Congenital Adrenal Hyperplasia (CAH)—One in 13,000 births
Two hormones are critical in normal sex differentiation. The testes of normal 46,XY males secrete both Müllerian Inhibiting Substance (also known as MIS or antiMüllerian hormone) and masculinizing androgenic hormones, while the ovaries of a normal 46,XX female secrete neither. In CAH, the absence of a critical enzyme allows a 46,XX fetus to produce androgens, resulting in ambiguous external genitals. A CAH individual may have an oversized clitoris and fused labia.
Testosterone Biosynthetic Defects—One in 13,000 births
In a condition related to CAH, some 46,XY individuals do not have the properly functioning enzymes needed to convert cholesterol to testosterone. When such enzymes prove completely incapable of creating testosterone, the genitals appear female; when the enzymes function at a low level, ambiguous genitals form.
Androgen insensitivity syndrome(AIS)—One in 13,000 births
AIS affects the section of the 46,XY population that is physically unable to react to androgens. In Complete AIS (CAIS), testes exist in the abdomen while the external genitals are female. The Wolffian, or male, duct structures do not form because of the lack of response to androgens. The Müllerian, or female, duct structures do not evolve because the testes still release MIS. At puberty, CAIS individuals grow breasts but do not menstruate. The testes are sometimes removed from the abdomen because they may develop cancer.
Partial AIS (PAIS) is marked by a limited response to androgens. The external genitals are ambiguous and duct development is incomplete. Depending on the selection of hormone treatment, PAIS individuals may exhibit partial male or partial female development at puberty.
Gonadal Dysgenesis—One in 150,000 births
In gonadal dysgenesis, the androgen receptors are intact while the androgen-secreting testes are not. Complete Gonadal Dysgenesis, in which neither androgens nor MIS are produced, yields female genitals and Müllerian duct formation, despite a genetic profile suggesting maleness. With estrogen treatment, female puberty can be achieved. Partial Gonadal Dysgenesis results in ambiguous genitals and duct development, as some androgens and MIS are produced. Like PAIS, the choice of hormone treatments determines the physical gender of the adult with Partial Gonadal Dysgenesis.
5-Alpha Reductase Deficiency—No estimate available
5-Alpha Reductase is the enzyme that facilitates the conversion of testosterone to another hormone, dihydrotestosterone (DHT). When a genetic male is deficient in 5-Alpha Reductase, the powerful DHT hormone is not produced. While testes and Wolffian ducts do exist, the male external genitals are similar in size to those of a normal female. If left intact, an adult 5-Alpha Reductase Deficiency individual will appear generally male but with small genitals and no facial hair.
Micropenis—No estimate available
In order to create a proper penis in a 46,XY individual, androgens must be secreted twice during fetal life. First, the androgens help to shape the basic structures into a penis and scrotum; later, the androgens enlarge the penis. A micropenis is the result of normal androgen secretion in the first stage and little or no androgen secretion in the second. The penis is normal in shape and function, but extremely small in size. While earlier surgeons often converted micropenises to female genitals, today micropenises are often left intact. Individuals with intact micropenises are often given testosterone to stimulate masculinizing puberty.
Klinefelter syndrome—One in 1,000 births
Sometimes chromosomes join but do not form standard 46,XX or 46,XY combinations. Individuals with Klinefelter Syndrome are genetically 47,XXY and live as men. Small penis and testes, low androgen secretion, and possible female breast development are characteristics of this syndrome.
Turner syndrome—No estimate available
Like Klinefelter Syndrome, Turner Syndrome is marked by an abnormal karyotype, 45,XO. While Turner women have female external genitals, the individuals lack properly formed ovaries. Without estrogen treatment, no breast growth occurs. Other possible features of Turner Syndrome include short stature, webbing of the neck, and misshapen internal organs.
Timing Defect—No estimate available
If all of the proper stages of normal male sex differentiation occur, but the timing is incorrect by just days, errors may arise. The occasional outcome in a 46,XY individual with this timing defect is ambiguous external genitals.
The information above was adapted from Syndromes of Abnormal Sex Differentiation, written by physicians at The Johns Hopkins Children's Center in Baltimore, Maryland. The statistics on frequency were obtained from the Intersex Society of North America (www.isna.org).
This feature originally appeared on the site for the NOVA program Sex: Unknown.