Biological sex determination in humans is a complex developmental process that begins at conception. It involves a precise sequence of genetic instructions and subsequent hormonal influences guiding the formation of reproductive structures. This intricate journey establishes the biological sex of a developing baby over several weeks of pregnancy.
The Genetic Starting Point
A baby’s biological sex is established at fertilization. Every egg carries an X sex chromosome, while sperm can carry either an X or a Y chromosome. If a sperm carrying an X chromosome fertilizes the egg, the resulting chromosomal combination is XX, leading to female development. Conversely, if a sperm carrying a Y chromosome fertilizes the egg, the combination is XY, which sets the genetic foundation for male development. This chromosomal pairing is the primary step in determining biological sex, directing subsequent developmental events.
Gonadal Differentiation
During early embryonic development, all human embryos possess “indifferent” gonads, which are structures with the potential to develop into either testes or ovaries. The presence of a Y chromosome introduces a specific gene called SRY (Sex-determining Region Y). Located on the Y chromosome, this gene acts as a master switch for male development. The activation of the SRY gene triggers the indifferent gonad to differentiate into testes, a pivotal event that begins around 6 to 7 weeks of gestation. This transformation marks when the primary male sex organs begin to form, steering the embryo towards a male developmental pathway.
Hormonal Influence and Organ Formation
Once the testes have differentiated, they become endocrine glands, producing hormones for male development. The two primary hormones involved are testosterone and Anti-Müllerian Hormone (AMH). Testosterone stimulates the Wolffian ducts, which are embryonic structures present in both sexes, to develop into internal male reproductive organs, including the epididymis, vas deferens, and seminal vesicles. Testosterone also influences the development of external genitalia, such as the penis and scrotum, with this external development completed by approximately 12 weeks of gestation.
Concurrently, the Sertoli cells within the developing testes produce Anti-Müllerian Hormone (AMH). AMH causes the regression of the Müllerian ducts, which would otherwise develop into female internal reproductive organs like the uterus, fallopian tubes, and upper vagina. The coordinated action of testosterone and AMH ensures the male reproductive system forms completely while preventing female internal structures. This hormonal orchestration leads to the masculinization of the embryo.