sex differentiation
the process of acquiring distinctive sexual features during the course of development. Human sexual differentiation is determined genetically at the time of fertilization, primarily by the presence or absence of a Y chromosome. Fertilized eggs containing a Y chromosome develop as male embryos, whereas ones lacking a Y chromosome develop as females. This is due to the presence on the Y chromosome of a particular gene, called SRY (sex reversal on Y). It encodes a testis-determining factor that, via a cascade of signals, triggers the development of testes and other male reproductive organs. In the absence of this gene, the embryo develops along the default pathway, with ovaries and other female organs.