Although the phenomenon of chimerism has been observed far more vastly in animals, several cases have been there in which human chimeras have been identified. Though it's a rare happening but human chimeras do exist and they contain two alternating forms of any feature or trait in their body. Solid experimentation, blood reports and DNA analysis have confirmed the results so far in particular humans.

Lydia Fairchild is one such example of a human in which chimerism has been observed. Her DNA appeared to have two distinct forms. In the terms of human biology, a chimera is an organism who possess two (or more) different types of cells. In such humans, when the process of fertilization occurs in female womb, two eggs that has been fertilized, fuse together in order to form one zygote/foetus that carries two distinct DNA codes. This happens normally when the birth of twins is expected, but the individual two eggs rather than developing separating into two foetuses, fuse together to form one single organism. The resulting foetus contains two typically different forms of DNA.

Lydia Fairchild came across this strange character about herself when she had to prove that her children were related to her in Washington court. When the DNA test results came, she was shocked to see them as they clearly pointed out that none of the DNA sequences matched with her kids whatsoever. Fairchild was astonished and so were others as they all remembered her bearing the kids and giving the births. The court suspected him of being a surrogate mother and ordered to take the custody of the children off her. But after many efforts by Fairchild, she was able to prove that the children were her own and that she was a twin of her own self.

Another woman, Karen Keegan from Boston was found to be a chimeric twin of her own self. The identification of Karen Keegan as a human chimera took place long before the case of Lydia Fairchild. Doctors reported that her children too didn't match her DNA. DNA samples were taken from all over the body of Keegan but none showed significant results to prove her motherhood. Later, doctors tested a thyroid nodule which was recently removed from the body of Keegan and was kept safely in a lab at Boston. According to Uhl, the DNA that would have matched her sons could have been located anywhere in her body but it was her removed thyroid nodule that matched the DNA of her sons. Her body appeared to have two distinct DNA forms that gave rise to chimerism in Keegan and the same applies to Lydia Fairchild.

Another discovery in human chimerism was made through a woman named Mrs. McK, aged 25 in which two distinct blood groups were found to be present. Work done by Ruth Sanger & R.R. Race points out that when the results first came into light, it was thought to be a mixture of A- and O- blood groups present in her body. Given the fact that the lady had never been transfused, it was hardly possible to believe what appeared to the eyes of the researchers. It seemed as if a large amount of O-blood group had been transfused to the lady for when observed with a naked eye, large agglutinates appeared to be forming but when the samples were observed under a microscope, no agglutination was observed whatsoever. The woman is the first homo sapien to have a two sets of blood group. This discovery brought the consideration of Owen's work on bovine twins that states that primordial red cells from one twin happen to take root in the other twin. Further, these primordial red cells continue to be present in the body of the other twin throughout the life and produce red cells that in nature genetically possess foreign antigens. This had been remarked as one of the cases of free martinism as postulated by Lillie in 1916. Other major implications that were observed in the woman include the inhibition of anti-A due to presence of A antigen, the presence of anti-B in serum and the blood group being O. Mrs. McK has a girl child too but no signs of chimerism or free martinism has been observed in her.

Over the past few decades, important discoveries has been made in the genetics of human chimerism that derive their basis directly from the theories propounded long ago, ever since the start of 19th century. Though, these cases are as rare as they could e, not much information has been gathered but based on the current specimens available to test the human specimen further, sufficient further research is in progress.

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