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View Full Version : Can there be a pile up on the x chromosome?



firemonkey
02-11-2017, 01:44 AM
I have 7 matches on the X from 123-136000000 ,12.9-13.2 cMs, that are not autosomal matches. I also have one from 123-134000000,11.5 cMs, that is a small autosomal match ie 7.4 cMs.
Could this be a pile up or does it suggest shared ancestry albeit very distant?

geebee
02-12-2017, 01:20 AM
It depends on the sexes involved. Since you're male, there can't be any "pile ups" on your side -- given the fact that you only have a single SNP at each position on the X. If you match another male, you know that neither of you has any additional "opportunities" to match each SNP.

That's why companies typically use three different thresholds to determine matching segments on the X chromosome. Male-male has the lowest threshold; female-female has the highest; and male-female is intermediate.

So to answer your question, it's likely that this match does indicate shared ancestry -- especially if it's to another male. But how ancient the match is likely to be, may depend on the combination of genders on each side.

Basically, there's only one time when the X chromosome is unable to recombine: when it's passed from father to daughter. If it's from mother to daughter or mother to son, recombination can occur. (It doesn't mean it necessarily will, but just that it can.)

It's possible that there are no males between you and the common ancestor who provided the segment. In that case, the segment wouldn't necessarily be any more ancient than a similar-sized segment on an autosome. Or, for either or both of you, there could be a male in every other generation. That would obviously also mean that recombination was skipped in every other generation, so the segment could be much older than a similar-sized segment on an autosome.

EDIT: Especially since you say you have seven of these matches, the segment is almost certainly "real". That doesn't mean for any two of you that your most recent common ancestor is the same person as for the others. But, it does mean that there presumably is someone who is the most recent common ancestor of the whole group.

How many generations back, as I was trying to say, may depend on the combination of sexes in each of the lines involved.