“Sex” is a biological quality or classification of sexually-reproducing organisms, generally female, male, and/or intersex, according to functions that derive from the chromosomal complement, reproductive organs, or specific hormones or environmental factors that affect the expression of phenotypic traits that are strongly associated with females or males within a given species. Hormonal (and environmental) effects, which may be organizational (differentiating) and essentially permanent, or activational, thus possibly reversible, are strongly influenced by the genetic make-up of the individual (Wallen, 2009). Therefore, a range of traits are expressed within each sex, with considerable overlap of “female” and “male” phenotypic traits, especially for “secondary sex characteristics.” Sex may be defined according to:

Problems to Avoid when Analyzing Sex

Problems can arise if researchers assume that:

  • all females (or males) are the same.
  • females and males are different
  • apparent differences between women and men are solely biological.
  • apparent differences between women and men hold across cultures.
  • Genetics: chromosomal make-up (female/male), such as ZW/ZZ (birds and some insects), XX/XO (insects) and XX/XY (most mammals). In mammals the sex-determining region of the Y chromosome, SRY, plays the greatest role in sex differentiation, but because of other transcription factors, such as DAX1 and FOXL2 in females and SOX9 in males, or translocation of the SRY to the X chromosome or an autosome, females and males may have karyotypes other than 46,XX and 46,XY, respectively (see Case Study: Genetics of Sex Determination). Regardless of karyotype, the presence of sex-determining genes means that every nucleated human cell has “sex.” (Note: many species have non-genetic sex-determination systems—see below.)
  • Gametes: germ cells. In species that produce two morphologically distinct types of gametes with each individual producing only one type, the egg-sperm distinction is the basis for distinguishing between females and males, respectively.
  • Morphology: physical traits that differentiate female and male phenotypes.
    • Primary sex characteristics in humans and many other animals include:
      • internal reproductive organs and genitalia, which derive from “bipotential” organs (e.g. “indifferent gonads” that become ovaries or testes) and dual structures. Usually, one structure is maintained and the other regressed. For example, human embryos have both mesonephric and paramesonephric ducts. The former become Müllerian ducts (and form the fallopian tubes, uterus and proximal vagina) in females, but regress in males. The latter become Wolffian ducts (and form the seminal vesicles, epididymis, and ductus deferens) in males, but regress in females.
      • external genitalia, which generally differentiate toward one of two basic forms: distal vagina, labia and clitoris in females; and scrotum and penis in males; nevertheless, external genitalia may not reflect karyotypical or internal genital sex (Fausto-Sterling, 2000).
      • sexually dimorphic prenatal neural structures. Many morphological and functional brain dimorphisms arise during late gestational and neonatal periods. They may be due to differentiating effects of fetal hormones and other sex-biased regulatory mechanisms, including genetic and environmental factors (McCarthy et al., 2011; Jazin et al., 2010).
      • other sexually dimorphic tissues under continuing study. As research on “sex” continues to expand beyond reproduction and neuroscience, sexual dimorphism in other fetal structures will receive increasing attention
    • Secondary sex characteristics are phenotypic traits strongly associated with females or males that become prominent at puberty when the ovaries and testes produce much higher levels of estrogens and androgens, respectively. Often referred to as “gonadal hormones” (though also produced by the adrenal gland and metabolized in many body tissues) or “sex hormones” (though other hormones and genetic factors influence female and male phenotypic traits and “sex hormones” have roles unrelated to sex differentiation), both classes of hormones have important biologic effects in both sexes. For example, estrogens are critical to skeletal development in both sexes, and androgens are responsible for pubic and axillary hair growth at puberty in both sexes.

      Examples of secondary sex characteristics in humans include shorter stature and wider pelvis, breast development, and more fat in the thighs and buttocks in women and broader shoulders, greater muscle mass, more facial and other body hair, and “male pattern” baldness in men. As noted above, these traits vary within each sex and ranges overlap. For instance, many women are taller than many men and some women are stronger than many men—see Analyzing Sex.

      These traits can also be promoted by exogenous hormones. For instance, muscle mass and facial hair will increase in women who take androgens, and breasts and other “female” traits will develop in men who take estrogens.

    Non-genetic sex determination systems are known in many species (Gilbert, 2010). These are diverse and include:
    • Thermal sex determination: In all crocodilians, most turtles, and some other reptiles, sex determination is partially or entirely temperature-dependent. In certain species, sex is genetically determined within a temperature range but environmentally determined outside that range.
    • Age-based sex determination: In some species, such as the slipper snail Crepidula fornicata, all young individuals are male, but some later change to female, depending on their position in a mound of snails.
    • Social sex determination: In many fish species, sex is determined through social interactions with other members of a school. In the echiuroid worm Bonellia viridis, sex is determined by physical environment: Larvae that land on the ocean floor develop as females (~10 cm. long), whereas larvae that are engulfed by a mature female through her proboscis develop as males (~2 mm. long) and live symbiotically.

Intersex may be defined as variations or combinations of what are considered XY male-typical and XY female-typical chromosomal, gonadal, and genital characteristics (Karkazis, 2008; Kessler, 1998).

Works Cited

  • Fausto-Sterling, A. (2000). Sexing the Body: Gender Politics and the Construction of Sexuality. New York: Basic Books.
  • Gilbert, S. (2010). Developmental Biology, 9th Edition. Sunderland: Sinauer Associates.
  • Jazin, E., & Cahill, L (2010). Sex Differences in Molecular Neuroscience: From Fruit Flies to Humans. Nature Reviews, 11 (1), 9-17.
  • Karkazis, K. (2008). Fixing Sex: Intersex, Medical Authority, and Lived Experience. Durham: Duke University Press.
  • Kessler, S. (1998). Lessons from the Intersexed. News Brunswick: Rutgers University Press.
  • McCarthy, M., & Arnold, A. (2011). Reframing Sexual Differentiation of the Brain. Nature Neuroscience, 14 (6), 677-683.
  • Wallen, K. (2009). The Organizational Hypothesis: Reflections on the 50th Anniversary of the Publication of Phoenix, Goy, Gerall, and Young (1959). Hormones and Behavior, 55 (5), 561–565.