Sex is determined at fertilisation, and depends on the individual’s genes. Those genes direct the body to grow in the womb, and later on in childhood and puberty, along either the male or the female developmental pathway.

Once a mammal’s body has developed along either male or female lines, most of that development cannot be undone. Moreover, many of the distinctive features of an adult body of the opposite sex cannot be developed without genes that are possessed only by people of that sex. A male person cannot grow a uterus, for example, and a female person cannot grow a penis.

People can alter the appearance of their bodies in order to disguise or change some (but not all) of the bodily features that differ between males and females.

Superficial and temporary interventions include clothing, makeup, wigs and prostheses (for example, fake breast forms or a “packer”, which creates the impression of penis and testicles when placed in a female person’s underpants). A boy or man may “tuck” his penis and testicles between his legs using specialist underpants. A girl or woman may bind her breasts to give the appearance of a male chest underneath clothing (if this is done often, it can cause serious damage to breast tissue, ribs, lungs and breathing capacity).

Some changes intended to give the appearance of the opposite sex are permanent. These include cross-sex hormones and surgery to alter or remove reproductive organs. Such measures are often referred to as “medical transition” or “sex change”. However, they do not actually change a person’s sex and are incapable of creating functional reproductive organs of the opposite sex. (See “What is medical transition?”)