This has consequences in terms of how much DNA siblings share . And even more significantly, what DNA they share.

DNA isn’t passed down from generation to generation in a single block. Not every child gets the same 50% of mom’s DNA and 50% of dad’s DNA. (Unless of course they are identical twins).

So yes, it is definitely possible for two siblings to get pretty different ancestry results from a DNA test. Even when they share the same parents.

But because of how DNA is passed on, it is possible for two siblings to have some big differences in their ancestry at the DNA level. Culturally they may each say they are “1/8th Cherokee” but at the DNA level, one may have no Cherokee DNA at all.

It seems like brothers and sisters should have the same ancestry background. After all, they both got half their DNA from mom and half from dad.

One way to think about this is to imagine DNA as a bunch of colored beads. Since we are interested in ancestry here, we will say that different colors mean different ancestries.

Imagine that a man from Japan marries a woman from Europe. Her DNA happens to be 100% European and his 100% Asian.

Let’s say that the European beads are red and the Asian beads are blue. Here is what this might look like:

When these two parents have a child, that child will get a random half of mom’s beads and random half of dad’s. The child might look like this:

This child is 50% European and 50% Asian. Here is what this child’s sibling might look like:

In this case you can see that each child actually shares the same ancestry even though they got some different DNA from each parent. They are each 50% European and 50% Asian.

This is the result if the parents happen to be 100% of an ethnicity. It is a different story if the parents are not so red and blue.

Lots of Colors, Lots of Possibilities

Let’s now imagine a couple of more complex situations. First we will add in some green beads from sub-Saharan Africa. Imagine these parents:

The difference here is that mom has a small bit of African ancestry in her family tree. This is actually pretty common in the U.S.

Otherwise everything else is the same. Red is European, blue is Asian.

Imagine this is their first child:

As you can see, by chance this child didn’t happen to get any green beads. A DNA test would say the child has 0% African DNA.

Now imagine a second child happens to get this arrangement:

As you can see, by chance the second child inherited both green beads from mom. In an ancestry DNA test, child 1 would be 0% African while child 2 would be around 4%. Even though they have the same parents.

It all has to do with which DNA you happen to get from each parent. It might have ended up that each child got one green bead each but it doesn’t have to be that way.

Now imagine the newer tests that split European, Asian, Pacific Islander, African, Native American and so on into many subcategories. Now even the first mom’s all red becomes a variety of different colors. And these can get passed down differently leading to different percentages.

Mom might be 23% Northern European, 46% Eastern European, and 31% Southern European. And these can be further subdivided. Maybe the Northern European is really 15% British, 5% Scandinavian and 3% German/French.

Here is what mom looks like now:

Now she has children with her Japanese husband. Here is what the children’s DNA might look like:

Now it is getting very confusing. By chance, child 1 has no British, French/German or Scandinavian DNA while child 2 does. Even with the same parents!

Now split the Southern and Eastern European DNA into subgroups and see what happens. Then start splitting dad’s DNA further to add more to the mix. Do all of this and you can see how siblings might wind up with very different ancestry results indeed.