The biology of reproduction is a story that is 4,5 billion years long. The origin of sexual reproduction can be traced to early prokaryotes, which first developed around two billion years ago. Animals that reproduce sexually have two biological genders, called sexes: female and male.

In general, an individual's biological sex can be distinguished based on its genitals, shape, size, appearance or behavior. In some animal species, gender is determined by the influence of the environment. For example, flea eggs develop into females in good conditions, whereas some of the eggs develop into males due to malnutrition or cold temperatures.

In many reptile species (e.g. turtles, lizards, alligators, and crocodiles), sex is determined by the temperature of the eggs during hatching. For these species, the temperature of the environment has the same function as the sex hormones in the XY system, where the gender of an individual is determined by its sex chromosomes.

Oysters are hermaphrodites, but their male and female genitals do not develop simultaneously. Usually, young oyster individuals are males. In old age, they become females. 

In mammals and birds, sex is an inherited characteristic. This means that an individual's sex is determined by the chromosomes it receives from their parents.