One of my favorite DNA-related topics is siblings. Why test them?


It’s true, if you have both parents tested, you don’t need to test your siblings, or even yourself, if your goal is to find relatives who may help you develop your family tree. But if you aren’t able to test both parents, siblings do have something to offer.

The goal is to connect with someone whose DNA matches yours. Let’s suppose there is someone out there named Abby Rhodes who kept a diary while she was on the Oregon Trail. Her descendant John now has that diary, with lots of genealogical info in it, and he’s taken an autosomal DNA test. (That’s the AncestryDNA test or the 23andMe test, or the FamilyFinder test at FamilyTreeDNA.) If John is my 3rd cousin, there’s a 90% chance he’ll be on my match list. There’s a 90% chance he’ll be on my sister’s match list too. But if he’s our 4th cousin, there’s only about a 50% chance he will match me. There’s a 50% chance he’ll match my sister. He might match both of us, or neither of us, or just one of us. If he matches my sister Margaret and not me, then if I only test me, I’ll never connect with him and his family treasures.

Suppose Abby Rhodes is the great-grandmother of my dad’s mother (Nana). The DNA that Dad got from Abby, which John got too, is on a segment of chromosome 6. The chart here shows that Dad has one copy of chromosome 6 from Granddad and another from Nana. Mom has a copy of chromosome 6 from Grandpa and another copy from Grandma. Dad’s two copies got recombined and he passed one of this blended version to me for my paternal copy of chromosome 6. Mom’s two copies did the same, and I got my maternal copy of the blended chromosome 6 from her. The same process occurred when Margaret came along, but the chromosomes recombined in a different way.


You can see that Margaret got a lot more DNA from Nana (light blue) on chromosome 6 than I did. It’s random; if it were exact, then 25% of my DNA would be from each of my four grandparents, across all the autosomal chromosomes. But it’s not exact; I could have 33% from Granddad and just 17% from Nana.

If the segment of DNA on chromosome 6 that John got from Abby Rhodes is in the middle of the chromosome, then he would match Margaret and would not match me. But if Margaret takes a DNA test too, then maybe we’ll get to see a copy of that (hypothetical) Oregon Trail diary!

In March, I’ll follow up on this example with an Irish-themed discussion on testing siblings!


For more information on autosomal statistics, see International Society of Genetic Genealogy, ISOGG Wiki, “Autosomal DNA Statistics,” ( : accessed 21 Feb 2016).

For the tool used to ‘map’ the chromsomes visually, see Kitty Cooper, Kitty Cooper’s Blog, “Ancestor Chromosome Mapper,” ( : accessed 21 Feb 2016.)


(c) Ann Raymont 2016

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