Freemartin

A freemartin or free-martin (sometimes martin heifer) is a female bovine with a masculinized behavior and non-functioning ovaries. Genetically and externally the animal is female, but it is sterilized in utero by hormones from a male twin, becoming an infertile partial intersex.

History
The 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 envelop a fetus) of a male and the chorion of a female bovine fetus fuse in the uterus.

In rural areas folklore often claimed this condition was not just peculiar to cattle, but extended also to human twins; this belief perpetuated for generations, as was mentioned in the writings of Bede.

Mechanism
In most cattle twins, the blood vessels in the chorions become interconnected, allowing blood from each twin to flow around the other. If both fetuses are the same sex this is of no significance, but if they are different, 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. The degree of masculinization is greater if the fusion occurs earlier in the pregnancy – in about ten percent of cases no fusion takes place and the female remains fertile.

The male twin is largely unaffected by the fusion, although the size of the testicles may be slightly reduced. Testicle size is associated with fertility, so there may be some reduction in bull fertility.

Freemartins behave and grow in a similar way to castrated male cattle (steers).

Diagnosis
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.

Examination of the animal may also reveal physical differences between a freemartin and fertile heifer.

Other animals
A freemartin is the normal outcome of mixed twins in all cattle species which have been studied. In most other mammals it does not normally occur.

Modern
Freemartins are occasionally used in stem cell and immunology research. During fetal development 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. Male-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.

Historical
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). 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.

Other uses of the term

 * The term "freemartin" has occasionally been used to refer to lesbians.
 * In the Aldous Huxley novel Brave New World, a "freemartin" (mentioned in chapters 1, 3, 11 and 17) is a woman who has been deliberately made sterile by exposure to hormones during fetal development; In the book government policy requires freemartins to form 70% of the female population.
 * In the crime novel Freemartin, by David Cohler, an FtM transgendered man is a murderer.