Freemartin
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A freemartin is a female bovine with a masculinized brain that behaves as would a bull, at least insofar as it will mount normal females. An 18th century physician, John Hunter, discovered that a freemartin always has a male twin. In 1916, several researchers independently discovered what happens when the chorion (the outer layer of the two membranes that completely envelope a fetus) of a male and the chorion of a female bovine fetus fuse in the uterus. The blood vessels in the chorions become interconnected and male hormones pass from the male twin to the female twin. The male hormones then masculinize the female twin, and the result is a freemartin.
Freemartins are occasionally utilized in stem cell and immunology research. During fetal developement cells are exchanged between the fused circulations of the bovine twins. Up to 95% of the freemartin's blood cells can be derived from those of its twin brother. Bull-derived cells and their progeny can be easily visualized in the freemartin tissues, as only they contain the male Y chromosome. Thus, by analyzing these tissues, one is able to investigate the capacity of hematopoietic stem cells or other circulating cells to produce other tissues in addition to blood. The freemartin model allows one to analyze perfectly healthy and unmanipulated animals, without resorting to transplantation often used in stem cell research.
If suspected, a test can be done to detect the presence of the male Y-chromosomes in some circulating white blood cells of the subject. Genetic testing for the Y-chromosome can be performed within days of birth and can aid in the early identification of a sterile female bovine.
The term "freemartin" has occasionally been used to refer to lesbians.
In the Aldous Huxley novel Brave New World, a freemartin is a woman who has been deliberately made sterile by exposure to hormones during fetal development; by government policy, freemartins form 70% of the female population.