Don’t get me wrong. Autosomal DNA testing is a very valuable tool. A match has the possibility of breaking through some very significant genealogical brick walls. It’s important to understand what a match means or doesn’t mean.
In a nutshell, we all have 46 chromosomes, 23 from mom and 23 from dad. Two of those chromosomes are the sexy kind, X and Y. We’ll ignore those for now. In an autosomal test, the DNA sequences in your chromosomes are compared against everyone in the testing company’s database. The goal is to find long matching sequences. Depending on how long the sequences are and the total number of matching sequences, a calculation predicts the cousin relationship.
Now here is where things get dicey…
Take two full siblings (not twins). At first glance, you might think that genetically they are a 100% match. Dad gives these two siblings 23 chromosomes each, half of his DNA. It’s not necessarily the same 23 chromosomes. Mom does the same. Let’s look at the two extremes.
Imagine mom’s DNA as two chunks of 23 chromosomes each – A & B. Dad has two chunks also – C & D. Mom gives each child chunk A and dad gives each chunk D. Both children will have A & D and will be exact genetic matches.
What if mom gave one child A and one child B. Then dad gave one child C and one child D. The full siblings would be A & C and B & D, showing no match at all. The truth is that a full sibling match will exist on a continuum somewhere in between.
The probability that a sibling match would be 0% or 100% is extremely low. Cousin matches are a different story. In a perfect world, two 1st cousins could share 25% of their DNA. Two 2nd cousins might have 1/8, 3rd cousins – 1/16, 4th – 1/32 and 5th cousins – 1/64 – a little more than 1% shared DNA. The possibility of two cousins not sharing DNA or not sharing a long enough sequence to make a match gets higher.
In my family, two Scottish brothers married two German cousins. I am the grandson from one of these unions. I have a cousin who is the grandson from the other marriage. We are both 2nd cousins and 3rd cousins. It is possible that we share 1/8 plus 1/16 for a total of 3/16th. That much shared DNA could be reported on a test as being 1st cousins.
The autosomal match game is not a perfect world. If you don’t get a match and you think you should have, then test different cousins. Adding more DNA samples could give a new set of results. If you do get matches, the degree of the relationship can help set a starting point in looking for that common ancestor.
DNA is just one of many tools we have as genealogists. In the case of autosomal testing, DNA is just the beginning. It will take traditional genealogy to get you to the prize.