DNA-based Ethnicity Between Siblings

I bought an Ancestry DNA kit awhile ago for one of my siblings and the results came in just before Christmas 2020. I had another sibling tested and those results came back more recently. 

Each child in a family gets different DNA from their parents. Although we receive 50 percent of our DNA from our mother and 50 percent from our father, each child does not receive the same 50 percent. For example, in the 50 percent DNA that I inherited from my mother, there could be DNA from one of her grandparents and no DNA from another of her grandparents. But one of my siblings could have DNA from a great-grandparent that I don't have. Or we might all have DNA from all our great-grandparents but in different amounts. It's a random trickle-down effect.

Statistically, full siblings share on average 50 percent of their DNA with each other (other than identical twins who share 100 percent). But because of the way algorithms work, it is possible that full siblings can show as sharing between 40 and 60 percent. And in fact, it is theoretically possible for full siblings to show that they share little to no DNA (but unlikely). One of the reasons for this is that some double segments are often not counted in the overall number. Different DNA testing companies use different algorithms. Uploading the identical sets of DNA to a different database may provide a different estimated percentage.

Matching centimorgans is considered to be a more accurate indication of relationship. Full siblings typically share between 2200 and 3400 centimorgans. I shared 2600 centimorgans (which showed as 37 percent) with one sibling and 2900 (which showed as 39 percent) with another. 

This random combination of DNA is the reason that all the children in one family don’t have the identical characteristics. Have you ever noticed how one child might look more like a grandparent than her siblings do? And one child may inherit musical talent from generations past that other children in the same family don't receive. But another child may have inherited other talents or abilities that his siblings don't have. 

Even with that randomness, typically, however, the ethnicity between siblings is very similar if not identical. Ethnicity is a larger more general interpretation of the DNA showing how your DNA compares to the DNA of people in various regions around the world. 

According to AncestryDNA, my sibling and I are quite different (and yes, we are full siblings in case anyone was wondering). The image below shows my DNA ethnicity estimates on the left and a sibling's on the right. 

As you can see, I have 26 percent Scottish, but my sibling has only eight percent Scottish. My sibling has 20 percent Irish, but I have only 12 percent (although we both have DNA traced to a tiny location in County Mayo called Ballycastle). My sibling shows 31 percent Germanic, while I have only nine percent. 

Even that 40 percent for England/Northwestern Europe that we share is different when you drill down into it. My sibling's map encompasses all of the Netherlands, where as my map covers only the southern tip of Holland. 

I have mentioned this in previous posts, but just as a reminder, these ethnicity percentages are estimates based on other people in the database. The larger the database, the more accurate the ethnicity estimates are — and these estimates change as the database grows. 

Because these are statistical estimates, it’s possible that I was just slightly less or more on one segment than my sibling, but the algorithm rounded up or down, which changed the estimate a lot. It is also possible that we inherited vastly different DNA segments from our parents. Again, have you ever known two children in a family who were so totally different in both looks and other characteristics that you have trouble believing they are full siblings? One is super tall, while the other very short. One has blond hair and blue eyes while the other has dark hair and brown eyes. Maybe the randomness of the DNA inheritance just happened to put them on opposite ends of the spectrum. 

I suspect Ancestry will be updating these ethnicity estimates over the next few months since apparently many people did DNA in 2020 or received a kit for Christmas. Will I become less Scottish and more Germanic and Irish? Will my sibling become less Irish and Germanic and more Scottish?