Twins: Identical, Fraternal, and Now Semi-Identical

While studying the genetics of a baby hermaphrodite, researchers discovered the twin brother shared only half the babies' father's genes, while sharing all of the genes on the mother's side. These "semi-identical" twins are the result of two sperm simultaneously fusing with one egg - a phenomenon that was previously unreported in producing twins. The genetic research showed "each twin contained some 'female' cells with two X chromosomes, and some 'male' cells with an X and a Y." The proportion of each type varied depending on the tissue and the twin.

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