People with only one sex chromosome (X, as a single Y can not survive development). This is defined as Turner's syndrome, a female with underdeveloped ovaries because there is no SRY gene present.

Turner's Syndrome

XX individuals can also be exposed to androgens in utero causing them to be masculinized. One possible cause is congenital adrenal hyperplasia (CAH) where the adrenal glands instead of producing corticosteroids, produce androgens. 

Congenital Adrenal Hyperplasia (CAH)

The androgen receptor is on the X chromosome. If the gene is dysfunctional in XY individuals, there is insufficient androgen receptors resulting in the wolffian ducts being unable to sense testosterone and regress. 

Androgen Insensitivity Syndrome (AIS)

Intersex refers to an atypical genital development that resembles an intermediate between typical male and female appearance. There are several possible causes for this condition. Hormone levels can be controlled after birth, but external appearance cannot be modified without surgical intervention.

Intersex

There are individuals with a rare genetic mutation that causes 5α-reductase, the enzyme that converts testosterone to DHT, to be disabled. An XY individual with this condition will have testes and other internal male structures, but the external genitalia will not fully masculinize. Affected individuals are typically raised female until puberty when androgen production increase and the external genitalia is more masculinized. 

Changing Sex at Puberty

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

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

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.

Gonadal Hormones

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.

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