Sex change

Sex change is a process by which a person or animal changes sex – that is, by which female sexual characteristics are substituted for male ones, or vice versa. Sex change may occur naturally, as in the case of the sequential hermaphroditism observed in some species. Most commonly, however, the term is used for sex reassignment therapy, including sex reassignment surgery, carried out on humans. It is also sometimes used for the medical procedures applied to intersex people. The term may also be applied to the broader process of changing gender role ("living as a woman" instead of living as a man, or vice versa), including but not necessarily limited to medical procedures.

Some species exhibit sequential hermaphroditism. In these species, such as many species of coral reef fishes, sex change is a normal anatomical process.Clownfish, wrasses, moray eels, gobies and other fish species are known to change sex, including reproductive functions. A school of clownfish is always built into a hierarchy with a female fish at the top. When she dies, the most dominant male changes sex and takes her place. In the wrasses (the family Labridae), sex change is from female to male, with the largest female of the harem changing into a male and taking over the harem upon the disappearance of the previous dominant male.

Natural sex change, in both directions, has also been reported in mushroom corals. This is posited to take place in response to environmental or energetic constraints, and to improve the organism's evolutionary fitness; similar phenomena are observed in some dioecious plants.

Chickens can sometimes undergo natural sex changes. Normally, female chickens have just one functional ovary, on their left side. Although two sex organs are present during the embryonic stages of all birds, once a chicken's female hormones come into effect, it typically develops only the left ovary. The right gonad, which has yet to be defined as an ovary, testes, or both (called an ovotestis), typically remains dormant. Certain medical conditions can cause a chicken's left ovary to regress. In the absence of a functional left ovary, the dormant right sex organ may begin to grow, if the activated right gonad is an ovotestis or testes, it will begin secreting androgens. The hen does not completely change into a rooster, however. This transition is limited to making the bird phenotypically male. The condition could also be caused by mycotoxins that can develop when animal feed is stored, and these have the same effect as synthetic hormones. In about 10 per cent of cases, if eggs fertilised with male chromosomes are cooled by a few degrees for three days after laying, the relative activity of the sex hormones will favour development of female characteristics. The sex chromosomes work by coding for enzymes that affect the bird’s development in the egg and during its life. This cooling will produce a chicken with a fully functioning and reproductively fertile female body-type; even though the chicken is genetically male.

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