Prenatal Hormone Exposure and Sexual Variation

mined? In high school biology class you probably learned a basic rule for mammals: Individuals with one X and one Y chromosome develop into males. Those with two X’s develop into females. Distinguishing between the resulting males and females should be straightforward. Unfortunately for those who like simple rules, the story is far more complex. X and Y chromosomes are only the beginning of sex determination. Biologists have recently taken a closer look at the events between fertilization and sexual maturity that establish an individual’s sexual characteristics. These events help explain variability among individuals in sexual anatomy, physiology and behavior. For example, an XX individual can exhibit some masculine traits if hormone production or sensitivity is abnormal during early development. In fact, there is almost a continuum of sexual traits between male and female. The ability of environmental influences during development to produce such a continuum demonstrates that another classical dichotomy, between nature and nurture, is in fact a synergy: Genes and environment work together to produce an organism. Specific genes are only turned on when the environment of the cell, tissue, organ or organism calls for them. In the case of sexual characteristics, one mechanism by which environmental variables exert their influence is through the activity of hormones. Hormones are substances released by cells into the bloodstream, where they travel throughout the body and influence the function of other, distant cells. Hormone molecules themselves, or the enzymes that produce those molecules, are encoded by the genome, but hormone concentrations can be modulated by a wide array of environmental factors, including stress, food consumption, temperature, and time of year. In turn, hormone concentrations modulate the expression of genes in a variety of different cell and tissue types, producing anatomical, physiological and behavioral differences. My own studies in rodents have helped show how hormones affect development in the womb, producing differences that later manifest themselves in adult morphology, physiology and sex-related behavior. A detailed understanding of the sensitivity of the fetus to hormones will play an important role in the evaluation of the controversial impact of hormone-mimicking environmental substances on animal and human health and development.