Shared match with my cousin's daughter but not the mother

I find this odd. I think I have found two "cousins" where "shared matches" list Clara, but not her mother Johanna, my first cousin. Surely if Clara has a bit of matching DNA to someone matching me, she must have got it from her mother. How can it skip a generation?

 

Hello Barbara, I'm wondering if your problem of shared matches with daughter but not mother is due to a combination of the 20 cM cut-off Ancestry impose on shared matches and the downgrading of matches due to the filtering system (TIMBER algorithm) they use. That's a bit of gobbledygook, so let me explain:

I have counted 8 cases where my son has a match of >20 cM with someone but where my match is <20 cM with the same person. So my son would show up in shared matches with these people but I wouldn't. Likewise my husband has 4 such matches where he has <20 cM but our son has >20 cM with the same person. In each case my (or my husband's) match has been downgraded from >20 cM to <20 cM but our son's hasn't. For example, one of my matches has an 'unweighted' shared DNA of 24 cM (for both me and my son). This has been reduced to 14 cM for me but 22 cM for my son. Similar situation for the others. I'm not totally sure why this is, something to do with statistics - maybe Peter could shed more light? But it would certainly have an effect on the 'shared matches' list. You don't know how many cM your two "cousins" share with Clara or her mother, just that it's >20 cM with Clara (and yourself) but less with her mother.

 

Thank you Helen, very helpful. Yes, down near the 20 cM line things appear to get pretty tricky!