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Two females tested on 23andme.com and come up as a 1st cousin match of 11.2% of their DNA.

They share zero X chromosomes with each other. Does this mean they are related to through their mothers and not their fathers?

  • I think that we need more information like how much shared DNA segments are (in cM) and how they are distributed across chromosomes. It is really very strange that there is no X match. But it may be normal as 1st cousins share the grandparent(s) and the inheritance may be more complex than you think. Also X chromosome is not very long. And we don't get precisely from each grandparent 25% of DNA – George Gaál Jan 16 '18 at 9:44
  • Maternal X2b K1c1 Paternal Not available Not available – Carlos Jan 16 '18 at 9:48
  • 11.2% of your DNA – Carlos Jan 16 '18 at 9:50
  • the first cousin does not share any dna testers 1/2 maternal sister. Which would make it be that the two that are first cousins must be share paternal x which they do not, right? – Carlos Jan 16 '18 at 9:57
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Females get one X chromosome from their mother and one from their father.

Their father only has one X chromosome and it was passed whole to him from his mother.

So two 1st cousin females who do share some part of their X chromosome can be related through their mothers or their fathers. But if they are related on their father's sides, then it must be through their father's mother. It cannot be through their father's father.

Now your question is different. You're asking what happens if they don't share any part of their X chromosome.

First realize that no sharing of an autosomal segment usually cannot tell you anything, because they might be related, but just didn't get the same random parts that get passed down. For example, it is possible for 2nd cousins once removed and further to not share any DNA at all on their 23 chromosomes. For a particular chromosome, it is possible for even siblings not to share any, since one could get the full chromosome from one parent if it happens not to recombine, and the other could get the full chromosome from the other parent if it as well happens not to recombine.

Let's look at the X chromosome in particular. The X chromosome is 196 cm long meaning it averages about 2 crossovers per generation. Using a Poisson distribution to estimate the probability of a specific number of crossovers gives:

enter image description here

This means there is a probability of 13.5% that there are no crossovers, meaning the X that was passed down was a complete copy of one of the two X's of the mother.

So here is what can happen in your situation of two female first cousins. It is complicated, so please bear with me:

Case 1: If they are both related on fathers' sides, then each of their fathers would have passed down to them a possibly recombined X from their grandmother. There is a 13.5% chance that the X did not recombine for one of their parents and a 13.5% chance that it didn't recombine for the other parent. And if they didn't, then there is a 1/2 chance they are from different X's of the grandmother and they won't match. That means there's a 13.5% x 13.5% x 1/2 = 1% chance that they don't share any X in this case.

Case 2: One is related on their father's side and one on their mother's. The father passed down the one X he got from his mother that had a 13.5% chance of not recombining. The mother passes down an X that is possibly recombined from her father and mother. There is a 13.5% that it is not recombined, 6.75% that it was from her father whole, and 6.75% that it was from her mother whole. If it was from her mother's father, then it will not match her cousin's X. If it was from her mother's mother, then if it did not recombine and her cousin's did not recombine, there is a 1/2 chance they are from different X's of the grandmother and they won't match. So the overall chance of not matching in this case = 6.75% + 1/2 * 6.75% = 10%.

Case 3: If they are both are related on their mothers' sides, then it is similar to case 1. Each of their mothers would have passed down to them a possibly recombined X from their grandmother. There is a 13.5% chance that the X did not recombine for one of their parents and a 13.5% chance that it didn't recombine for the other parent. And if they didn't, then there is a 1/2 chance they are from different X's of the grandmother and they won't match. That means there's a 13.5% x 13.5% x 1/2 = 1% chance that they don't share any X in this case.

In summary, the chance that there is no X DNA in common between two 1st cousin females is:

  1. 1% if they are both related through their fathers.
  2. 10% if one is related through their father and one through their mother.
  3. 1% if they are both related through their mothers.

The likelihood is greater that it is case 2, but anything is still possible.

So I'm sorry, but you can't eliminate any sides from this.

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