Sex determination refers to the mechanisms employed by organisms to produce offspring that are of two different sexes. First we present an overview of the sex determination mechanisms used by mammals. Then we discuss the great variety of mechanisms used by animals other than mammals.

Mammalian Mechanisms

A developing mammalian embryo's gender is determined by two sequential processes known as primary and secondary sex determination.

Primary Sex Determination.

Early in an embryo's development (four weeks after fertilization, in humans), two groups of cells become organized into the gonad rudiment that will eventually develop into either the ovaries or testicles. These gonads will eventually be the source of gametes in the adult. However, at this early stage they are unstructured organs that lack sex-specific features but have the potential to develop into gonads.

The first visible indication of sex-specific development, occurring in week seven in humans, is...