Gender:  In the Genes or in the Jeans?

Section A

Just how does the Y chromosome determine maleness? 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 can’t conduct comparable experiments on humans. But once again, we can look for "experiments of nature," where these conditions naturally occur.

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.

Questions

1. 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?
    
   1. An individual who has XX chromosomes, with a translocated SRY gene attached to one X chromosome.
       
   2. An individual who has XY chromosomes, but the SRY gene has been permanently deleted from the Y chromosome.
2. What do these results tell you about the factors that are normally responsible for the differentiation of male and female gonads? Fill in the gonad part of the flowchart to reflect your answer.

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