The Y chromosome contains the SRY gene, a sex-determining region responsible for the development. The SRY protein causes the gonad's core cells to reproduce rapidly at any expense of the outer layers to form testes. Without the Y chromosome or a defective SRY gene, no such protein is produced, so instead, the outer layers of the gonad to reproduce rapidly to form ovaries.

Rutgers University

The very first display of sex determination begins in the indifferent gonads, these are part of an early vertebrate fetus that may develop into either testes or ovaries. The genetic makeup determines what the gonads develop into. The Y chromosome creates testes through the SRY gene, the lack of such creates ovaries.

Indifferent Gonads

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

There are various individuals that do not have either a XX or XY genetic makeup, or even those that do are still influenced by androgens causing an internal or external difference from those with a typical development. These include: 

•  Turner's Syndrome

•  Congenital Adrenal Hyperplasia (CAH)

•  Intersex

•  Androgen Insensitivity Syndrome (AIS)

•  Changing Sex at Puberty

Possible Changes in the Sexual Differentiation Sequence

The indifferent gonads at this point have been developed into either the ovaries or testes. The testes produce hormones that develops more masculine features, and the lack of these hormones allow the body to develop more feminine. The main difference is which duct system develops, which connects the gonads to the outer body wall. As one system begins development, the other degenerates.

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