Dictionary of Human Evolution and Biology

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Freemartin Effect

Consequence of prenatally shared circulating blood in nonhuman, opposite-sex (and therefore fraternal twin pair) mammals, especially cattle; occurs when the blood vessels of a fused placenta become interconnected when chorions fuse, resulting in the masculinization of female members of the pair due to prenatal exposure to male hormones. Opposite-sex twins in the higher primates develop in a bicornuate uterus, but partial chimerism nevertheless occurs in about 10% of human fraternal twin pairs.

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