Genetic tests can be wrong if one of the tested people is a human chimera.
Imagine a welfare mother denied custody of her children because her DNA points to her not being their mom. Or another mother who cannot get a kidney transplant from her son because they do not share enough DNA.
Strange stories like these are becoming increasingly common in this new age of DNA testing. In both cases, the moms really were the moms. They each are just made up of two different sets of cells with different DNA—they are chimeras.
How Chimeras Happen
In Greek mythology, a chimera was a beast made of a lion, a goat and a dragon. Human chimeras aren’t as drastic as this but they actually exist.
A human chimera is made up of two sets of cells with DNA as different as any two siblings’ DNA. This is because a chimera is the result of fraternal twins fusing together at a very early stage in development.
This can happen because early on, embryos are made up of embryonic stem (ES) cells. ES cells have not yet committed to being any specific kind of cell so having a few extra or missing cells is well tolerated. At this stage, it is OK to remove a cell for genetic testing (called preimplantation genetic diagnosis or PGD). Or to combine two sets of ES cells to create a chimera.
Combining two sets of ES cells with different DNA has no consequences for the new chimera. Unless they are of different sexes in which case the chimera may end up a hermaphrodite. Or unless the chimera is getting a DNA test done.
Getting the Wrong DNA Results
A DNA test is usually done by taking some blood or cheek cells and looking at the DNA. This is fine if the person has the same DNA in every cell (which is usually the case). But a chimera can run into problems.
Because different body cells will have different DNA, it is possible that a DNA test can give incorrect results for a chimera. Imagine that a woman has one set of DNA in her cheek or kidney cells and a different set in her eggs.
Now she has a DNA test done on her children and one done on her kidney or cheek cells. When the results are compared, she looks like the children’s aunt instead of their mother. This is what happened in the custody and kidney transplant cases.
The children looked like nephews or nieces with one set of cells. And like sons and daughters with the other set of cells.
If chimerism is more common then most scientists think, then they’ll need to be more careful in how they interpret DNA tests. As more and more genetic tests are done, scientists will be able to figure out how rare chimerism really is.
