Artwork By Luis Irurita Juarez
Ever met a person with two different eye colors and thought Wow, this is so cool?
Hold on to your hats because there is a possible explanation for this phenomenon that’s incredibly fascinating: they absorbed their twin before birth! Curious as to how this bizarre event happens?
The answer can be found in the fables of Greek mythology. ‘Chimeras’ were grotesque, fire-breathing hybrid creatures consisting of more than one animal: usually a lion with a goat’s head growing from its back, and a tail that punctuated with the head of a snake. Now, with that in mind, it makes sense why people with two different sets of DNA are referred to as human Chimeras; after all, it is two different beings combined. 
There are three different scientific explanations for this biological circumstance. One explanation involves the embryo of one fraternal twin absorbing its dead sibling’s DNA. This can occur in extreme surgical cases like after a bone marrow transplant, or even during a very normal pregnancy. However, the chances for this to happen during the latter situation are very small. In the case of a bone marrow transplant, doctors use chemotherapy or radiation in order to suppress the patient’s affected region’s blood cell production and replace it with healthy stem cells. Since the bone marrow donor contains different DNA than the sick patient, the donor’s stem cells will produce blood cells with that DNA within the patient, thus the patient becomes a chimera.
Second, if a woman is expecting fraternal twins and one of the embryos dies early during the pregnancy, the other one can absorb its twin’s cells which results in the newborn carrying two different sets of DNA. This explains why some people you might have met had one blue and one green eye!  In reality, many individuals aren’t even aware of being a chimera. Take this incident for an example: in 2002, a woman named Karen Keegan needed a kidney transplant, so she and her family had to undergo a couple of genetic tests to find a possible donor. However, when the results arrived, everyone was stunned. They concluded that Karen genetically couldn’t be her son’s mother! Later on, it was discovered that Karen was carrying a different set of DNA in her blood than she did in other tissues of her body.
Now, while most chimeras happen to exist naturally, scientists have also been working on creating them in the laboratory, bringing us to our third scientific possibility. Remember those mad scientists in movies, mixing their glowing liquids and giving life to mysterious mutants? That is kind of what’s happening behind the scenes in modern science! There are various combinations that experts have been working on: experiments such as creating human-human, human-pig, human-monkey, human-mouse, and even virus chimeras! In fact, in 2017, scientists tried growing an embryo consisting of human cells and pig cells. However, only 186 out of 2075 implanted embryos actually began to develop up to the 28-day stage. In the same year, Portuguese researchers created a mouse virus with a human viral gene that allows them to investigate different ways to treat cancer caused by human herpes infection.
In March 2021, Japan lifted its time limit for growing human-animal chimeras to over the two-week period it was originally confined to. These laws allow the transplantation of chimeric embryos into animals (human incorporation isn’t allowed). A Japanese scientist is currently planning on inserting human stem cells into mice or rats in an attempt to grow a human pancreas. To make sure the experiment doesn’t go too far, it will immediately be stopped if too many human cells are found in the rats’ brains. In April 2021 an international team consisting of China, Spain, and the U.S. created the first human-monkey chimera ever. They kept the embryos alive for around 20 days. This is geared towards helping with research on human diseases, making it easier to understand early development, and perhaps even to find methods on how to grow organs inside of animals that could be used as transplants.
Although this sounds straight out of an exciting science fiction movie, all of these executions come along with various ethical questions. With regards to these guidelines, none of the created chimeras have been implanted into a uterus, nor did they actually create living animals. But it is about time to discuss this controversial topic if scientists want to continue growing chimeras to help human beings. What do you think? Should we be allowed to create chimeras or try finding a different way to save human lives?
- Wikipedia contributors. (2021, July 4). Chimera (mythology). Wikipedia. https://en.wikipedia.org/wiki/Chimera_(mythology)
- Praderio, C. (2018, February 22). It’s possible for one person to have two different sets of DNA — here’s how it happens. Insider.
Reporter, G. S. (2019, August 12). The five: chimeras created by science. The Guardian. https://www.theguardian.com/technology/2019/aug/11/the-five-chimeras-human-monkey-hybrid-genetic
- STAT. (2021, April 15). International team creates first chimeric human-monkey embryos. https://www.statnews.com/2021/04/15/international-team-creates-first-chimeric-human-monkey-embryos/