What is a chimera?
A chimera in biology refers to an organism that possesses two sets of DNA instead of just one.
This can happen both naturally and through human intervention, and is quite rare to find throughout the animal kingdom. Chimerism is not commonly diagnosed and when it is, it’s usually by chance, such as when carrying out genetic tests for transplants. In fact, many people will live all of their lives oblivious to the fact that they have chimera DNA, as it does not impact their daily lives.
One natural occurring way that this can happen is through twin foetuses. When for some reason one of the twins dies earlier on during development, the surviving twin gradually absorbs the other twin’s remaining cells. The surviving twin will then have two sets of DNA throughout all his or her life.
As stated previously, this will not necessarily impact one’s daily life. However, in the exception that this particular individual undergoes a DNA test, there may be a chance that the sample taken holds the DNA of the absorbed twin. Therefore, when analysed, the test results may state that the parent has an avuncular relationship with the child, due to having their twin’s DNA along with their own, making it seem as if they are the uncle or aunt rather than the parent.
Some people may receive a second DNA when undergoing medical treatments, such as bone marrow transplants, stem cell treatment or blood transfusions. In blood transfusions, this is only temporary, as the body renews blood regularly. On the other hand, with bone marrow transplants the patient receiving the treatment will permanently have two sets of DNA, because the bone marrow is where blood cells are created so a transplanted bone marrow will continue making blood cells with the donor’s DNA inside the patient’s body. Depending on the situation, the patient’s blood may either be a mix of the original DNA and donor DNA or be completely made up of the donor’s DNA. This is called artificial chimerism.
There are also cases of micro-chimerism, where a person has small amounts of another DNA throughout their body. An example of this can be found in pregnant mothers, where some fetal cells from the baby pass on through the mother and remain in her organs such as the brain and liver for long periods of time. Vice versa, the baby may also receive small amounts of DNA from the mother.
Most often, chimerism is found in the blood, however it is possible that it may be found in other areas as well, such as in the reproductive system. Therefore, it is possible for a parent with chimerism to pass on more than one set of DNA, therefore the child will have 3 or even more sets of DNA. This can make it tricky to obtain accurate results in DNA testing.
For example, in 2002 a woman having chimerism was mentioned in the New England Journal of Medicine. The maternity tests that the woman had undergone concluded that she was not the biological mother of her child. However, it turned out that the sample analysed had a different DNA than that passed on to her child.
Although a rare phenomenon, having knowledge of chimerism could overcome doubts when certainty of parentage is absolute. It could also prove beneficial to test from different samples or different parts of the body if a parent is suspected of having chimerism.