How an individual's sex is determined has been one of the great questions of embryology since antiquity. Aristotle, who collected and dissected embryos, claimed that sex was determined by the heat of the male partner during intercourse. The more heated the passion, the greater the probability of male offspring. (Aristotle counseled elderly men to conceive in the summer if they wished to have male heirs.) Aristotle (ca. 335 b.c.e.) promulgated a very straightforward hypothesis of sex determination: women were men whose development was arrested too early. The female was “a mutilated male” whose development had stopped because the coldness of the mother's womb overcame the heat of the father's semen. Women were therefore colder and more passive than men, and female sexual organs had not matured to the point at which they could provide active seeds. This view was accepted by the Christian Church and by Galen (whose anatomy texts were to be the standard for over a thousand years). Around the year 200 c.e., Galen wrote:

Just as mankind is the most perfect of all animals, so within mankind, the man is more perfect than the woman, and the reason for this perfection is his excess heat, for heat is Nature's primary instrument … the woman is less perfect than the man in respect to the generative parts. For the parts were formed within her when she was still a fetus, but could not because of the defect in heat emerge and project on the outside.

The view that women were but poorly developed men and that their genitalia were like men's, only turned inside out, was a very popular one for over a thousand years. As late as 1543, Andreas Vesalius, the Paduan anatomist who overturned much of Galen's anatomy (and who risked censure by the church for arguing that men and women have the same number of ribs), held this view. The illustrations from his two major works, De Humani Corporis Fabrica and Tabulae Sex, show that he saw the female genitalia as internal representations of the male genitalia (Figure 17.1). Nevertheless, Vesalius' books sparked a revolution in anatomy, and by the end of the 1500s, anatomists had dismissed Galen's representation of female anatomy. During the 1600s and 1700s, females were seen as producing eggs that could transmit parental traits, and the physiology of the sex organs began to be studied. Still, there was no consensus about how the sexes became determined (see Horowitz 1976; Tuana 1988; Schiebinger 1989).

Figure 17.1

Vesalius’ representations (1538, 1543) of the female reproductive organs. (A) Vesalius’ rendering of Galen's conception of the female tract from the vagina to the uterus. (B) Rendering of the female reproductive system. (Reprinted in Schiebinger (more...)

Until the twentieth century, the environment—temperature and nutrition, in particular—was believed to be important in determining sex. In 1890, Geddes and Thomson summarized all available data on sex determination and came to the conclusion that the “constitution, age, nutrition, and environment of the parents must be especially considered” in any such analysis. They argued that factors favoring the storage of energy and nutrients predisposed one to have female offspring, whereas factors favoring the utilization of energy and nutrients influenced one to have male offspring.

This environmental view of sex determination remained the only major scientific theory until the rediscovery of Mendel's work in 1900 and the rediscovery of the sex chromosome by McClung in 1902. It was not until 1905, however, that the correlation (in insects) of the female sex with XX sex chromosomes and the male sex with XY or XO chromosomes was established (Stevens 1905; Wilson 1905). This finding suggested strongly that a specific nuclear component was responsible for directing the development of the sexual phenotype. Thus, evidence accumulated that sex determination occurs by nuclear inheritance rather than by environmental happenstance.

Today we know that both environmental and internal mechanisms of sex determination can operate in different species. We will first discuss the chromosomal mechanisms of sex determination and then consider the ways by which the environment regulates the sexual phenotype.

Contents

• Chromosomal Sex Determination in Mammals
• Chromosomal Sex Determination in Drosophila
• Environmental Sex Determination
• Snapshot Summary: Sex Determination
• References