Chimerism is rare in humans. There are few medically documented cases. Throughout history there were likely more incidences, but usually people do not discover chimerism. DNA testing and knowledge is so recent.
A famous modern case involves a woman named Lydia Fairchild. In 2002 she was in a custody dispute for her children, and discovered that her three children, that she gave birth to, did not share her DNA. Doctors and lawyers were flabbergasted, and Lydia almost lost custody of her children.
Eventually it was discovered that Lydia was meant to be a twin, but in utero the two eggs fused into one. As a result Lydia's twin's DNA lived on inside her. Remarkably, Lydia's children were essentially her sister's children as well. Certain organs had Lydia's DNA, and other organs had her twin's DNA.
Microchimerism is far more common, but it is difficult to detect because it is at the cellular level. Microchimerism is when the fetus' and the mother's cells get transferred during pregnancy. Many adult children have their mother's cells inside certain organs. Mothers also have their children's DNA.
Fraternal twins and identical twins also exchange DNA in utero. In fact with fraternal twins, there sometimes can be sexual development complications (or differences) at puberty, because male or female chromosomes have been exchanged. Pretty fascinating.
Scientists are studying microchimerism, as this phenomena has affects on the development of diseases and autoimmunity, both positively and negatively.
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