Daughter matches but her mother does not

[ikas]

I have a new match on Anc (CD) at 32cm x 2 segments longest 21cM. Looking at our shared matches I can see my 1st cousin twice removed (FC) who I match at 139cM x 8 segments. Her mother (1st cousin once removed) who I match at 386cm x 16 segments is not on my shared match list with CD. Is a connection between FC and CD via her paternal line the only explanation for this or are there other possibilities?

[AnotherDay]

On AncestryDNA, a shared match is defined as you and your match both sharing at least 20 cM with the third person. So if the mother only shares 19 cM with the new match, the mother won't be in YOUR shared match list with the new match.

[ikas]

So if the mother only shares 19 cM with the new match, the mother won't be in YOUR shared match list with the new match.

But that means the daughter (FC) is a higher match than her mother which seems impossible to me. The daughter being a generation lower should have a lower match than her mother.

[TreeDigger]

But that means the daughter (FC) is a higher match than her mother which seems impossible to me. The daughter being a generation lower should have a lower match than her mother.

In pure genealogy terms, it is indeed impossible to share less DNA with a parent than a child if the line of shared ancestry only runs through that parent. There are several options where that would be different:

1. Both you and the match have a segment of so-called Identical By State (IBS; DNA shared among a group of people that have their roots in the same geographical location) in exactly the same place that got 'tacked on' to the (small) segment of DNA that's Identical By Descent (IBD; shared DNA due to a common ancestor) causing the two segments (IBS & IBD) to be pinged by Ancestry as a complete IBD segment.

2. There is a double line of descent and you & your 1st cousin 2x removed also share an (unknown) ancestor through her paternal line.

3. Ancestry made a booboo.

With regard to the latter: have you checked on your 1st cousin 1x removed page to see whether this match shows up?

[ikas]

Good point, TreeDigger, about IBS. Is that feasible at 21cM longest segment?
As regards Anc booboo - yes I have checked my shared match list with the mother (AC) and no, CD does not show up.

I have looked into it further since my OP and AC's brother's daughter is also on Anc. Again a first cousin twice removed (sharing a common pair of grandparents with FC) and CD is not on my shared match list with her.

I am inclining towards the double line of descent.

[AnotherDay]

Until you know how much DNA the new match shares with mother and with daughter, you have no way of knowing what is going on. If daughter shares 22 cM and mother shares 19 cM - that means they share the same amount (a difference of less than 5 cM can be treated as the same - the family history DNA tests are not precise to that extent).

[AnotherDay]

On Ancestry, if two people both share at least 20 cM with a 3rd person, then the 3rd person will appear in their shared match list.

I (Person A) share 10 cM with a 3rd cousin (Person C) on Ancestry.

My 1st cousin (Person B) shares 60 cM with Person C.

When I look my shared matches with Person B, Person C is not listed (as I only share 10 cM with C).

When I go to Person B's account and look at their shared matches with me, Person C is not listed (as I only share 10 cM with C).

But there is an interesting flip side...

When I look at Person C in my match list (i.e. the people C and I share), Person B is listed as a shared match - as I (800 cM with B) and Person C (60 cM with B) both share at least 20 cM with Person B.

When I look at Person C on Person B's match list (i.e. the people B and C share), I am not there - as C shares less than 20 cM with me (Person A).

[TreeDigger]

Good point, TreeDigger, about IBS. Is that feasible at 21cM longest segment?

Yes. It's even possible for +/- 20cM to be completely IBS, though that happens in only 25% of cases. I would therefor not rule out the possibility of it being partially or even completely IBS instead of IBD.

I am inclining towards the double line of descent.

Definitely research that as well.

[TreeDigger]

If daughter shares 22 cM and mother shares 19 cM - that means they share the same amount (a difference of less than 5 cM can be treated as the same - the family history DNA tests are not precise to that extent).

Yes and no. If the tests are done by the same company, a difference of 5cM is quite significant.