Select Page

Why we test everyone we can

It’s the phrase every single child, ever, hates to hear.

The Legal Genealogist sure knows it, and heard it often enough.

You know the one.

“Why can’t you be more like (fill in the blank with the name of the particular sibling or cousin who was the role model of the day)?”

So here’s my answer.

Because, dear parent, you didn’t give me those genes.

You, or your sibling, had those genes from your parents, and you didn’t pass them on to me.

We see that all the time when we work with autosomal DNA — the kind of DNA inherited from both parents by way of our autosomal chromosomes.1 An autosomal DNA test works across genders and helps us find cousins with whom we share DNA from all parts of our family tree, paternal and maternal alike.2

And autosomal DNA changes — a lot! — with each and every generation because of a process called recombination. That’s a process where all of the pieces of autosomal DNA we inherited from our parents — what’s in each pair of chromosomes we have — get mixed and jumbled before half (and only half) of those pieces get passed on to the next generation.3

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. And since Mom and Dad resulted from the same sort of random mixing of their parents’ DNA, the brothers and sisters each inherited different bits and pieces from their grandparents.4

That’s why we say that siblings share 50% of their DNA, and that they share 50% of their parents’ DNA — but it won’t be the same 50% from sibling to sibling.

Reading that, it’s hard to imagine, isn’t it? Words just don’t paint a clear enough picture.

So let’s look at the picture instead.

Here’s a set of results from my mother’s siblings — five of them to be exact — at Family Tree DNA. The test shown is one of her brothers, David, so everything in the background light grey is his. The areas in color are where he matches his siblings. The darker blue is where he matches my aunt Carol, the red my uncle Mike, the blue-green my uncle Jerry and the orange my aunt Trisha.

recombination in siblings

Boy, does that ever show how this works, doesn’t it? Every area on every line that’s the light grey means those two siblings did not inherit that chunk of DNA together. They got different pieces from their parents.

And if that’s not clear enough, take a look when we get to first cousins. On average, we should share 12.5% of our DNA. But it sure isn’t the same 12.5%.5

Here’s an image of results for me and two of my first cousins — each of them the daughter of one of my mother’s siblings. The grey in the background is where I do not match that cousin — or either of them.

recombination in cousins

Really easy to see where I share with one cousin and not the other, and even easier to see the vast amounts of my DNA that I don’t share with either of them, isn’t it?

Which is why one cousin may end up matching a third cousin, and another — sigh… usually me… — won’t. And why we want to test every single person that we can test, to make sure we’re getting all those matches who are in fact our third cousins.

And of course why I’m not more like that sibling or that cousin.

Because I’m really not, not at the genetic level, because you didn’t give me those genes.

So there.

Cite/link to this post: Judy G. Russell, “Recombining relations,” The Legal Genealogist ( : posted 10 Oct 2021).


  1. See ISOGG Wiki (, “Autosomal DNA,” rev. 21 Oct 2020.
  2. See generally Judy G. Russell, “Autosomal DNA testing,” National Genealogical Society Magazine, October-December 2011, 38-43.
  3. See ISOGG Wiki (, “Recombination,” rev. 14 Apr 2019.
  4. Ibid.
  5. See ISOGG Wiki (, “Autosomal DNA statistics,” rev. 21 May 2021.
Print Friendly, PDF & Email