Table of contents:
- One human being has two different DNA structures, recognize chimerism
- What causes chimerism in humans?
- How do doctors diagnose chimerism?
Every human being generally only has one set of DNA in his body, which is passed down from father and mother. So, is it possible if a human being has two different DNA structures? Doesn't that mean it's the same as one body inhabited by two different people? Psstt… This is possible, you know!
One human being has two different DNA structures, recognize chimerism
Put simply, DNA is a long structure containing the unique genetic code that constructs who you really are - including physical characteristics and basic, irreversible qualities - and which differentiates you from other people. DNA also holds the instructions for the development or reproduction of every cell and tissue of a living being, managing your life, and ultimately death.
The phenomenon of two different sets of DNA structures in one living body is called chimerism. This term is taken from the word "Chimera", a monster in Greek mythology with the head of a lioness, goat, and snake in one body.
Chimera illustration (credit: Josh Buchanan)
In the real world, chimerism generally only occurs in animals. Maybe you've come across a photo of a cat or dog that has two different coat colors on its body, as well as different eye colors - like the example below.
What causes chimerism in humans?
The body of a chimera is made up of cells from different people. So, certain cells have the DNA structure that belongs to one person and other cells contain the DNA of others. According to Melissa Parisi, a pediatrician in the U.S. National Institutes of Health, chimerism can occur due to several causes.
Some people get bonus DNA from twins who failed to be born or died in the womb. When a mother is carrying fraternal (non-identical) twins, one of the embryos may die early in the pregnancy. The other embryo can absorb cells and chromosomes from the deceased throughout the pregnancy. Remember that each zygote (potential embryo) carries its own unique DNA sequence.
So, the baby who manages to survive is finally born with two sets of DNA - hers and that of the twin. This is what happened to singer from the United States, Taylor Muhl, who recently learned that he is a chimera. In Muhl's case, he has two different DNA structures because he absorbed a twin while he was in the womb (vanishing twin syndrome).
Taylor Muhl, the dark birthmark on the left side of his body is the "absorption" of his twin (source: dailymail)
Chimerism can also occur in twins who are both alive, because they sometimes exchange chromosomes with each other while in the womb. Parisi said this could have happened because the blood supply that the twins received was also shared. If the twins in the womb are of different sex, then there is a possibility that one or both of the children have half male chromosome and half female chromosome.
However, cases of chimerism can not only occur in twins. In a single pregnancy, the unborn child may exchange cells with the mother. A small portion of the cells belonging to the fetus travels into the mother's bloodstream and travels to a different organ. The baby's DNA can be in the mother's bloodstream because the two are connected together via the placenta. Conversely, babies can also get some of the mother's DNA. A 2015 study shows that this happens to nearly all pregnant women, at least temporarily.
A person can also become a chimera if they have had a bone marrow transplant, for example, to treat leukemia. After the transplant, the person will have their own leftover bone marrow that has been destroyed (due to cancer) and replaced with healthy bone marrow from someone else. Bone marrow contains stem cells which develop into red blood cells. This means that the person who gets a bone marrow transplant will have blood cells that are identical to those of the donor, whose genetic code is not the same as other cells in their own body.
How do doctors diagnose chimerism?
Chimerism in humans is a rare genetic condition. It is difficult for scientists to determine how many people in the world have chimerism because the condition usually causes no symptoms or significant problems. So, it is possible that there are many people who do not realize that they are chimeras until they actually receive genetic tests, DNA tests, or other medical tests.
Dr. Brocha Tarsis, a clinical geneticist at Nicklaus Children's Hospital, Miami, states that without undergoing medical tests, it can be difficult to determine whether someone has chimerism or not.
However, some cases of chimerism can be seen with certain physical signs. For example, different colored eyeballs, different skin colors in one part of the body, or two different types of blood groups. In addition, it is difficult to predict which tissues in the body will be affected and what condition the chimeras will be like.
Several cases of chimerism have been reported to cause disturbances in the genital development of children. For example, a girl who is born has testicular tissue, because her twin who died in the womb was a boy. However, Parisi said this was rare. Usually the condition of chimerism does not show signs with easily observable characteristics.
In Taylor Muhl's case, having two different DNA structures means he has two immune systems and two different blood groups. Muhl is also known to have an autoimmune disease that makes him allergic to food, drugs, supplements, jewelry, and insect bites.
