Sex change is a term often used for gender reassignment therapy, that is, all medical procedures transgender

Transgender

Transgender is a general term applied to a variety of individuals, behaviors, and groups involving tendencies to vary from culturally conventional gender roles....

ed people can have, or specifically to sexual reassignment surgery, which usually refers to genitalia surgery only. It is also sometimes used for the medical procedures intersexual people undergo, or, more often, are subjected to as children.

"Sex change" is sometimes also used for the whole process of changing gender role

Gender role

Gender roles refer to the set of social and behavioral norms that are considered to be socially appropriate for individuals of a specific sex in the context of a specific culture, which differ widely between cultures and over time...

 and the medical procedures associated with it. Since changing of gender role, i.e. living as a woman instead of living as a man, or living as a man instead of living as a woman, is much more important to almost all transgendered people than any medical procedures, this use is even more inaccurate. (Of course, medically induced changes and surgeries are often needed to make a change of gender role at all possible, both socially and legally. Also, they can have a very significant impact on the well-being of people having them!)

Sex change in animals

In some species, such as many species of coral reef fishes, sex change is a normal anatomical process. Clownfish, wrasses, moray eels and other fish species are known to change sex including reproductive functions. A school of clownfish

Clownfish

Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Twenty-eight species are recognized, one in the genus Premnas, while the remaining are in the genus Amphiprion. In the wild they all form symbiotic mutualisms with sea anemones...

 is always built into a hierarchy

Hierarchy

A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...

 with a Female fish at the top. When she dies, the most dominant male changes sex and takes his place. In the wrasses (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 his disappearance. See Hermaphrodite for more details.

Sex change in humans

Sex change is a term often used for sex change in humans

Sex reassignment therapy

Sex reassignment therapy is an umbrella term for all medical procedures regarding sex reassignment of both transgender and intersexual people...

, that is all medical procedures transgender

Transgender

Transgender is a general term applied to a variety of individuals, behaviors, and groups involving tendencies to vary from culturally conventional gender roles....

 people may pursue, or specifically to sex reassignment surgery

Sex reassignment surgery

Sex reassignment surgery is a term for the surgical procedures by which a person's physical appearance and function of their existing sexual characteristics are altered to resemble...

, which usually refers to genital surgery only. It is also sometimes used for the medical procedures intersex

Intersex

Intersex, in humans and other animals, is the presence of intermediate or atypical combinations of physical features that usually distinguish female from male...

 people undergo, or, more often, are subjected to as children.
"Sex change" is sometimes also used for the whole process of changing gender role

Gender role

Gender roles refer to the set of social and behavioral norms that are considered to be socially appropriate for individuals of a specific sex in the context of a specific culture, which differ widely between cultures and over time...

 and the medical procedures associated with it. Since changing of gender role, i.e. living as a woman instead of living as a man, or living as a man instead of living as a woman, is much more important to almost all transgender people than any medical procedures, this use is even more inaccurate. (Of course, medically induced changes and surgeries are often needed to make a change of gender role at all possible, both socially and legally. Also, they can have a very significant impact on the well-being of people having them.)

Many people also see "sex change" as factually inaccurate. Sex

Sex

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

 in humans is usually determined by four factors:

• Chromosome
  Chromosome
  A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
  
  s
• Gonad
  Gonad
  The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
  
  s (Ovaries
  Ovary
  The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...
  
   and/or testicle
  Testicle
  The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...
  
  s)
• Hormone status
• Primary sex characteristics
  Sex organ
  A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; flowers are the reproductive organs of flowering plants, cones are the reproductive...
  
  , sometimes also secondary sex characteristic
  Secondary sex characteristic
  Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive...
  
  s

Not all of these factors can be changed, however:

• Chromosomes cannot be changed.
• Gonads can be removed, but not replaced
• Hormone status is easily changed
• Existing secondary sex characteristics
  Secondary sex characteristic
  Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive...
  
  can to some extent be changed; existing ones mostly through surgery, non-existing ones can be induced to grow through hormones.
  For example: Changing a male genital anatomy
  Sex reassignment surgery male-to-female
  Sex reassignment surgery from male to female involves reshaping the male genitals into a form with the appearance of and, as far as possible, the function of female genitalia. Prior to any surgeries, trans women usually undergo hormone replacement therapy and facial hair removal...
  
   into a good or even excellent female appearance with functioning is complicated, but entirely possible; changing a female genital anatomy
  Sex reassignment surgery female-to-male
  Sex reassignment surgery from female to male includes a variety of surgical procedures for transsexual men that alter female anatomical traits to provide physical traits more appropriate to the trans man's male identity and functioning....
  
   into an even reasonably male appearing one however is extremely complicated and not successful very often; function is always limited.

Gender reassignment is usually preceded by a period of feminization or masculinization. This is accomplished by the use of either synthetic/natural hormones (such as estrogens, androgens, progesterones and anti-androgens) . The most common period before gender reassignment surgery is two years, as specified by the WPATH standards of care. Body changing hormones are normally started after sufficient counselling, or a period of living 'full-time' (in the target gender) for a minimum of three months.

Natural sex change in humans

Several medical conditions can result in a natural sex change in humans, where the appearance at birth is somewhat, mostly, or completely of one sex, but changes over the course of a lifetime to being somewhat, mostly or completely of the other sex. The overwhelming majority of natural sex changes are from a female appearance at birth to a male appearance after puberty, due to either 5-alpha-reductase deficiency

5-alpha-reductase deficiency

5-Alpha-reductase deficiency is an autosomal recessive intersex condition caused by a mutation of the 5-alpha reductase type 2 gene.-Normal function:...

 (5alpha-RD-2) or 17-beta-hydroxysteroid dehydrogenase deficiency

17-beta-hydroxysteroid dehydrogenase deficiency

17-beta-hydroxysteroid dehydrogenase III deficiency is a rare disorder of sexual development affecting testosterone biosynthesis, which can produce impaired virilization of genetically male infants and children and excessive virilization of female adults.It is an autosomal recessive condition and...

 (17beta-HSD-3). A relative handful of male to female changes have been reported, and the etiologies of these are not well understood.

Genetic females(has two X chromosome

XY sex-determination system

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...

) with congenital adrenal hyperplasia

Congenital adrenal hyperplasia

Congenital adrenal hyperplasia refers to any of several autosomal recessive diseases resulting from mutations of genes for enzymes mediating the biochemical steps of production of cortisol from cholesterol by the adrenal glands ....

 lack an enzyme needed by the adrenal gland to make the hormones cortisol

Cortisol

Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...

 and aldosterone

Aldosterone

Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

. Without these hormones, the body produces more androgen, a type of male sex hormone. This causes male characteristics to appear early (or inappropriately).

Genetic males (has one X and one Y chromosome

XY sex-determination system

The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...

) with androgen insensitivity syndrome

Androgen insensitivity syndrome

Androgen insensitivity syndrome is a condition that results in the partial or complete inability of the cell to respond to androgens. The unresponsiveness of the cell to the presence of androgenic hormones can impair or prevent the masculinization of male genitalia in the developing fetus, as...

 is resistant to male hormone androgen

Androgen

Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...

. As a result, the person has some or all of the physical characteristics of a woman, despite having the genetic makeup of a man. The degree of sexual ambiguity varies widely in persons with incomplete AIS. Incomplete AIS can include other disorders such as Reifenstein syndrome which is associated with breast development in men.