Negative evidence: autosomal DNA

Last time I posted about negative findings and negative evidence, in traditional document-based genealogy and in genetic genealogy. Link here.

Now for the fun stuff – a cool use of negative evidence in *autosomal* DNA. For this exercise, the research question is: from which grandparent did the DNA segment in question originate?

 

 

Let me introduce you to my four grandparents. Dad’s parents were Granddad and Nana; Mom’s were Grandpa and Grandma.

Last year I illustrated how DNA from those 4 grandparents might be passed down to two children on chromosome 6, as seen in Image 1.

Image 1.

chart

This time, we’re looking at chromosome 11.

Dad has two copies of chromosome 11: one from Granddad and one from Nana.  These recombined to create one copy of chromosome 11 (potentially with large segments from each of his parents), which I got as my paternal copy of chromosome 11.

Ditto for Mom. She has two copies of chromosome 11: one from Grandpa and one from Grandma.  These recombined to create one copy of chromosome 11, which I got as my maternal copy of chromosome 11.

Mom and Dad have five kids: Brian, Ann, Margaret, Sheila, and Alice. We each got different combinations of their DNA.

See image 2 for a map of the paternal chromosome 11 that Brian, Ann, and Alice got. It shows a 41 cM segment where they all match distant cousin Paul. Margaret and Sheila do not match Paul on that segment.

Image 2.

chrom 11

Traditional genealogy and shared DNA matches have confirmed that this DNA segment came to the three siblings from their dad. Dad got it from Granddad, who got it from his dad John Harrigan (1868-1922), who got it from his mom Barbara Conneely (1828-1923). Barbara is the sister of Cousin Paul’s ancestor.

Remember, Dad had two copies of chromosome 11, and each of them included a segment at this location, from start position 104,374,441 to end position 130,920, 477. Let’s call it 104-131. DNA in 104-131 on his paternal chromosome 11 came from his dad Granddad, which we’ve tracked back to Barbara Conneely in Ireland. DNA in 104-131 on Dad’s maternal chromosome 11 came from his mother, our Nana. (We don’t know the origin of this segment further back than that yet.)   

When Dad’s two copies of chromosome 11 recombined to create one copy to pass down to each child, this segment 104-131 came from Granddad on the copy created for Brian, Ann, and Alice.  Margaret and Sheila didn’t get this segment (since they don’t match Paul here), so the absence of a match tells us something useful: they must have DNA from Dad’s *maternal* chromosome 11 on this segment 104-131, i.e. from Nana.

And thus, we have a kind of negative evidence. Suppose Sheila matches a new cousin’s DNA at this segment location, and we have determined from shared matches (say, a paternal first cousin) that the new match is related on Sheila’s paternal side. If only Sheila of the siblings had tested, we wouldn’t know which grandparent it came from. But her siblings have tested and their paternal copy of this segment on chromosome 11 is known to come from Granddad. Since Sheila doesn’t match them, we know that her paternal DNA in that location came from Nana. She and her new cousin have a common ancestor in Nana’s line.

It’s a great tool to help you narrow down which quarter of your family tree to focus on, when trying to find the common ancestor with a DNA match.

To take advantage of this strategy, you need the following:

  • a chromosome browser that tells you where the matching segment occurs. Most DNA test companies offer this. AncestryDNA does not, but those users can copy their AncestryDNA raw data to the free tools at GEDmatch.com to get that information.
  • DNA test results from an aunt, uncle, or first cousin on your maternal side and paternal side, if possible. You can do this with less close kin. For example, distant cousin Paul also matches Dad’s paternal first cousin, although on different chromosomes. The important thing is, you want to be sure you know whether your new match is a maternal match or paternal match before you start trying to decide which grandparent the DNA came from.
  • Finally, you need one or more siblings with DNA test results! As much as possible, identify which grandparent passed them which DNA segments, as shown with Cousin Paul, above. Then when a new distant cousin matches one sibling but not another, if you’ve mapped that segment to a grandparent, you’ve narrowed down your possible shared ancestral lines with the new match! \o/

Ann Raymont © 2017

 

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