Most of the time, animals develop from single zygotes; a human develops from one fertilized egg. Genetic chimerism or a chimera is an organism that develops from at least four parent cells. This can occur in a variety of ways, from two fertilized eggs, for instance, or two embryos fusing. The exact effect chimerism has on a person also varies. Due to the rarity of the condition and restrictions on human-based studies, the medical community is still seeking information about human chimerism.
Medical experts cannot predict how many human chimeras exist in the world. Not only is the condition incredibly rare, but obvious signs and symptoms may never develop. It is possible for a person with chimerism never to realize their genetic difference. Modern medical literature records only around 100 cases of human chimerism. A study from the late 1990s states fertility treatments such as in vitro fertilization may increase the likelihood of a child being born with chimerism. However, there is not enough research into this topic to verify the accuracy of these claims.
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