As would be predicted since many hormones are involved, each of which
is produced via biosynthetic pathways, many different mutations can
affect normal sexual development. Most of these genes are not sex-linked
or holandric, but are located on somatic chromosomes.

Examples of genetic defects that alter sex development include:

Tfm, an X-linked gene that codes for testosterone receptors. Females who
are XTfmXtfmare normal. Their XtfmY progeny, expected to be males, develop
into sterile females (the condition is called testicular feminization or
complete androgen insensitivity). Although the ovotestes become testes
and produce testosterone, it has absolutely no effect, so the outward and
mental development becomes female. Unfortunately, the Mullerian ducts
disintegrate so there is no chance of fertility, and the undescended testes
should be removed after puberty to diminish the risk of cancer.

In guevodoces, infants are classified as female at birth based on external
genitalia. At puberty they develop into fully functional males ; which they
generally have claimed to be all along.

When hormone therapy or surgery is used, the best bet is to make the
appearance match the "brain-sex" of the individual.