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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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What Is Sex?

Roughly speaking, sex can be considered in terms of three categories: genotypic sex, phenotypic sex, and gender. Genotypic sex refers specifically to an individual's two sex chromosomes. Most people have either two X chromosomes (genotypic female) or an X and a Y chromosome (genotypic male). Phenotypic sex refers to an individual's sex as determined by their internal and external genitalia, expression of secondary sex characteristics, and behavior. If everything proceeds according to plan during development (Box A), the XX genotype leads to a person with ovaries, oviducts, uterus, cervix, clitoris, labia, and vagina—i.e., a phenotypic female. By the same token, the XY genotype leads to a person with testicles, epididymis, vas deferens, seminal vesicles, penis, and scrotum—a phenotypic male. Gender refers more broadly to an individual's subjective perception of their sex and their sexual orientation, and is therefore harder to define than genotypic or phenotypic sex. Generally speaking, gender identity entails self-appraisal according to the traits most often associated with one sex or the other (called gender traits), and these can be influenced to some degree by cultural norms. Sexual orientation also entails self-appraisal in the context of culture. For purposes of understanding the neurobiology of sex, it is helpful to think of genotypic sex as largely immutable, phenotypic sex as modifiable (by developmental processes, hormone treatment, and/or surgery), and gender as a more complex construct that is determined culturally as well as biologically.

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Box A

The Development of Male and Female Phenotypes.

Clearly, then, genotypic sex, phenotypic sex, and gender are not always aligned. Variations in alignment can be minor, or they can challenge the usual definitions of female and male and lead to psychosocial conflicts and sexual dysfunction (see Box B). Genetic variations include individuals who are XO (Turner's syndrome), XXY (Klinefelter's syndrome), or XYY. Each of these genotypes has its own particular phenotype. Other genetic variations arise from mutations in genes coding for hormone receptors or for the hormones themselves. For instance, a metabolic disorder that leads to overactive adrenals during maturation, called congenital adrenal hyperplasia (CAH), causes abnormally high levels of circulating androgens and hence, along with severe salt imbalance, an ambiguous sexual phenotype. In addition to having a large clitoris and fused labia at birth, females with CAH typically exhibit “tomboyish” behavior as children and tend to form homosexual relationships as adults. High levels of circulating androgens from the adrenals may cause sexually dimorphic brain circuitry to have a male rather than female organization, leading to more aggressive play and the eventual choice of a female sexual partner.

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Box B

The Case of John/Joan.

An example of a mutation in a gene responsible for hormone receptors is androgen insensitivity syndrome (AIS), also called testicular feminization. The receptor deficiency leads to the development of the internal genitalia of a male and the external genitalia of a female in an individual who is genotypically XY. Thus, people with androgen insensitivity syndrome look like females and self-identify as female, even though they have a Y chromosome. Since they are generally not aware of their condition until puberty, when they fail to menstruate, they see themselves and are experienced by others as female. Thus, their gender identity matches external sexual phenotype, but not genotype. Although this syndrome is relatively uncommon (about 1 in 4000 births), there are some well-known examples of individuals thought or known to have had AIS (e.g., Joan of Arc and Wallis Simpson, the woman for whom King Edward of England gave up his throne).

Another variation in the alignment of genotype, phenotype, and gender is genotypic males who are phenotypic females early in life, but whose sexual phenotype changes at puberty. As infants and children, these individuals are phenotypic females because they lack an enzyme, 5-α-reductase, that promotes the early development of male genitalia (see Box A). Such children have somewhat ambiguous but generally female-appearing genitalia (they have labia with an enlarged clitoris, and undescended testes). As a result, they are generally raised as females. At puberty, however, when the testicular secretion of androgen becomes high, the clitoris develops into a penis and the testes descend, changing these individuals into phenotypic males. In the Dominican Republic and Haiti, where this congenital syndrome has been thoroughly studied in a particular pedigree, the condition is referred to colloquially as “testes-at-twelve.” Such individuals generally exhibit male gender behavior at puberty, and most eventually live as males.

The term used to describe all these variations is “intersexuality.” Taken together, these individuals make up approximately 1–2% of all live births. In addition to the more clearly defined categories of Klinefelter's syndrome, Turner's syndrome, CAH, AIS, and 5-α-reductase deficiency, subtle permutations and combinations of genes, hormones, and environment give rise to a large number of biological and behavioral possibilities. In all these permutations and combinations, the relevant brain circuitry established early in development generally determines sexual behavior and identity (Box B).

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10943

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