The son of my wife's half-sister recently uploaded his DNA (from Ancestry) to GEDmatch where my wife's DNA (from 23andme) is already recorded. There was a high (846cm) autosomal match, but ZERO match on X-DNA.

My wife and her half-sister are from the same mother. My research so far suggests that my wife and her half-nephew should share a large amount of X-DNA which would have been passed from his GM and her mother. Can you help me understand this apparent inconsistency? Thanks!

  • Please write the particular number of kits on GEDMATCH. Also the incosistency may arise because of the different testing labs. The differet labs checks the different positions in DNA, so some of positions are overlapped and some not and they can't be counted by the GEDMATCH service. Maybe the tuning of comparison parameters will help. Or better - to pass the test in the same lab almost in the same time (because labs revise their test chips sometimes). – George Gaál Nov 22 '17 at 16:47

Your wife got one X Chromosome from her mother and one from her father. Her half sister also got one from her mother and one from her father.

Their fathers are different so those chromosomes won't have anything in common.

Their mothers are the same, but the X Chromosome is taken from a combination of their mother's parents. It is about 150 cM long and will on average crossover 1.5 times. Of course, it can't crossover 1.5 times but will crossover 0, 1, 2, or 3 and occasionally more, but the average is 1.5 times.

So each half sister may get any part of their maternal X from their mother's mother or their mother's father. There is a 1/2 chance they get the same part. That means that the two half sisters will on average share about half of their X chromosome.

Now we take the son of your wife's half sister. He gets his one X chromosome from his mother. But what he gets is a combination of his mother's mother and his mother's father. His mother's father will not match with your wife. So on average, you would expect 1/4 of his X chromosome to match your wife.

But due to recombination, the chromosome may have had 0 crossovers, and he may have got his entire X from his mother's father. In that case there will be zero match.

Or the chromosome may have had 1 or 2 or more crossovers, and the parts he got from his mother's mother may not have been among the approximately 50% that your wife shares with his mom.

Probability-wise, there may be a 20% chance that this case of no X sharing might happen, and you seem to have encountered this situation.

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  • Thank you for the detailed answer. However, I don't understand why the son of my wife's half-sister would not also match my wife's X chromosome at 50% since he would receive an exact copy of his mother's X chromosome? It seems that I may not understand how the X chromosome may vary from his mother's. So the chance of zero overlap would seem very low. – D Cuozzo Nov 24 '17 at 15:34
  • The son does not receive an exact copy of his mother's X chromosome. His mother has two X Chromosomes, one from her mother and one from her father. He usually receives a combination of his mother's two X chromosomes, or in about 15% of the time, receives just one of them. – lkessler Nov 24 '17 at 21:18

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