Q. My mom and I have both tested with AncestryDNA ancestrydna.com. Should she be a shared match for all my matches from her side of the family, because I received all that DNA from her? I have hundreds of matches, and she and I share only 12 of them. I thought things would be more equal between the two sides of my family.
A. Because of the way autosomal DNA is inherited—you get half of your DNA from your mom and half from your dad—every person on your match list should match one of your parents if your parents have been tested. But there are a few reasons why your match page might not look like you’d expect:
If you’re using the Shared Matches tool, which displays matches who share DNA with you and another person (in this case, your mom), remember that AncestryDNA shows you only shared matches who are related to both you and your mom as a fourth cousin or closer. So all those more-distant cousins won’t show up on that shared matches list.
If your dad’s side of the family is significantly larger than your mom’s, or for some reason a lot of people from his side are testing their DNA, you’ll probably see more matches from that side. If possible, test your dad or a close relative of his to shed more light on the close cousin matches you don’t share with mom.
If a branch of your family arrived in the United States relatively recently, their DNA is less likely to appear in a genetic genealogy database. Currently, due to database demographics—mostly Americans descended from European immigrants—test-takers whose families are recent immigrants or have roots outside Europe will have fewer matches than test-takers whose families have been in the United States for generations. This will change as the diversity of DNA databases grows.
Every so often, “ghost matches” crop up as distant cousins, causing you to have matches that don’t seem to come from either parent. But these matches are a result of DNA rearranging, instead of shared ancestors.
You actually have two copies of each bit of DNA, one from Mom and one from Dad. But when testing companies look at your DNA, all they see initially is a big pile of values. They don’t know which bits go together. The process of figuring that all out is called phasing.
This illustration shows a simplified example of how a phasing error could happen. Each word is like a bit of your DNA that’s being analyzed. In section A, your mom and dad each have a sentence that makes sense. You carry around these two sentences in your DNA. But when the testing company analyzes your data at each of the colored locations, the words have the chance to be mixed up, as you can see in section B.
Some mixups wouldn’t make all that much difference: Switching the words favorite and best doesn’t significantly impact the meaning of either sentence. But what if we switched some other words, as in section C? The new sentence makes a big difference, especially to the mom. DNA testing companies do have double-checks in place to help prevent this situation, but mistakes occasionally happen. These errors don’t appear in close relationships, though.
When something doesn’t make sense like this, you can always ask for help in the Ancestry Support Center. Click Support Center at the bottom of the AncestryDNA site, then DNA.
From the May/June 2017 Family Tree Magazine