Do you have to have a mixa of Chimera?

Chimera is a terrifying hybrid monster in Greek mythology. Like the legends, Chimera also has many versions, but the most common is a lion that has undergone plastic surgery with a goat's head growing on its back and a snake head at the end of its tail. He is the son of Echidna and Typhon, two snake-type monsters. So where did the lion's head and the goat's head come from? Most mythical monsters are products of a mixture of the characteristics of many animals. But the Chimera is the creature that best exemplifies this synthetic department store. That's probably why people use the name of this hybrid monster to describe the genetic phenomenon we are about to dissect.

Chimera is a phenomenon in which an organism contains cells with different genetic makeup. For example, a person may be derived from two or more zygotes and, therefore, have blood cells of two or more different blood types. The question is how the Chimera phenomenon could have happened. Doesn't it mean that if you transfuse type B blood into a person with blood type A, it will trigger a life-threatening immune response?

The cells in a normal person's body have only one DNA, which is their DNA. Humans have hundreds of types of cells and trillions of individual cells, but they all contain exactly your unique DNA. That's the basic understanding of genetics. But with the Chimera, this basic understanding becomes rubbish. A chimera will have two sets of cells with two distinct DNA. This can happen when the mother is pregnant with twins or more. If an embryo dies in the first trimester, its cells will be absorbed by the mother, by the placenta, or by its surviving twin brother or sister. The macrophages will march in and begin to break down the cells of the dead embryo. The broken-down cellular components are then reabsorbed into the mother's circulatory system through the placenta, and the amniotic fluid is then distributed back to the living embryo.

In the process, most of the cells will be smashed. But some more robust cells can survive intact when they pass through the placenta. They will divide and become parts of the organs in the new body. One of the most common types of cells of Chimera is blood cells. The blood cells formed in the bone marrow are very strong and flexible. They can survive the process of absorption and redistribution from one embryo to another. This is called "vanishing fetus" or Vanishing Twin Syndrome and it explains the Chimera phenomenon in people who are not twins. They could have had a twin brother or sister, but that didn't happen. If the mother passes 3, 4, or 5 embryos and only one of them disappears, the surviving pairs of twins and triplets can also become Chimera.

According to statistics, blood chimera occurs in about 8% of fraternal twins and 21% of triplets. But chimera doesn't necessarily occur only when one of the embryos dies early in pregnancy and is absorbed by the other. Such cases are called twin chimeras. In most cases, Chimera occurs naturally when two fertilized eggs, i.e., two zygotes, fuse before or shortly after implantation in the uterus. If you are born from two fused zygotes, your body will be made up of cells derived from two zygotes, carrying two different sets of genes. Taylor Muhl is an American singer and model. Her skin has a pronounced difference in pigmentation due to the phenomenon of chimera. She has two sets of DNA and confidently introduces herself as her own twin sister.

When Chimera people are created from two fused zygotes, they are called chimeras. The same is true if the placenta of another pair of twins whose eggs stick together. They can exchange hematopoietic stem cells and thus create a pair of twins, each of whom has two types of blood cells. But you might ask, why can Chimera people live with two blood types, while a normal person transfusing blood from different groups is life-threatening? It is true that usually, the immune system will recognize and attack foreign cells, such as different blood group cells. But in Chimera, because both sets of cells come from a very early stage in pregnancy, the immune system may have accepted both types of blood cells as family members and does not launch the same attack as it would with a wrong blood type transfusion. Additionally, two cell groups of the chimera are evenly distributed and balanced in the blood and throughout the muscles, from the liver and heart to the skin, as a mixture of two types of cells carrying two DNAs in a certain proportion. The body has been accustomed to this distribution since very early, so there are no problems growing up.

Chimera is often considered extremely rare, with only about 100 cases ever reported, accounting for 0.00000125% of the population. But there may be many more undiagnosed cases. In most cases, Chimera individuals have no obvious signs or symptoms, although some may show hyperpigmentation, hypopigmentation, asymmetrical body growth, curved thumbs on one side, differently growing hair on two sides of the body, or two eyes with different colors.

One classic example of the Chimera phenomenon occurred in 2002. An American woman named Lydia Fairchild, born in 1976, was pregnant with her third child when she decided to separate from her husband. When Fairchild filed for child support, the court requested a DNA test. The results shocked everyone: her husband was the father of the children, but Fairchild's DNA did not match her own children. Accused of fraud, Fairchild's children were almost taken from her. When Lydia gave birth to her third child, a judge ordered a person to supervise the process and take a blood sample immediately afterward. The results again showed that Fairchild's DNA did not match her child's. The turning point came when her lawyer learned about Karen Keegan, a chimera in Boston, Massachusetts. Further DNA testing showed that Lydia's children's DNA matched their grandmother's (Lydia's mother). This confirmed that Lydia Fairchild is a genuine chimera with two different sets of DNA.

There are real and fake chimeras. In addition to blood chimeras, twin chimeras, or scattered chimeras, there is also a very interesting form called microchimerism. This occurs when stem cells from the embryo pass through the placenta and divide in the mother's body, producing a small number of cells carrying the child's DNA in the mother's body for many years. It is estimated that about 50-70% of mothers carry some fetal cells in their bodies. Conversely, children also carry a small number of their mother's cells, but this rate is smaller. When a person has a marrow or organ transplant, the donor cells develop from a completely different genome, which is called an artificial chimera.

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