For very little money, you can now look at a million different spots on your DNA instead of the 20 or maybe 30 that is looked at in a conventional test. And as you’ll see below, this is enough to tell whether a man and woman are brother/sister or son/mother.

Of course the obvious answer is to look at more DNA. But until recently, that was way too expensive. This is no longer true.

The reason a paternity test can’t tell which relationship is right is that it looks at too little DNA. When you look at that little of the DNA, a test can’t tell the 50% of a mother/son relationship vs. the 50% of a brother/sister. In fact, it may have trouble telling that the two are brother/sister even if they are!

You have heard right. If you just test the two people, then a run of the mill paternity test would indeed have trouble telling a mother and son from a brother and sister. Testing more relatives would help but still might not be enough.

Two Halves Aren’t Always Equal

What I thought I would do is cut right to the chase and show you what the results would look like for a son versus a brother with one of these more extensive tests. These are actual results from a company called 23andMe.

One more thing. This is comparing a woman to her two brothers and her father, but the exact same ideas hold for a man versus sisters and a mother. For ease we’ll just ignore the X and the Y.

Now it is pretty obvious that this test can tell the difference between a parent/child relationship and a sibling one. The parent is all light blue (except where they didn’t get any data) and the brothers are a mish mash of white, light blue and black. So if someone has this test done, it will be pretty obvious if their DNA is being compared to a parent or a sibling.

What I’ll do for the rest of the answer is try to explain why we get these results. As you’ll see, it has to do with the fact that each of us has two sets of chromosomes, one from mom and one from dad. And with something called recombination.

Parents First

OK let’s start out with the easier of the two. Here is the parent/child comparison again:

This is a comparison of the daughter’s DNA to her father. As you can see, the test results are that the two are half identical everywhere they could get reliable data. To understand what that means, let’s take a step back and go over how DNA is passed from parent to child.

We get half our DNA from mom and half from our dad. This DNA is passed down in the form of chromosomes.

Most people have two sets of 23 chromosomes for a total of 46. One set comes from mom and the other from dad.

Here is what that might look like for a mother and father passing their DNA down to their child:

There is a lot going on here but let’s take this one step at a time. First off, we’ll focus on just mom’s contribution.

You may notice that mom doesn’t pass either her blue or green chromosome from each pair. Instead, she passes a chromosome that is a mixture of the two. This process, called recombination, means that the child has a brand new, never before seen chromosome. This will be important for the next section.

What we can definitely say is that one of each pair of chromosomes comes from mom and one of each pair comes from dad. So why does the test call this half identical? It has to do with a limitation of the test itself.

See, the test can see big swaths of DNA but it can’t tell which parts are on which chromosomes. What this means is that it can’t tell the difference between any of these:

In the test, all five of these kids would be termed half identical. Each one has a chromosome that completely matches a part of one of mom’s chromosomes. The other chromosome (the colorless one from dad) doesn’t match either of mom’s chromosomes. And so the child is half identical with mom at this chromosome.

This will be true of all of the child’s chromosome pairs. All of them will be half identical with mom.

OK now let’s tackle siblings. As you’ll see, that recombination we talked about earlier is really important for this part.