Cousins DNA

Genetic genealogy is the application of genetics to traditional genealogy. Genetic genealogy involves the use of genealogical DNA testing to determine the level and type of the genetic relationship between individuals.
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darkerhorse
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Cousins DNA

Post by darkerhorse »

I’m trying to determine the expected percentage of shared DNA between two male cousins: S and P.

S’s parents were 3rd cousins to each other and shared the same surname, and P’s parents were 2nd cousins to each other and shared the same surname as both of S’s parents.

In sum, all four parents had the same last name as they were descended from the same ancestral couple of great-great-grandparents. Of course, the surname was that of their common great-great-grandfather.

S’s father and P’s father were 1st cousins, S’s father and P’s mother were 2nd cousins, S’s mother and P’s father were 3rd cousins, and S’s mother and P’s mother were 3rd cousins. Again, all bearing the same surname from the same surname line.

This means that S and P were 2nd cousins through their fathers, 3rd cousins through S’s father and P’s mother, 4th cousins through S’s mother and P’s father, and 4th cousins through their mothers.

So, S and P are 2nd cousins, 3rd cousins, and double 4th cousins – all through their common surname line.

I know that the expected percentages of shared DNA are 3.125% for 2nd cousins, 0.780% for 3rd cousins, and 0.195% for 4th cousins. So, S and P should be expected to share more than 3.125% but I don’t know how to calculate the percentage. If you simply add the figures, the result is 4.295%, still within the range of 2nd cousins.

I know S and P would almost certainly have the same Y haplogroup, but I wonder if we can say anything more about their shared DNA especially given all of their blood relationship is in the same surname line?

As a side note, has anyone seen an elevated percentage of shared DNA among DNA matches who are known to be double, triple, or quadruple cousins?
darkerhorse
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Posts: 3360
Joined: 11 Jun 2020, 18:31

Re: Cousins DNA

Post by darkerhorse »

I read that double 1st cousins share double the amount of DNA or the same amount as half-siblings.

So, in my example, would the result be 8.59%, more than a half-1st cousin and less than a full 1st cousin?

In that case, it would be significant.
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