Not sharing all the same DNA
A reader of The Legal Genealogist was a bit confused on finding an unexpected DNA result.
No, not that kind of unexpected result.
The kind of unexpected result that just needs a bit of explaining.
Because it’s the kind of unexpected result that really should be expected — and welcomed.
Reader Angela is, as expected, a DNA match to both of her father’s sisters. But she didn’t expect to find that she shares more autosomal DNA with one than with the other — and she wonders how that can be.
Great question, and one every new DNA tester is likely to have.
So let’s go over this.
First off, the kind of DNA test we’re talking about here is autosomal DNA testing — the kind we do with Ancestry DNA, 23andMe, MyHeritage DNA or the Family Finder test from Family Tree DNA. It looks at the kind of DNA we all inherit equally from both of our parents — 50% from our fathers and 50% from our mothers — so both males and females can take this kind of test. It lets us find close relatives to distant cousins that we can work with collaboratively to expand our knowledge of our family trees and — sigh — yes, this is the kind that gives us those percentages we all like to look at to see whether we’re Vikings or not.1
And just as we inherit 50% of our DNA from each of our parents, each of our parents inherited 50% from our grandparents, and our grandparents from our great grandparents, and so on back through time.2
And each time that 50% gets passed on, it goes through a process of getting jumbled and mixed up called recombination. That process happens with each child separately, so even full blood siblings — brothers and sisters — won’t get exactly the same pieces from their parents. Which means that we won’t share all of the same DNA with a sibling: on average, full siblings share about half of their autosomal DNA. And which parts we do share will be different from sibling to sibling. And since Mom and Dad resulted from the same sort of random mixing of their parents’ DNA, they will also have inherited different bits and pieces from their parents, our grandparents, than their brothers and sisters, our aunts and uncles.3
So, just as an example, I’ve tested at Family Tree DNA along with four of my full siblings. The amount of autosomal DNA I share with those four varies from a high of 3,075 centiMorgans (cM) with one younger brother to a low of 2,760 cM with another.
And in some areas of my chromosomes, I match with anywhere from all to none of them, as you can see in this screen capture of just the first six chromosomes (grey is no match, each color shows a different sibling matching to me):
The same is true among the five aunts and uncles — siblings of my mother — who have tested at Family Tree DNA: I share anywhere from 1809 to 1965 cM with them, and they share different amounts with each other. The smallest match between any two of those five siblings is 2475 cM, and the largest is 3016 cM:
| D.C. | C.C. | J.C. | M.C. | P.C. | |
| D.C. | self | 2977 cM | 2690 cM | 2869 cM | 2475 cM |
| C.C. | 2977 cM | self | 2582 cM | 2708 cM | 2595 cM |
| J.C. | 2690 cM | 2582 cM | self | 2634 cM | 2761 cM |
| M.C. | 2869 cM | 2708 cM | 2634 cM | self | 3016 cM |
| P.C. | 2475 cM | 2595 cM | 2761 cM | 3016 cM | self |
Quite a variety, isn’t it?
Absolutely to be expected, because of recombination.
And that’s a good thing!
Why? Because that one bit of DNA that one sibling has and another one doesn’t is just the kind of cousin bait we need when we cast our DNA out there on the match waters, hoping to reel in the one connection we need to help us with our research.
One of my mother’s siblings shares more DNA in common with folks from their mother’s, my maternal grandmother’s Robertson and Gentry lines than the other siblings do. We’d have missed some of those key matches if he hadn’t tested. And the other siblings share more DNA in common with folks from their father’s, my maternal grandfather’s Baker and Johnson lines. We’d have missed some of those key matches if each of those others hadn’t tested.
So those differences in how much DNA we share with our close relatives aren’t unexpected at all.
And they’re something to be celebrated.
Yep.
Vive la difference!
Cite/link to this post: Judy G. Russell, “Vive la difference!,” The Legal Genealogist (https://www.legalgenealogist.com/blog : posted 13 Mar 2022).
SOURCES
- See generally “Introduction to Molecular Genealogy: Autosomal DNA,” Learn.Genetics, University of Utah Genetic Science Learning Center (https://learn.genetics.utah.edu/ : accessed 13 Mar 2022). And see ISOGG Wiki (https://www.isogg.org/wiki), “Autosomal DNA statistics,” rev. 27 Feb 2022. ↩
- Ibid. ↩
- See ISOGG Wiki (https://www.isogg.org/wiki), “Recombination,” rev. 27 Oct 2021. ↩
Our DNA is the most remarkable thing. On the one hand it has to be extraordinarily precise to maintain all the genes necessary to make us human beings. On the other hand it also allows for infinite variation so that each of us is unique. Boggles the mind!
Hi Judy… I have a fun DNA story to tell. I have an identical twin sister and we come from a large family (ten children). Neither my sister nor I could carry a pregnancy full term, and so we both adopted our children. As DNA tests became more available, our interest in them grew. (Through these tests, two of our adopted children have found their birth families.) One day, my sister’s daughter teased us and said she wanted proof that we were identical twins. (We do look a great deal alike, but not exactly alike.) So she bought us each a test and we sent them in. When the results came back, we all had a great laugh. Were our results exactly the same? Actually, no. Though they were very close. We kind of looked at each other and raised our eyebrows. We wondered how that could be. (Evidently, there also needs to be taken into account variations that are a part of the testing process itself.) However, the truth became clear when we looked at the relationships page. Yes, it told me that my sister and I were related, but the way the association was described said it all. Under her name read, “This is either you, or you have an identical twin!” So, there it was! All doubt was banished… and the fun of fooling our kids when they come up behind us and think we’re their mom, only to turn around and surprise them when they realize we are their aunt (or “other mom!”) lives on!
That is a fun story! Thanks for sharing it!