches on chromosome 23 come up more frequently than other chromosomes.

Is a match on chromosome 23 less statistically significant for the same cM match length?

2 Answers 2


There is no autosomal chromosome 23. The human cell has 22 pairs of autosomal chromosomes (termed chromosomes 1 through 22). The remaining two chromosomes of the cell nucleus are the sex chromosomes, the X and Y chromosomes.

The sex chromosomes are called the X chromosome and the Y chromosome. There might be some software that labels the X chromosome as "chromosome 23" but that would be a programming shortcut and is not an accepted term. (The Y chromosome in a male is just as much a member of the 23rd pair of chromosomes, but it would be even more confusing to also label it as "chromosome 23"). So it seems likely you were misdirected and your question is really about the X chromosome.

X chromosome matches behave quite differently from autosomal matches because the X chromosome is inherited differently. Down an ancestral path the X chromosome never passes from a father to his son, and passes unchanged and without recombination from a a father to his daughter. This means that the X chromosome pieces get chopped up more slowly than autosomes do over the generations, and so the sizes of matches in centiMorgans (cM) tend to be larger than with the autosomal chromosomes.

More about the inheritance behavior of the X chromosome can be found under Interpreting X DNA?, and in this blog post about 'That Unruly X'.

  • It is GEDmatch.com that is reporting on chromosome 23.
    – WilliamKF
    Nov 16, 2016 at 23:36
  • The sex (X and Y) chrosomosomes are often referred to as chromosome pair 23 by the DNA companies.
    – lkessler
    Nov 17, 2016 at 14:38

Robert is correct that X passes unchanged from a father to daughter, but that would likely only reduce the overall number of crossovers by about 25%, as a son's X is from his mother and recombines normally. So that reduction in crossovers shouldn't increase the number of matches by very much. Update Dec 25: I posted some analysis I did that discusses the reduction of the crossovers for X by only 25%.

I expect that the criteria that the company is using for X matching is what is causing your larger number of X matches. For example, at FamilyTreeDNA, the criteria for an autosomal match requires at least one segment matching at least of a certain length. The company doesn't go out of its way to post its criteria, but Blaine Bettinger in his Family Tree Guide to DNA Testing and Genetic Genealogy says it may be between 5 and 7 cM. He says for X-DNA, the criteria is twofold: the individual must be an autosomal match AND must share 1 cM on their X-DNA.

Since the X only need share 1 cM to show up, a higher proportion of X matches may exist than you'd expect. Of course, this could mean more false positives (matches that really aren't) for the X than the autosomal, but still it may account for your observation that matches on X occur more frequently.

This lower matching criteria for X, may also be true for other companies, 23andMe, GEDmatch, etc.

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