As a follow up from last week's post, I thought I'd go into more detail about the phenomenon of chimeras, specifically tetragametic chimeras. Chimeras are organisms that contain cells that are genetically different from each other. As the name suggests, tetragametic chimeras form from the fusion of two separate eggs that have been fertilized by two different sperm. More simply put, it is the fusion of two non-identical twin embryos during early development. Chimerism can be easily seen in several non-human animal species, such as the well-known example of Venus the Cat (right). However, in humans it can be less obvious (an exception would be true hermaphrodites).
In chimeric humans, different cell lines have one of the sets of DNA. For example, liver cells may have the same DNA found in saliva, but not the same DNA in brain cells or gametes. A famous case study of this phenomenon was regarding Lydia Fairchild (click on name to see video of her story), who found out she was not genetically the biological mother of her three children when she filed for welfare, which requires the parents to prove that the kids are theirs. Since she failed the maternity test, she was charged with welfare fraud and almost lost the case until doctors decided to test her DNA from multiple cell lines, such as her blood, hair, saliva, and cervical cells. Only the cervical cells provided a match to her children. As in vitro fertilization becomes more common, tetragametic chimeras have become more abundant, due to the practice of implanting several embryos into the mother to increase the chance of pregnancy, which increases the probability that two fertilized eggs might fuse (Strain 1998). To prevent future failed paternity and maternity tests, children who are the product of in vitro fertilization should have multiple cell lines screened to see if they are chimeric.
References:
Lisa Strain, Ph.D., John C.S. Dean, F.R.C.P.(Edin.), Mark P.R. Hamilton, F.R.C.O.G., and David T. Bonthron, M.R.C.P. A True Hermaphrodite Chimera Resulting from Embryo Amalgamation after in Vitro Fertilization. N Engl J Med 1998; 338:166-169. January 15, 1998 DOI: 10.1056/NEJM199801153380305
WEIRD OR WHAT? -- TETRAGAMETIC CHIMERA
I commented on your previous post as well, but I do find this area of research particularly interesting. I still think that even without the incidences caused by in-vitro implantation, this phenomena might be much more common than acknowledged; I know that I have one maternal aunt for which this is the case, and potentially a couple other aunts or uncles (since my grandmother had a predisposition to twins, and had that case with my aunt and another with an uncle where there was twin-to-twin transfusion and other complications, and she had 12 children). Since most of us do not have our genetic profile mapped at all, let alone from multiple cell line sources; I just think we really have no understanding of the possible variations that can be seen at the population level.
ReplyDeleteThis is still very interesting. Screening multiple cell lines is a great solution.
ReplyDeleteChimeras are so interesting. There was a guy recently that found out that he was not the father of his kid but instead the twin that he absorbed in the womb was the father. Its crazy to think that one person can hold two different types of DNA in their body and still function perfectly and maybe even never know they were different.
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