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Chimeras, Mosaics, and Other Fun Stuff

How is a paternity test affected if the child is a chimera?

-A curious adult from Indiana

April 23, 2009

With the right testing company, it shouldn't affect the results at all. The company should be able to tell that the child is a chimera and whether or not the child came from the two parents being tested. As long as the twins that make up the chimera had the same dad that is.

Chimeras are Fused Fraternal Twins

Chimeras start out as fraternal twins. At some point very early on in development, the two embryos fuse together to create a single embryo.

So chimeras have two sets of DNA. Some of their cells have the DNA from one twin and the rest of their cells have DNA from the other twin.

Since chimeras are fused twins, the two sets of DNA are related like any other siblings' DNA. Just like a paternity test can tell two brothers have the same parents, it can tell that a chimera has the same parents too.

The one tricky part is that the DNA testing company has to be careful when reading the results. A DNA sample from the chimera would look like two siblings' DNA samples mixed together. The company has to be aware of this and, if they get a "weird" result, have to interpret it correctly.

To give some idea of what the DNA testing companies are up against with DNA samples from chimeras, let's go over how a paternity test works. Then we can compare the results of a typical paternity test with one where the child is a chimera.

Paternity Tests Compare Only Some DNA

In a paternity test, a DNA company compares the DNA of a child to adults who may be the parents. The company cannot, however, look at all of everyone's DNA. This is too time consuming and way too expensive.

So what the companies do is compare only certain areas of DNA. The companies focus on particular regions that are more likely to be different between any two unrelated people. This is important when even unrelated people share 99.5% of their DNA.

These regions are places where a certain bit of DNA is repeated. Different people can have a different number of repeats at these positions. So one person might have 9 repeats while another might have 8 or 11 or whatever.

People Have Two Copies of Most of Their DNA

We need one more bit of information to be able to interpret the results of a paternity test with a child who is a chimera. That piece of information is that people have two copies of most of their DNA. One copy comes from their mom and the other comes from their dad.

What this means is that when a company looks at a piece of a person's DNA, they are actually looking at two pieces of DNA. And the results for each piece may be different.

Let's look at a specific DNA spot to make this clearer. We'll use a real one that DNA companies actually look at called D3S1358.

Imagine a DNA test comes back with the result D3S1358, 17/18. What this means is that this person got a D3S1358 from one parent that had 17 repeats and a D3S1358 from the other parent that had 18 repeats.

And when this person has a child, he or she will pass on either a 17 or an 18. Unless that person has a child who is a chimera in which case the child could end up with both.

Sample Paternity Test

Now we're ready to see how a paternity test might go if the child was a chimera. To make things simpler, I'll give a concrete example and focus on a single DNA site.

Let's say that mom has a 17 and an 18 at D3S1358 and dad has a 14 and a 15 at the same spot. This means that any child of theirs has four different possibilities for this piece of DNA (click here for exceptions):

17, 14
17, 15
18, 14
18, 15

These are the possibilities because one repeat has to come from mom and the other repeat has to come from dad. A quick way to figure this out to is to set up a small table called a Punnett square. What we do is put mom's possibilities along the top and dad's along the side. Then we fill in the boxes like this:

17 18
14 17,14 18,14
15 17,15 18,15

Now if the child is a chimera, then that child is going to get two repeats from each parent. Sometimes the chimera might get the same two from a parent and sometimes the chimera might get one of each. This means there are many more possibilities"¦I think this is all of them:

17, 14
17, 15
17, 18, 15
17, 18, 14
17, 18, 15, 14
18, 14
18, 15
18, 14, 15

Notice that all of these possibilities are shared with these two parents.

If the DNA testing company has experience with this sort of thing, they'll be able to recognize that the sample is either contaminated or comes from some sort of chimera. One company I spoke with said they had had two chimeras so far and were able to tell the paternity.

Same Mom, Different Dads

The one case where things could get very complicated is if the fused twins have different dads. This is called superfecundation.

Basically in superfecundation, one egg is fertilized by one man's sperm and a second egg is fertilized by another man's sperm. If these twins fused, you would have very complicated DNA test results because the chimera would essentially be a mix of three people's DNA.

Luckily this wouldn't be that common. Superfecundation is pretty rare in people and so are chimeras. So the odds of two rare events happening at the same time are, well, really rare. So rare that odds are we'll never see one but not so rare as to be impossible.


Chimeras happen when twins
fuse early in development.


We have two copies of most of
our DNA. One copy comes from
mom and the other from dad.