Health Talk: Human Chimeras

The fundamental rule of genetics states that parents pass genes onto their children, but recently, a 52-year-old woman from Boston was told that she was not the biological mother of her children, after years of raising them. The case, reported in The New Scientist, has the medical community at its wits’ end searching for an answer.

The woman who claimed to have given birth to her children had a set of genes completely different from those of two of her three children. What was even more intriguing about this case was the fact that the DNA of the children matched their father’s DNA.

After working for nearly two years on the case, doctors came up with an answer to the problem: the woman was a “human chimera.” Named after the legendary Greek monster with the head of a lion, body of a goat, and tail of a snake, human chimeras have two or more distinct sets of genes in their bodies. If the DNA from the skin is compared to the DNA in the blood, the two sequences will be completely different. In this woman’s case, the DNA in her gametes was different from the DNA in the rest of her body. DNA testing is usually done by analyzing blood samples or by analyzing samples from cheek cells. The children received the unique DNA from their mother’s gametes, but it was compared to other tissues in the mother’s body. Since the mother was a chimera, these two sequences did not match.

The fact that each person possesses a unique set of genes has been crucial in the development of modern biotechnology. Human chimerism seems to prove all of the known rules of genetics wrong. However, after a good amount of research into this issue, researchers were relieved to discover that the painstakingly laid down rules of genetics were not wrong after all. Human chimeras are the result of a rather common occurrence during pregnancy. A male and female gamete fuse to form what is known as a zygote.

A zygote is the first stage in the life of a human and consists of DNA derived partially from the mother and partially from the father. In some cases, two different male gametes (called sperm) fuse with two female gametes (called ova). In such a case, two zygotes are formed instead of one and this gives rise to fraternal twins. Such twins have two distinct sets of DNA. In fact, in a number of cases, the two babies are of the opposite sex. Sometimes, the two embryos fuse at an early stage and at the end of the pregnancy, the mother gives birth to only one child. This phenomenon is also known as “vanishing twin syndrome.” Such children now have two different cell types in the same body. They are formed from not two, but four gametes and are hence called “tetragametic chimeras.”

In such cases, half of the cells in the body will have one set of DNA, and the other half will have another set of DNA. In rare cases, chimeras have more than two distinct sets of DNA. This can be attributed to the formation of multiple zygotes and the eventual fusing of all of these zygotes.
More commonly, fraternal twins undergo swapping of blood during pregnancy. In such cases, after the twins are born, each will have blood with a DNA sequence different from the rest of the body. However, unlike tetragametic chimeras, such types of chimeras do not have different DNA sequences in any other parts of the body.Chimerism is usually not a problem, but in cases where the embryos of fraternal twins of opposite sexes fuse, problems have been encountered. These are usually reproductive problems in which the body contains the reproductive organs of both sexes. Chimerism rarely manifests itself in visible abnormalities.

Certain cases have been reported in which chimeras had eyes of different colors, however no other physical manifestations of this syndrome have been noted. In most cases however, people can lead normal lives without ever knowing that they are chimeras.