The Journal of Human Genetics recently reported on a rare type of twins, who have been dubbed “semi-identical twins.” In this case, a single egg was fertilized by two sperm simultaneously, and then the egg split into twins. One of the twins was born as a “true hermaphrodite,” which means that the child had both ovarian and testicular tissue. (She’s being raised as a female.) The other child is a male. Both children are otherwise normal mentally and physically, and are now three years old.
This condition differs from “polar body twins,” which occurs when a single egg splits, and then each egg is fertilized by two different sperm. This is not the same as “semi-identical twinning” (described above), in which fertilization occurs before the egg splits.
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