I am a bit puzzled, is there a dna expert who can help me?

+4 votes
There are some of my mom's and my 23andme dna relatives/cousins that we are equally % related to, and my mom's half brother is always more closely related to these same cousins. How can this happen?
in Genealogy Help by Allison Betz G2G Rookie (250 points)
retagged by Allison Betz
Is there a significant difference in the percentages for the half brother?
Hi Allison this can happen when Ancestor's are also 2nd, 3rd Cousins ect.- related to one another when they get Married. This Happened in my Mother's Lines in New Brunswick, Canada, it was common in that Province. My Maternal Grand Parents where 2nd Cousins, but that was due to there common ancestor getting Married Twice also making my Mom's Paternal Grand Parents related 3rd Cousins when they got married. it was an ancestor of my Great Grand Parents that married Twice making them all related .So it may be Possible that your Mom's Half Brother may also be a Cousin to her.

i know this all seems funny but it is what it is Circles in a Family Tree
Thanks, so much for your help. I was hoping to use this situation of cousins to help determine who my uncle's father is. Usually, he is .20% or a bit more closely related to these particular cousins.

2 Answers

+3 votes
Hi Allison, I suggest you add the tags DNA and Mentor to your question to bring this to the attention of those with DNA experience.
by David Selman G2G Astronaut (1.3m points)
+4 votes

Hi Allison. Not sure how closely these cousins are related to your mother, but if they are related via the parent that your Mom and her half-brother share, then this is not uncommon at all. Your Mom and her half brother each got half of their respective DNA from their shared parent, but that parent passed along different random segments to each of them. So roughly 25% of your Mom's DNA came of each of her four grandparents - same for her half-brother, but he would only share two of her four grandparents. It just appears that her half brother and these cousins happened to inherit more common segments than your mother and the cousins. And you have a similar percentage shared with the cousins because you happened to inherit all (or nearly all) of the common segments from your Mom. 

Hope this makes sense -- or if I have misunderstood your qusetion, let me know.


by Janis Tomko G2G6 Mach 1 (18.6k points)
Thanks, that does help.

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