Suppose you are carrying out a series of crosses with an insect where the mechanism of sex determination is unknown. You discover a mutant female with short bristles and decide to cross it with a wild type male that has normal bristles. Half of the F1 progeny have short bristles but all of these short-bristled F1 progeny are males. Based on these results, a valid hypothesis would be

Question

A.  Males are ZW, females are ZZ, and short bristles are caused by a dominant allele on the Z chromosome.
B.  Males are ZZ, females are ZW, and short bristles are caused by a recessive allele on the Z chromosome.
C.  Males are ZZ, females are ZW, and short bristles are caused by a dominant allele on the W chromosome.
D.  Males are ZZ, females are ZW, and short bristles are caused by a dominant allele on the Z chromosome.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain?

  Gather Content
· What do you already know about sex determination systems?

  Consider Possibilities

· Consider the different answer options. Which can you rule out?
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

D.  Males are ZZ, females are ZW, and short bristles are caused by a dominant allele on the Z chromosome.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses “X-linked inheritance”, but in the context of the ZZ/ZW sex determination system.
· What type of thinking is required?
        o This answer requires you to weigh and judge evidence – to Evaluate – and choose the best of the possible answers.
· What key words does the question contain?
        o Sex determination is the mechanism by which the gender of the individual is specified.

  Gather Content
· What do you already know about sex determination systems?
        o Remember, there are two common types of chromosomal sex determination systems: XX/XY and ZZ/ZW.
        o In the ZZ/ZW system, the female is the heterogametic sex, with Z and W chromosomes.
  Consider Possibilities

· Consider the different answer options. Which can you rule out?
        o Looking at the answer options, we see they all involve the ZZ / ZW system.
        o You may recall that in the ZZ / ZW system, it is the females that are ZW. (Otherwise we just call it XX and XY.) So you can rule out the answer that says males are ZW.
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o If the insect uses the ZZ / ZW system, then the female (with the short bristles) is ZW. The short bristles allele is likely on the Z. Let’s call her genotype ZS / W.
        o So the male with normal bristles is Z+/ Z+.
        o Only the sons exhibit the short bristles phenotype.
        o Yet they are ZS/ Z+. Therefore the mutant allele is dominant.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question required you to weigh and judge evidence – to Evaluate – and choose the best of the possible answers.
        o Did you remember that in the ZZ / ZW system, males are ZZ?
        o Did you try writing out the cross to help answer the question?         o Are there other possible explanations? Could the allele have been on the W chromosome? Could the allele have been on an autosome?