Inheritance of DNA Segments

DNA is randomly inherited. As a result, a match that shares 100 cM DNA with a parent will likely NOT share exactly 50 cM with the parent’s child; rather, there are a range of possibilities (100 cM, 50 cM, 0 cM, and everything in between, for example). On average it will be about 50%, but there is lots of room for variation.

Prompted by a great question in the Genetic Genealogy Tips & Techniques Facebook group, I used the “People who match one or both of 2 kits” tool at GEDmatch to look at the random inheritance pattern of DNA between my father and myself with regard to matches sharing about 35 cM (the examples here worked out great, but you can pick any size).

We can see the randomness of inheritance in this table. And we see a surprise (that I just discovered today with this exercise!) that reminds of the fact that matching DNA can come from BOTH parents!

And if you’re interested in even more detail, here’s how my siblings inherited DNA from these same matches.

7 Responses

  1. Cindy Norton 5 October 2017 / 9:34 am

    I have a brother who is homeless. He has been a crack cocaine addict for years. Would his DNA test be altered because of his drug abuse?

    • Blaine T. Bettinger 5 October 2017 / 12:04 pm

      Hi Cindy. I’m so sorry to hear about your brother. It is unlikely that the DNA collected by these tests is going to be affected by drug abuse, but I don’t know if anyone has done any such study.

  2. Dana Prather 5 October 2017 / 12:00 pm

    That is really interesting! I always thought that every child would have a lot more that just 50% from a parent. I have been looking for my father’s birth parents and decided to do DNA testing from 3 different places. I have a half brother, and I just knew that I could find my father’s birth father, but I think the DNA results have really confused me. A lot of what I see in his DNA are his mothers side and the birth grandmothers side. I thought it was suppose to give me the male side. Why am I finding the maternal side?

    • Blaine T. Bettinger 5 October 2017 / 12:05 pm

      An autosomal DNA test will share information about both sides of a family. However, if one side has tested more, or if that side comes from a region of the world that is more likely to test, you’ll see more matches to that side.

  3. Amanda 3 January 2018 / 1:39 am

    I see this wide variance with my Grandma, my mom, and her 3 half siblings a lot with dna matches from my grandma’s side. It’s really neat to see how the segments are passed down differently to each sibling.

  4. Lauren Robinson 11 August 2019 / 4:57 pm

    My husband a DNA match with someone that has 251cM’s across 14 segments. Are we looking in the area of a second cousin?

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