Experiments in Life

Hang on tight, you’re in for a wild ride on the biology rollercoaster

Okay. Here’s the deal with the whole I am my own twin thing.

Basically, what happened was the pregnancy started out as a pair of fraternal twins. About 4 days into the pregnancy, the two eggs fused, making one embryo with two different sets of DNA. This results in a condition known as chimerism. (Yes, there was an episode of CSI in which this condition came up and threw everyone for a loop or two!) This condition is pretty easy to spot when the two eggs are a different sex. Usually, the resulting child is a hermaphrodite. There can also be some patterns of different skin pigmentations that can be characteristic of chimerism. However, in the cases we saw referenced, the chimeric individuals were both women, meaning that both the eggs were female. They did not display some of the usual characteristics of chimerism so the condition was not considered until much later.

In one instance, the woman needed a kidney transplant and they were testing her children for matches. They found that two out of her three sons did not match her genetically. In fact, once they started running DNA tests on the extended family, they found that the woman’s husband and brother (genetically speaking) were a perfect match for her two sons. Isn’t that bizarro!!

Sadly, the other case did not go so well. In this poor woman’s case, she was applying for welfare and they needed to do a paternity tests on the children. Well, they tested her DNA against the children and they found that her DNA was different than her children’s. Well, her children were very young and everyone jumped to conclusions. They figured she had kidnapped the children and was trying to commit welfare fraud as well. They took her kids away and she was stuck trying to explain to the DA that she was their biological mother. No one believed her. Luckily, she was pregnant with her 3rd child at the time. And luckily, this child did not match her genetically either. She got her kids back and she was diagnosed (not that it’s a disease mind you) with chimerism.

In the case of both women, the two sets of DNA were dominant in different tissues. By blood tests alone, it was not obvious that two sets of DNA were present. Only by taking other samples, such as hair, saliva, and organ samples, were doctors able to discover the second set of DNA.

It was a pretty interesting show to watch. I suggest watching the actual show if you get a chance. It would probably do better explaining it in laymen’s terms than I can. I think Jon still has questions as to why they didn’t figure it out sooner. I am having to brush the dust off my books in order to answer those questions….if there is an anwer.

This condition is quite rare. There are only about 40 cases known worldwide if I am remembering the show correctly. Of course, documented cases more than likely consist of the pregnancies which produce a hermaphroditic child. Who knows how many of us out there could be considered our own twin And what does this do to the reliability of DNA testing, especially when used to solve crimes? Who knows…..only the future will tell

Hope you found that as interesting as we did. Tune in next week, same time, same channel, for another lesson in the wonderful world of biology

This entry was posted on Tuesday, July 5th, 2005 at 1:54 pm and is filed under Weird Science. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.