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Sex determination is the process by which an organism's sex is determined, typically at the time of fertilization. In most species, including humans, sex is determined by the presence of specific sex chromosomes.

In humans, sex determination is based on the presence of two sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The presence of the Y chromosome triggers the development of male characteristics, while the absence of the Y chromosome results in the development of female characteristics.

Sex determination can also be influenced by environmental factors, such as temperature or social cues. For example, in some reptiles, the temperature at which the eggs are incubated can determine the sex of the offspring.

In some species, such as birds and some fish, sex determination is not based on sex chromosomes but on other genetic factors. For example, in birds, the presence of two different sex chromosomes (ZW in females and ZZ in males) determines sex.

Sex determination is a complex process that can be influenced by a variety of genetic and environmental factors. It plays a crucial role in reproduction and the development of sexual characteristics in organisms.