How does DNA pass on over four generations?

DNA cM Across Four generations


by Andy Lee

Blaine Bettinger has done some great work in gathering data on shared cM as part of his Shared cM project. But how about looking at a single family? How do some of those Shared cM amounts line up?


Before Blaine’s shared cM project, we just had percentages based on DNA halfing each generation. Because of the Shared cM project, we can see from actual data what those DNA amounts look like between relationships.

In my family, I have been able to test 4 generations, my grandparents, parents, myself and siblings, and my children. So how do our family numbers stack up to what the Shared cM project is showing us? I’ll try to keep relationships unconfusing by referring to my father, my children, my grandfather, and my siblings.

Comparing Parent - Child DNA
Comparing Parent - Child DNA

First, when looking at DNA, comparing parent to children is not that interesting. It is always going to be 50% for autosomal DNA because you get one set of chromosomes from your mother and one set from your father. But once you start comparing 3 generations and more it can be interesting.

Brothers cM shared with same grandparent

First off, let’s look at a chart of me and my siblings compared to my Grandfather. The average amount should be around 25% and you can see that my 4 brothers and I are tightly packed around 25 percent, there really isn’t much variability even though the Shared cM project shows a range of about 20-30%,

Grandchildren cM shared with same grandfather


So when we look at my children compared to their grandfather, my dad. It shows a different story. There is a wide range of from as low as 20% to as high as 27.5% which is amazing with 5 samples that it covers most of the range that the Shared cM project found.

So seeing the results of my children could you predict which one shares the least and which shares the most amount with their great-grandfather, a 4 generation spread?

Well, if you are tempted to choose the highest and the lowest, from the 3 generation spread, you’re getting ahead of yourself. The DNA that my children share with my father, is a mixture of my grandfather and grandmother’s DNA. And because I happen to share exactly 25% of my DNA with my grandfather, I also share exactly 25% of DNA with my grandmother.


But here is the results, C3 shares the most with my grandfather, not C1 although there is only a 1cM difference.

On the flip side, C4 shares the least amount with my grandfather, and in fact, the amount of shared DNA is on the very low end of what the Shared cM project has gathered. Its very possible that 2 or 3 generations from now, there may be some descendants of C4 that share no DNA with my grandfather.

Also of note, even though I have exactly 25% of DNA from my grandfather, none of my children shared 12.5% or more of DNA. DNA is random and unpredictable like that.

Brother comparison of DNA to same Aunt


Let’s next look at my father’s sister, my aunt. My siblings and I share around 25% of DNA with her, again in a fairly narrow range.

great niece/nephew comparison of DNA to same great-aunt


My children, on the other hand, share a broader range, from 8.7% to 11.6% of DNA with my aunt, but again nobody shares 12.5% or more DNA.

shared DNA between niece/nephew and uncles


Lastly, I’ll look at my children compared to my siblings. As expected now, we can see that there is a lot of variabilities. S2 stays in a narrow range below 25%. S1 stays in a narrow range centered at 28%. And S3 has both the most shared DNA with any of my children and the least amount of shared DNA with any of my children.

In summary, except for parents and children, shared DNA is not going to be the same for every relationship. The more distant the relationship is, the more variability might see in the amount of shared DNA. If you have any questions, put them in the comments below. And if you enjoyed this episode, hit the thumbs up and share the video with your friends.


This post is part of the Research Over My Shoulder video series. 
To watch this video in full, click on THIS LINK

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