Sex determination is the biological process that establishes the sex of an organism. While often associated with humans, it's a fundamental aspect of reproduction across diverse species, involving a fascinating array of genetic and environmental mechanisms.

In humans, sex determination is primarily governed by sex chromosomes. Females typically possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). The presence or absence of the Y chromosome, and specifically a gene on it called the SRY gene (Sex-determining Region Y), plays a crucial role in male development.

While chromosomal sex determination is common, many other fascinating mechanisms exist in nature. These include environmental factors, genetic ratios, and even social cues.

In many reptiles, such as turtles and crocodiles, the sex of the offspring is determined by the temperature at which the eggs are incubated. Different temperature ranges trigger the development of either male or female reproductive organs. This mechanism highlights the significant influence of environmental factors on genetic expression.

In some insects, like bees and ants, sex determination follows a system called haplodiploidy. Fertilized eggs develop into diploid females (possessing two sets of chromosomes), while unfertilized eggs develop into haploid males (possessing a single set of chromosomes). This system has significant implications for their social structures and colony dynamics.

Some species exhibit sex determination influenced by environmental or social factors. A classic example is the clownfish, where the largest and most dominant individual in a group is the female, and the next largest is the breeding male. If the female dies, the breeding male transforms into a female, and the next largest juvenile becomes the breeding male. This demonstrates a remarkable plasticity in sex determination.