In some ancient mythologies chimeras were incredible beasts with a lion's head and a serpent's tail, a composite of two or more creatures. Human chimeras do exist, but they are nowhere near as fantastical. In fact the reality is rather more prosaic.
What are Chimeras?
Chimerism refers to organisms possessing more than one genetic identity. The term “chimera" comes from a mythological composite creature made from parts of other animals. In humans, a chimera can be the result of two embryos becoming fused. This is considered to be very rare.
It all takes place during pregnancy. Normally a male gamete (sperm) and a female gamete (ova) fuse together to form a zygote (the initial cell that will ultimately give rise to the embryo). This contains DNA from the mother and DNA from the father. Sometimes two male gametes will fuse with two female gametes which gives rise to fraternal twins.
These twins are not identical twins and so their DNA is distinct from each other. Now in rare occasions these two embryos can fuse and instead of there being two children a mother gives birth to just one which has been formed from four gametes instead of the usual two. This is a chimera and it will have two distinct sets of DNA.
Recognition of Chimerism
Occasionally human chimeras hit the headlines. For example, there was the case of a woman from Boston who was told that she was not the biological mother of her children, even though she claimed to have given birth to them and had been raising them for years. Her genes were completely different from two of her three children. After several rounds of genetic testing with doctors working on the case for more than two years, it was eventually decided that she was indeed a chimera.
The woman had two distinct sets of DNA with the DNA in her gametes being different from the DNA with the rest of her body. The reason that scientists were confounded for so long was that the DNA tests typically compare DNA from blood samples or from cheek swabs. The children had received their DNA from their mother's gametes but it was being compared to other tissues in the mother.
Manifestation of Chimerism in Humans
A person could be a chimera without ever knowing. For example, a chimera could have one organ with different DNA to that in the rest of their body. In some cases, they may have visible signs such as differently-colored eyes. Chimerism can also result in a person with both male and female sexual organs (hermaphrodism).