Gender:  In the Genes or in the Jeans?

Section A

Terry enjoyed biology lab. She was especially thrilled that today’s class involved looking at her own cells under the microscope. Like the other ninth graders, she and her lab partner, Robert, had each taken a swab full of cells from inside their cheeks, smeared them on a few microscope slides, and stained them with cresyl violet. Now, as they examined them under the microscope, they searched for those dark bodies that Mr. Wilson had told them to look for.

"Okay, which one of us is supposed to have that Barr body?" Terry asked.

"Whatever body you have is fine with me," answered Robert.

Terry rolled her eyes and ignored him as she grabbed another slide to put under the microscope. "Hey, I still can’t find any," she said. "What are they supposed to look like again?"

Finally, Terry and Robert called Mr. Wilson over to look at their slides. "Don’t worry about it," he said. "The way the cells are put on the slide makes it difficult to see the inactivated X chromosome in all of a girl’s cells. Look at Melissa’s sample to see what the Barr body looks like."

Mr. Wilson called the class to attention. "So, what did you learn? Robert, how are male chromosomes different from female?"

"We’re better," Robert said with a smirk, "cause we’ve got something the girls don’t -- a Y chromosome."

"Oh, no," Terry retorted. "You guys are defective! You’re missing something awesome that we’ve all got -- that second X chromosome!"

Mr. Wilson laughed as the bell rang. "Okay, it seems like you’ve all gotten the point. Females have two X chromosomes, while males have an X and a Y chromosome. Now, write up your lab reports, and I’ll see you tomorrow."

Questions

1. Terry and Robert have mentioned two alternative hypotheses that might explain the role of sex chromosomes in determining the sex of an individual.
    
   • Hypothesis 1:  Male development occurs because of the presence of a Y chromosome.
      
   • Hypothesis 2:  Male development occurs because of the absence of a second X chromosome.
   
   Fill in the table below with the words "male" or "female" to indicate which phenotype you’d expect to see if Hypothesis 1 were correct, and which you’d expect if Hypothesis 2 were correct.
    
    

   Gametes		Zygote	Predicted phenotype if "maleness" is determined by the ...
   Egg	Sperm	Genotype of sex chromosomes	presence of a Y chromosome (Hypothesis 1)	absence of a second X chromosome (Hypothesis 2)
   X	X	XX
   X	Y	XY

   
    
2. Do the appearances of males and females allow us to distinguish between these two hypotheses?

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