Module 3

Module 3—The Male and Female Reproductive Systems

Lesson 4—Chromosome and Hormone Influence

Get Focused

The first couple in “A Fertility Case Study,” Jane and John, decided not to have children until they were in their early thirties. Their first pregnancy produced gender-different twins, also called fraternal or dizygotic (di, meaning “two;” zygotic, meaning “zygote”) twins. In fraternal twins, each embryo has a unique makeup because it comes from a separate egg fertilized by a separate sperm. Gender is dependent on the chromosomal makeup of the embryos, and fraternal twins are often different sexes.

To maintain the variation and continuation of a mammalian species, such as the human species, two separate genders are required. In a species with two genders, there are two different sex chromosomes at play—the X and Y chromosomes. These are referred to as sex chromosomes because they determine the gender, or sex, of the offspring. To be a male, an organism must have one X chromosome and one Y chromosome (XY). To be a female, an organism must have two X chromosomes (XX). Further information about the structure and function of chromosomes will be provided in Unit C.

The gender of a child is determined at conception by the genetic material carried in the sperm. While all eggs carry the X chromosome, a sperm can carry either an X or a Y chromosome. Although it was once believed that female development would just occur unless “maleness” was stimulated by the presence of the Y chromosome, researchers now think the process of becoming female is much more intricate.

While the genders of Jane and John’s twins were determined at conception, they remained identical in gender until about the sixth or seventh week of their embryonic development. At this point, the sex-determining region Y (SRY) carrying the testis determining factor (TDF) gene, found on the Y chromosome of their male fetus, began the process that leads to the sexual development of a male individual. The TDF gene started the production of the male sex hormones, collectively called androgens, which began the development of the male sex organs. The lack of the Y chromosome with the TDF gene and the resulting male sex hormones allowed the other fetus to develop as female.

To develop the male twin, the TDF gene caused differentiation of the supporting cells into Sertoli cells 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 caused the gender 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, it does appear that without the presence of testosterone and other hormones to suppress the development of female sex characteristics, other hormones then cause female reproductive structures to develop.

Although the ways in which embryos develop as male or female differ, both are controlled by genes and hormones. For the twins, their genders were determined at conception, and sex characteristics developed as they underwent fetal growth. They will not display all of their gender-specific characteristics until they reach puberty. You will learn more about puberty later in Module 3.

In Lesson 4 you will learn the roles of the X and Y chromosomes and the hormones they produce to better understand gender differentiation and general hormone changes during puberty. You will investigate the following focusing questions:

• How is gender determined at conception?

• How do the X and Y chromosomes and hormonal influences form gonads and reproductive organs in the female and male embryo and fetus?

Module 3: Lesson 4 Assignment

There is no assignment for this lesson. As the influence of the male and female chromosomes and their hormones is not complete in forming male and female characteristics until after puberty, the assessment for the material presented in this lesson will be included in a later lesson assignment.

You must decide what to do with the questions that are not marked by the teacher.

Remember that these questions provide you with the practice and feedback that you need to successfully complete this course. You should respond to all of the questions and place those answers in your course folder.

While you are completing this lesson, there will be many opportunities for you to acquire, understand, and practise the concepts that are presented to you. As you complete these activities, as well as your summary notes, you will file everything in your course folder to reference when you are preparing for exams.

Remember that you also have the option of trying additional questions from the textbook for further practice. Consult with your teacher for the answers to these questions. The Key will also provide you with many Diploma Exam-style multiple-choice, numerical-response, and written-response questions that will be an excellent review of the module. Practising your responses to these types of questions is good preparation for the Diploma Exam.