Gonad, indifferent

In embryonic life, the gonad in males and females is initially identical.

This gonad is said to be 'indifferent' before it differentiates into a definitive testis or ovary.

An indifferent gonad becomes a testis if the embryo has a Y chromosome, but if the embryo has no Y chromosome, the indifferent gonad becomes an ovary.

Thus, an XY chromosome complement leads to testes while an XX chromosome complement leads to ovaries.

People with just one X chromosome and no Y are females with Turner syndrome, and are infertile.

The absence of a Y chromosome permits the indifferent gonad to become an ovary, but both X chromosomes are needed for the ovary to function normally