Once a freemartin attempted to mount a cow, that cow would then be isolated from the rest of the herd and allowed to breed with the desired bull, artificially inseminated, or prevented from breeding, as desired. Prior to the wide availability of inexpensive testing, freemartins were valued as a way to identify cows in estrus, without risking injury (as would occur if a bull were used). 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. 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. The freemartin model allows one to analyze perfectly healthy and unmanipulated animals, without resorting to transplantation often used in stem cell research. 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. Bull-derived cells and their progeny can be easily visualized in the freemartin tissues, as only they contain the male Y chromosome. Up to 95% of the freemartin's blood cells can be derived from those of its twin brother. During fetal development cells are exchanged between the fused circulations of the bovine twins. Uses Modernįreemartins are occasionally used in stem cell and immunology research. The male hormones then masculinize the female twin, and the result is a freemartin. The blood vessels in the chorions become interconnected and male hormones pass from the male twin to the female twin. In 1916, several researchers independently discovered what happens when the chorion (the outer layer of the two membranes that completely envelop a fetus) of a male and the chorion of a female bovine fetus fuse in the uterus. The 18th-century physician John Hunter discovered that a freemartin always has a male twin.