In most species on Earth, sex is determined by the presence or absence of certain sex chromosomes. In humans, for example, females have two X chromosomes, while males have one X and one Y chromosome.

However, there are a few exceptions to this rule. In some species, sex is determined by environmental factors, such as temperature or the presence of certain chemicals.

In flies of the species Drosophila miranda, for example, some cells can't tell whether they're male or female. This is because these cells lack a functional copy of the sex-determining gene Sxl. As a result, these cells can adopt either male or female characteristics, depending on the environmental conditions.

If the flies are raised in a warm environment, the cells will develop into males. If the flies are raised in a cool environment, the cells will develop into females.

The ability of some fly cells to change sex is a remarkable example of how environmental factors can influence the development of an organism.