Genetic tests are not perfect. Especially when dad is a chimera--a mix of two twins.
Paternity tests are powerful but not perfect. They can sometimes give results that make a man look like he isn’t the father of the child even when he is.
For the past six months I have been working with a Washington couple trying to solve this question. They had used a fertility clinic and found through DNA testing that the child was unrelated to the dad. Since I write a column called, “Ask a Geneticist”, they wrote me to get some clarification on the test results.
Normally this would be chalked up to a mix up at the clinic but that didn’t really seem possible in this case. This is why I had the man take a second, more powerful test from 23andMe. The results showed that he was probably the child’s uncle.
This didn’t really make sense either. Which led to the idea that he might be a chimera.
Chimeras start out as fraternal twins. One egg is fertilized by one sperm and a second egg is fertilized by a second sperm.
But instead of growing separately into twins, the two embryos fuse together to form a chimera—a single person with two sets of DNA. Some of his cells have DNA from one twin and the rest have DNA from the other twin.
This would explain why he looked like an uncle in the test—his twin brother provided the DNA for his child. Great idea but we needed more than his DNA looking like the child’s uncle.
This is where genetic counselor Kayla Sheets of Vibrant Gene Consulting and Dr. Michael Baird of DNA Diagnostics Center come in. Together we can now show that the father was indeed a chimera. All we had to do was look at the DNA in his sperm cells.
Two Sets of DNA, One Man
While companies like 23andMe and AncestryDNA offer powerful relationship testing as part of their services, they only look at the DNA in your spit. This is why we turned to the DNA Diagnostic Center to look at the DNA in this man’s sperm.
When we took a close look, we could see two sets of DNA there. Around 90% of the sperm cells had DNA that matched the DNA in his cheek cells. We’ll call this Twin 1.
The other 10% of his sperm had DNA that was clearly related to the first but different. This was Twin 2’s DNA.
Comparing the child’s DNA to Twin 2’s DNA showed a father/child relationship. So mom’s egg was fertilized by a sperm that had the DNA of Twin 2. In the end, the man was both father and uncle to the child!
To make things even more interesting, the couple also has another child who matches Twin 1. So one child was fathered by Twin 1 and the other by Twin 2. Except that Twins 1 and 2 are the same man. Is genetics cool or what?
He Is Not Alone
There are more chimeras roaming the streets than you might think.
For example, anyone who has had a bone marrow transplant is a chimera. Their blood cells have the DNA of the donor while the rest of their cells have their original DNA. Except it isn’t as simple as that.
Sometimes when a company does a genetic test using cheek cells, they can see the donor DNA there too. In other words, the DNA from this man-made chimera can influence genetic tests too.
As stem cell treatments other than bone marrow transplants become more commonplace, more and more of us will be chimeras. And sometimes this may affect a genetic test.
Luckily reputable DNA testing companies ask if you’ve had a bone marrow transplant before doing any testing. But that doesn’t help the “natural” chimeras out there like our dad/uncle.
There was no procedure that could be reported that caused it. He was just born that way!
We still don’t have a good idea about how many more people like him are out there. There have been around 100 or so reported cases but no one knows how many there really are.
All of these 100 have been found by chance—scientists just stumbled on them. There may be many, many more out there. We simply don’t know.
What we do know is that many pregnancies start out as twins but end up ultimately as singletons. Some people estimate that these “vanishing twin” pregnancies may make up as many as 1 in 8 pregnancies. If even a fraction result in chimeras, there are a whole lot more of them out there!
And they could be getting wrong DNA test results. Only with lots of DNA testing of lots of different parts of lots of people will we get a better idea about just how rare this kind of human chimerism really is.
Imagine the results Maury Povich might get if he used a genetic test on his show that could easily identify a human chimera.
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