Special cells that can develop into gonads where meiosis takes place are
set aside very early in embryology.

The tissue is called "ovotestes" since it can develop into either ovaries
or testes.

Mullerian ducts are present that can develop into female internal
components.

Wollfian ducts that can develop into male components are present.

If a Y chromosome is present all the cells will display Hy antigen, a cell
surface protein coded by a gene on the Y chromosome.

After about 7 weeks, another Y encoded gene (called Tdffor testes
determining factor, or Sryfor sex-region of Y) is activated and directs
ovotestes to develop into testes.

Testes produce androgens, including testosterone, which induces
development from the Wolffian ducts and regression of the Mullerian
ducts

After about 11 weeks, in the absence of a Y chromosome (actually lack of
a functional TDF gene), female development is initiated with the
conversion of the ovotestes into ovaries, regression of Wollfian ducts, and
development of Mullerian ducts.

Hormonal changes at puberty further advance sexual development.
Gonadotropic hormones (FSH and LH) from the pituitary, via the
hypothalamus, start monthly cycle of egg maturation and release in
females which increases estrogen, which decreases FSH and LH, etc. In
males, trace levels of the same hormones lead to induction of secondary
sex characteristics, (beards, voice, etc.). Adrenal hormones lead to
puberty changes in females. Very few if any of the genes needed for
sexual development/identity may be environmental. It becomes difficult
to separate genetic effects from non-genetic effects since males and
females often are treated with "different expectations".