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Chimeras, Mosaics, and Other Fun Stuff

The other night on CSI they used a medical term (I think it started with a C) for a person that had two different DNA's. Have you ever heard of this?

-A curious adult from Alabama

June 8, 2004

What you are thinking of is "chimera." In the TV show, CSI, a woman claimed a man raped her, but DNA taken from his blood did not match the DNA of the suspect. The test also revealed that the most likely suspect was a relative of the man. When further DNA tests cleared his relatives, the man's DNA was tested again. This time it was from a hair sample and this time it was a perfect match to the suspect's DNA. Can one person have two types of DNA in different parts of his body?

Yes, this can happen. People with two types of DNA are called chimeras after a mythical creature with a lion's head, a goat's body, and a serpent's tail (individuals are also called mosaics). These people have two different sets of DNA in different parts of the body.

How does someone become a chimera? There are a number of ways this can happen:

First, it is possible to become a chimera if developing fraternal twin embryos fuse together to become one embryo. (Think of this is as the reverse of identical twins where a single embryo splits into two.) This happens very early on when the embryos are just unspecialized cells, so a healthy baby can still be made. Fraternal twins do not have the same DNA, so a mixture of two embryos will give a chimera.

Second, chimeras can arise when developing fraternal twins share a blood supply. This happens when the twins (who have different DNA) share a placenta and cells from their blood mix. The twins will be chimeras only in terms of their blood since other cells in the body are not affected by the blood supply.

Third, sometimes chimeras can happen through an error in the way cells divide in the developing embryo. (These people are technically called mosaics but the concept is similar.) Cells split into two to make more of themselves--something embryos need to do a lot of to grow into a baby. For this, cells need to double their DNA and divide it between the two new halves. Sometimes this goes wrong and some new cells end up with different DNA. If this happens early on, the tissues that come from these cells end up with a different genotype.

How common are chimeras? We don't really know. We generally only find out about chimeras when their DNA is analyzed. It sure makes crime solving difficult, but it can also complicate finding organ donors.


This is the Chimera
of Greek mythology.