Reproductive Case Study 7
Terry and the Spotted Hyena
After school that day Terry went to the mall with her friends Tiffany and Melissa. They were excitedly getting ready for summer camp and shopping for swimsuits. They always had a great time at the mall, trying on makeup and perfume, picking out clothes, and checking out guys. Terry came out of the dressing room modeling a shocking pink bikini.
"Hey, whatÕs that?" asked Tiffany, pointing to some small scars just above TerryÕs bikini line.
"ItÕs nothing," Terry answered. "Some kind of operation I had as a baby. I donÕt even remember it."
Just then Melissa dashed over with a turquoise swimsuit. "Oh, Ter, this is so you!" she squealed, interrupting their conversation.
Terry loved it, and ended up buying both suits.
Later that night as Terry was lying in bed, she ran her fingers idyly across the scars on her abdomen. What was it her mother had told her about that operation? Something about "boy stuff" that got into her by mistake, before she was even born, and they had to take it out.... "Boy stuff?!?" What could that mean? What do boys have inside there? Terry pulled her biology book off the shelf and flipped to the picture of the reproductive system. Gonads.... Testes.... Did she have testes before she was born...? "But that canÕt be," she thought, "IÕm a girl!"
In 1990, scientists found that the crucial part of the chromosome was a segment on the short arm, which they called SRY, for Sex-determining Region of the Y chromosome. Subsequently this was narrowed down even further, with the discovery of a crucial gene. When this gene was injected into a fertilized mouse egg with an XX genotype, the eggs developed into a mouse with testes. (In fact, mice produced in this way were male in all respects, not just their gonads--they had male genitals and, as adults, they behaved like normal males would when an adult female was in the vicinity.) The protein that this gene coded for was called TDF, for Testes Determining Factor, and this single gene seemed to act like a switch, turning on the development of the male gonad.
Does the same gene cause the gonads to develop into testes in humans, too? Obviously, we canÕt conduct comparable experiments on humans. But once again, we can look for "experiments of nature," where these conditions naturally occur.
Sometimes errors occur during the meiotic divisions that lead to gamete formation (sperm and eggs). Instead of each sex chromosome going to a different gamete, two of these chromosomes might fail to separate, and so end up going to a single gamete, while the sister gamete would then get no sex chromosome. In other words, gametes may occur that either lack any sex chromosome or that have extra sex chromosomes.
During meiosis, a process called translocation sometimes occurs in which a gene on one chromosome may move to another chromosome that happens to be nearby. After screening thousands of people, scientists have found that this process may occur with the SRY gene.
Different processes are involved in differentiation of the gonads and the ducts. The gonads begin as a single tissue that can differentiate in either the male or female direction, depending on whether or not a Y chromosome is present. But when it comes to differentiation of the ducts, the Y chromosome is not important. Rather, the ducts begin as two sets of tissues, each of which can differentiate in only one direction (either male or female) depending on whether or not testicular endocrine and paracrine signals are present.
What about the external genitals? In the early embryo, we see that their development seems to be like the gonads, that is, the male and female genitals are indistinguishable until about four to eight weeks of fetal life, at which time the tissue develops in either a male or female direction.
1.) Desribe the mechanism by which a peptide hormone initiates a cell response; a steroid hormone initiates a cell response. How are they the same? How are they different? Give a specific example of each and state their effect on the human body.
2.) Compare the hormonal controls of the male reproductive cycle with the hormonal controls of the female reproductive cycle. How are they the same? How are they different?
1. Discuss the role of chromosomes and hormones in the determination of sex (gender- male/female).
Assume that differentiation of gonads in humans works in a way similar to
that in mice. What kind of gonad would you expect to develop in each of these conditions?
a. An individual who has XX chromosomes, with a translocated SRY gene attached to one X chromosome.
b. An individual who has XY chromosomes, but the SRY gene has been permanently deleted from the Y chromosome.
3. What do these results tell you about the factors that are normally responsible for the differentiation of male and female gonads?
4. Terry doesnÕt realize it yet, but she did have testes when she was born. Suggest at least three scenarios that might explain TerryÕs situation.
5. Is either of these scenarios a satisfactory explanation of TerryÕs biological condition? Why or why not? (You may want to read again about the mouse experiment that was described earlier.)
6. Most people have a sex that is consistent on all levels: genetic, gonadal, internal genitals, external genitals. But sometimes there are discrepancies, as weÕve seen. In these cases, how should one determine whether a person is considered male or female? Is Terry a male or a female? Why do you think so?
7. What does any of this have to do with the Spotted Hyena (Crocuta crocuta)?
adapted from Gender: In the Genes or in the Jeans?
William J. Hoese, California State University Fullerton
Judith Gibber, Columbia University
Bonnie Wood, University of Maine Presque Isle
National Center for Teaching Science Using Case Studies