Module 3S1 Reproductive Systems

Lesson 2.3.4S1

Lesson 4—Chromosome and Hormone Influence

 

Get Focused

 

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

 

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 and one Y chromosome (XY). To be a female, an organism must have two X chromosomes (XX). Further information on 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’ is 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 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, hormones, and reproductive structures 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 developed as they underwent their fetal growth. They will display all of their gender-specific characteristics until they reach puberty. You will learn more about puberty in Module 3: Section 2.

 

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 puberty hormone changes. You will investigate the following focusing questions:

• How is gender determined at conception?

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

Module 3: Section 1—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 the Module 3: Section 2 Assignment.

Here is a tutorial video for this lesson that you can watch if it suits your learning style.  Bio30 tut#2.3.4S1

 

** The Self-Check and Try This questions in this lesson are not marked by the teacher; however answering these questions will help you review important information and build key concepts that may be applied in future lessons. You can respond to these mentally, write out your response, or record your answer in any other way that works for you.