Genetic Chimerism (Chimera)

A genetic Chimerism or Chimera is a single organism composed of cells with distinct genotypes. In animal, an individual derived from two or more distinct “Zygotes” from possibly blood cells of different types (phenotype) or if the zygotes were of different sexes, the individual can heritage of both male and female sexual organs (intersexuality). Chimeras can be produced in the animal by merging multiple fertilized eggs while in plants it deals with different type of tissue from the same zygote. “Genetic Chimerism” is not detectable on casual inspection like bone marrow. Chimerism can occur also by organ transplantation.

An animal Chimera can have two or more different populations of distinct genetic cells from the sexual reproduction and if the cells emerge from the same zygote, the organism is called a “Mosaic”. They are formed at least of two fertilized eggs or embryos fused together. Cases of Human Chimerism have been documented. This condition is either acquired or inherited in non-identical twins. Blood vessel anastomosis is responsible of the phenomenon during the in-vitro fertilization but the type of offspring depends on the type of cells from ovaries or testes bringing intersex differences genetically male or genetically female.

The congenital Chimerism can be “tetragametic” through the fertilization of two separate ova by two spermatozoids with aggregation of both eggs at the stage “Blastocyst or “zygote”. In other way, this is the merging of two non-identical twins and as such, they can be male or female or have mixed intersex characteristics. These chimera organisms develop organ which may have different sets of chromosomes like a kidney or a liver in the same individual may present different set of chromosomes. Nowadays, this phenomenon which was considered extremely rare, is diagnosed more often.

“Marmosets” are chimeras sharing DNA of their fraternal twins after trading blood through chorionic fusions (Hematopoietic Chimeras). Most Chimeras will not realize the changes except when a difference in the phenotype is subtle, like a hitchhiker’s thumb, eyes of different colors, different hair growth on the opposite side of the body. They may have distinctive marks visible to the ultraviolet light and resembling to an arrow pointing down called “Blaschko’s lines”.

Other signs to identify the Chimeras are two populations of red cells, or zygotes of opposite sex, or ambiguous genitalia. They may have hairy patch skin, hair or eye pigmentation (Heterochromia). They may show the genitals of both sexes if the blastocysts were of different sex or simply demonstrate a combined “ovotestes” as previously known to be a true case of hermaphroditism. We have seen some cases during our training back home and these cases have always been an object of curiosity and the approach has always been to wait at puberty to know what sex is the predominant and then remove the less dominant organ. Evolution in medicine has shown that we may have “missed the boat” because of our incapability to perform any genetic testing.

It is only when they exhibit abnormalities like “male-female’ or “hermaphrodite” or skin pigmentation etc. that they may be examined and diagnosed. A variant of Chimerism is more noticeable on the “Calico cat” or “tortoisehell cats” with an extra X Chromosome. One can understand the legal ramifications with the implication for the family and the criminal law overseeing the DNA testing when it needs to be performed.

Two famous examples of Chimerism were bought to court after DNA testing and showing apparently in 2002, that the children of Lydia Fairchild were not hers. Her custody rights were challenged and fraud charges were filed. She was denied public assistance in Washington State and later dismissed when the DNA was matched in her cervical tissue. Another case in the New England, involving Karen Keagan, suspected of not being the biologic mother of her adult son in need for a kidney transplant was challenged in court. This is the lawyer for the prosecution in this second case who was able to state to the court that Lydia Fairchild had two set of DNA and one was able to match her kid’s DNA. I will leave the debate to Dr Rony Jean Mary for a profound explanation in the cases involved.

Foekje Dillema, a Dutch sprinter was expelled in 1950 from the national team for refusing a mandatory sex test. Later in 1953, tests demonstrated a Y-chromosome in her body cells showing she was 46,XX / 46,XY mosaic Female. Another woman was described in the British Medical Journal as having blood containing two different blood types.

I tried to define through the best of my reading, how Genetic Chimerism is presently seen in the world of Genetic and Dr Rony Jean Mary will explain in more details some famous cases. I can’t conclude without talking about some restrictions imposed on the laboratories. The Western Europe and the United States have strict regulations and code of ethics forbidding experimentations using human cells. Moral and legal issues creating controversies are raised by humans about concern about the right to create human chimera with respect to human dignity. In 2008, the house of Commons in the United Kingdom concluded that embryos should not be allowed to be made in laboratories. In 2005, the United States voted on a bill “The Human Chimera Prohibition Act” knowing that science reached a point where human and non-human life can be easily created in laboratories, showing blatant disrespect to human dignity. It was found also that the creation of new life brings with it zoonotic diseases detrimental to Humans. Unfortunately, this bill died in Congress and the issue has never been re-considered in a legislation.

Maxime Coles MD


1-     Norton, Aaron; Ozzie Zehner (2008). “Which Half is Mommy? Tetragametic Chimerism and Trans-Subjectivity”. Women study Quarterly. Fall/Winter (3-4): pp. 106-127.

2-     Friedman, Lauren. “The Stranger-Than-Fiction Story of A Woman Who Was Her Own Twin”. August 2014

3-     Gengozian, N; Batson, JS; Eide, P; (1964). “Hematologic and Cytogenetic Evidence for Hematopoietic Chimerism in the Marmoset, Tamarinus Nigricollis” Cytogenetics. 10 (6): 384-393

4-     Strain, L; John CS Dean; Mark PR Hamilton; David T Bonthron (1998). “A True Hermaphrodite Chimera Resulting from Embryo Amalgamation after in-vitro Fertilization”. The New England Journal of Medicine. 338 (3) pp.166-169.

5-     “The twin Inside Me: “Extraordinary People”. TV Show. (March 2006)

6-     Zimmer, Carl (2007-3-27). “In The Marmoset Family, Things really Do appear to Be all Relative”. The New-York Times. 2010-4-01.

7-     Hooper, Rowan (26 March 2007). “Marmosets may carry their siblings sex cells”. New Scientist.

8-     Ballantyne, KN; Kayser, M; Grootegoed, JA (2011) “Sex and Gender Issues in competitive sports Investigation of a historic case leads to a viewpoint”. British Journal of Sport Medicine. 46 (8): 614-617.

9-     Barry, Star” In the chimera episode, the fact that the subject was a chimera first established through a camera flash on the skin illuminating skin abnormalities: Ask a Genetician. Stanford School of Medicine (2011-7-25)

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