Module 3 Lesson 4 - 2

Sex Development

The genetic sex of a child is determined at conception by the genetic material carried in the sperm. Although all eggs carry the X chromosome, a sperm can carry either an X or a Y chromosome. In the past, scientists believed that female development occurred unless 'maleness' was stimulated by the presence of the Y chromosome, researchers now think the process of becoming female is much more intricate.

At about the sixth or seventh week of the embryonic development, the sex-determining region Y (SRY) carrying the testis-determining factor (TDF) gene found on the Y chromosome of the male fetus begins the process that leads to the sexual development of a male individual. The TDF gene starts the production of the male sex hormones, collectively called androgens, which begin the development of the male sex organs. The lack of the Y chromosome with the TDF gene and its resulting male sex hormones allow a fetus to develop as female.

To develop as male, the TDF gene causes differentiation of the supporting cells into Sertoli and interstitial cells. The interstitial cells begin to form the hormone testosterone by about the eighth week of gestation. From the eighth to the twelfth week of gestation, the testosterone causes the sex-identical structures to differentiate into male structures such as testes, penis, and scrotum. Although the way in which an embryo becomes female is not fully understood, evidence is that, without the presence of testosterone and other hormones to suppress the development of female sex characteristics, female hormones and reproductive structures develop.

Although the ways in which embryos develop as male or female differ, both are controlled by genes and hormones.  Sexes are determined at conception and develop throughout the fetal growth. Infants display all their sex-specific characteristics until they reach puberty, which is a topic later in this unit.

Read "Sex Hormones and the Male Reproductive System" on pages 492 and 493 of the textbook and "Sex Hormones and the Female Reproductive System" on page 495 of the textbook to begin your understanding of sex development in the fetus.

Try This

Beginning in the mid-1960s, female athletes competing at the international level were required to undergo sex-verification tests to ensure that they were not men posing as women, for example. Although widespread sex testing of athletes has been suspended, individuals in certain sports are still screened. Do you think that the determining an individual’s sex should be straightforward?

You may have noticed that there are often exceptions to the rules in biological studies. This is the case with respect to the X and Y chromosomes that determine sex. During gamete formation, the TDF gene can be transferred incorrectly or it can be missing.

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