By Macy Osborne-Frazier
If you watched the recent Percy Jackson and the Olympians show, you might remember a scene where he fought a monster called the chimera. In Greek mythology, the chimera is a monstrous combination of a lion, goat, dragon, and snake. From the legend of the Greek chimera, we get our name for chimeric animals used in genetics today. Just like the legendary chimera, chimeric organisms are genetic combinations from different sources, and you have likely seen a genetic chimera at least once in your life. For example, if you’ve ever been around a tortoiseshell or calico cat, you were likely hanging out with a chimeric cat.
Drawing of the Greek chimera (image source)
Most organisms are made up of cells that all have the same genetic material. Typically, organisms develop from a single fertilized egg, meaning every cell in the entire organism should have the exact same genome. This means that a cell taken from your eye should have the same genetic information as a cell taken from your heart. However, not every cell in the chimeric organism has an identical genome. Therefore, a cell taken from one eye of a chimeric organism could have different DNA than a cell taken from the other eye. In fact, if you know a person with two different colored eyes, there’s a good chance they are a chimeric human. Chimeric humans can be identified because of eye color, but many people might have chimeric gene expression and never know because there are no visible consequences.
Chimeric human with different colored eyes (image source)
Chimeric animals also naturally occur. Chimerism is especially common and noticeable in cats. Chimerism in cats most often is seen in the unique fur pattern on their faces. On chimeric cats, the fur on one half of the face is one color and the fur on the other half of the face is a different color.
Chimeric cat (photo courtesy of author)
Chimeric humans often develop during pregnancy as fraternal twins. If one fetus fails to develop correctly in early pregnancy, the other fetus can absorb its twin. When the remaining child is born, they will contain cells with their unique DNA and cells with their absorbed twin’s unique DNA. Chimerism in humans can also be created by organ transplants or bone marrow transplants. In these cases, adding cells with the organ donor’s DNA to the body of a recipient patient creates the chimerisms. Although this type of chimerisms doesn’t create any noticeable effects in the recipient patient, they technically have two different cell populations with two unique genomes in their body.
As our understanding of genetics has grown, people now utilize chimerisms for science and agriculture. Scientists create chimeric organisms in the lab to use for experiments. Chimeric mice are generated in the lab by isolating cells from mice, genetically engineering the cells, and then introducing the genetically modified cells back into mice. Scientists then use these chimeric mice to study development of specific organs or to understand how certain genetic modifications can cause diseases. Another way that people create chimeric organisms is by grafting plants. Grafting is a method to combine two different plants together into one single plant. Grafting can be very useful for crop plants, like fruit trees, to help repair damaged plants or improve pollination.
Using chimeric mice for experiments and generating chimeric plants in agriculture has both improved our understanding of human health and increased the efficiency of crop production. Chimerism is a perfect example of how furthering our understanding of a naturally occurring genetic phenomenon has helped humanity grow in our ability to treat diseases and to sustain our growing populations.
