So what's up here? Why did we miss the fact that she is a chimera? Well, there are lots of possibilities. She has her blood and cheek cells tested and gets this result: In our final case, let's imagine Sally is a chimera. Because her skin and blood have different DNA, Jane is almost certainly a chimera. But she did the right thing and had both samples tested.Īgain once we rule out lab error or contamination, the best explanation is chimera. If we tested just one sample, then we would miss that she was a chimera. All of her skin cells came from one twin and all of her blood cells from another. This is why testing multiple tissue samples is so important. Of course, if John's skin is all made up of one twin's DNA and we had only done a cheek swab, then we would miss the fact that he was a chimera. These tests can almost always tell siblings apart and so would detect fused siblings. But it is very unlikely that he would have only two at all 12 or 15 markers tested. So it could have been that John had only two versions of this particular marker. People share some of the same versions of markers, and the more related you are, the more you tend to share. It is important to note that we would not get such clear cut results at every marker tested. Once we've ruled out lab error or contamination, the most likely possibility is chimera. He has four different versions of this marker which raises a red flag immediately. The results of a DNA test on his blood might look like this: Half his blood cells come from one twin and half from another. A normal paternity test would have eleven or fourteen more of these markers but I am just going to focus on a single one. This means that at marker D16S539, you have a 9 from one parent and a 16 from the other. So your results at one marker might look like this: ( Click here to learn more about how they are different.)Įveryone has two sets of these markers, one from your mother and one from your father. These spots have DNA that tends to be different between different people. Before doing this, we need to take a quick step back and go over what a paternity test looks at and what the results look like.Ī typical paternity test looks at 12 or 15 different spots on your DNA. What I thought I'd do is go now is go over a few possible scenarios in more detail. A DNA test might find out one person is a chimera and miss this fact with a second person.Ī DNA test can often figure out if you are a chimera. So if a DNA test comes back that you are not a chimera, you still might be one. Unless you had reason to dig up a bit of liver or kidney to test. But if you had skin and blood from one twin and internal organs from another, a DNA test would still miss it. If you had skin cells from one twin and blood cells from another, then your chimerism might be detected if you tested both types of cells. But if you are a chimera whose blood comes from just one twin, a DNA test on your blood wouldn't be enough. This means that to find out if you are a chimera or not, you have to test the right tissues.įor example, if you are a chimera whose blood is made up of cells from both twins, then a DNA test on just your blood would reveal that fact. Via Wikimedia Commons DNA Testing of ChimerasĪs I said before, each chimera is unique in which cells have one DNA and which have another. And that is what makes interpreting a DNA test so tricky.Ī chimera happens when fraternal twins fuse at a very early stage in development. We can't predict which cells will have which DNA. The same is true for all the different tissues in a chimera's body. Or even a chimera whose blood and skin are both made up of both DNAs. Or one whose blood and skin have the same DNA. You could end up with a chimera whose blood has one DNA and whose skin has another. Which cells have which twins' DNA is totally random. As the embryo grows, some cells become liver, others blood, etc. They have the potential to become anything. Nothing bad happens because the cells at this stage are totally undefined. Two separate eggs fertilized by two separate sperm.īut instead of growing into separate people, the two bunches of cells fuse together and end up as a single person. (Shutterstock) Chimeras RevealedĬhimeras start out as fraternal twins. But not always - chimerism is hard to detect. DNA tests might be able to reveal that someone is a chimera.
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