Sharing more autosomal DNA with second cousin's child than with second cousin?

Question

In my shared matches on AncestryDNA I am used to seeing matches to the children of my 2nd cousins that are less than the match I am to their parents.

However, when examining my sister's DNA I see what seems to be a surprising result where she shares more DNA with the child of a 2nd cousin than she does with our 2nd cousin.

I'll refer to our female 2nd cousin as 2C and to her son (our 2nd cousin once removed) as 2C1R.

• I have 169 centimorgans shared across 10 DNA segments with 2C, and 104 centimorgans shared across 6 DNA segments with 2C's son 2C1R
• my sister has 68 centimorgans shared across 6 DNA segments with 2C, and 94 centimorgans shared across 6 DNA segments with 2C's son 2C1R

Is it possible to explain what I think seems to be an anomaly?

Answer 1

The anomaly is your sister's 68 cM in 6 segments with your 2C as it appears too low.

Since you're on Ancestry, there's not a lot you can do to figure this out. Try comparing the sets of Shared Matches of you versus 2C, your sister vs 2C, you versus 2C1R, and your sister versus 2C1R. See if that sheds any light on the problem.

Answer 2

There could be several reasons:

1. You and/or your sister are matching a segment with the 2C1R that was inherited from his other parent.

2. Your sister and the 2C1R have some "identical by chance" markers which make the segments appear longer than they truly are by descent.

3. The Ancestry algorithm is not fully reporting the shared cMs between your sister and the 2C, due to no-calls or some other anomaly.

Uploading them to a site with a chromosome browser would tell you.

Answer 3

This resolved itself today when I had email contact with my second cousin's son for the first time, and he told me that I had recorded him as being the offspring of the wrong one of three sisters in that family.

Once I switch in the correct lady, and call her 2C instead:

• I have 106 centimorgans shared across 5 DNA segments with 2C, and 104 centimorgans shared across 6 DNA segments with 2C's son 2C1R
• my sister has 218 centimorgans shared across 10 DNA segments with 2C, and 94 centimorgans shared across 6 DNA segments with 2C's son 2C1R

This no longer seems anomalous.