Adoptee as first cousin on mother’s side, Haplogroup does not match?

+1 vote
If 23andMe matches an adoptee as a “ first Cousin” with 11.2% DNA match and different maternal Haplogroups ( r vs T)— how can this person be showing DNA matches to distant cousins withIn the same maternal surname ancestry? We are both females and about 10 years apart in age. Please help!

As I understand this she can’t be related maternally. But other common ancestors are in my more distant maternal line.
in Genealogy Help by A. Holmer G2G Rookie (160 points)

Hi, there! I'm not completely certain I follow the scenario. Mitochondrial DNA is passed along exclusively from the mother. We all have mitochondria, but they come to us only in the ovum.

So you and your discovered 1st cousin (since you're 10 years apart in age, the only other feasible relationships for that amount of shared DNA would be great aunts/uncles or great nieces/nephews) would share the same mtDNA only if your mother is the sister of her mother. If the connection is through your uncle, the mtDNA would be different.

Works the same way for prior generations, too. Just one male anywhere in the inheritance chain breaks the mtDNA continuity. And because most western surname traditions are patrilineal, it often makes it very difficult to accurately determine the mtDNA inheritance sequence on paper.

Too, because autosomal DNA is divvied up 50/50 at conception, any pedigree collapse along the way could account for atDNA sharing and different mtDNA, even if the matrilineal paper-trail is rock solid. For example, if your 2g-grandparents were cousins, you might expect to see some of your 2g-grandfather's DNA showing up in some of your 3rd cousins who look on paper like they should be only on your matrilineal line.

And last, you didn't indicate how distant the other shared matches are. It's in the testing companies' best interests to make it seem like a piece of cake to reach back to your Xg-grandparents via our inexpensive microarray DNA tests. In truth, things get really iffy past 4th cousins, or 3g-grandparents. It requires significant understanding of DNA, multiple test takers, and a good deal of legwork--things like comparing the shared segment to population and haplotypic pile-up regions; evaluating viable SNP density in the segment; traditional trio phasing if possible; determining if the segment resides in the exonic region, which is less reliable for genealogical comparisons; etc.--in order to arrive at an accurate assessment. If any of those distant matches are showing very small segments, they may just be muddying the water; they may not be actual evidence of a genealogical relationship.

2 Answers

0 votes

Perhaps she’s not a first cousin then? There are other options if you share that number of cM:

by Katie Fuller G2G6 Mach 3 (34.8k points)
0 votes
From my interpretation of your question, I'm going to assume the following is true: 1) your match is a full first cousin, i.e. you share a set of grandparents, 2) they match on your mother's side, 3) they do not match your mitochondrial haplogroup.

The simplest explanation that satisfies all these assumptions is that their father was your mother's sibling.  They would have inherited their mitochondrial DNA from their own mother, who was likely not closely related to you.
by Brian Lamothe G2G6 Mach 3 (34.1k points)
edited by Brian Lamothe

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