In Drosophila, dosage compensation is controlled by the male-specific lethal (MSL) complex consisting of MSL proteins and roX RNAs. Based on what you know about dosage compensation, the role of the MSL complex in males would be to

Question

A.  double the level of expression of genes on the X chromosome.
B.  increase the level of expression of genes on the X chromosome by 50%.
C.  decrease the level of expression of genes on the X chromosome by 50%.
D.  decrease the level of expression of genes on the X chromosome by 100%.
E.  double the level of expression of genes on the Y 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 dosage compensation?

  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

A.  double the level of expression of genes on the X chromosome.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses dosage compensation.
· What type of thinking is required?
        o Although you may not have seen this example before, you know enough about dosage compensation to Apply your knowledge and understanding to this unfamiliar situation.
· What key words does the question contain?
        o Dosage compensation is the process that ensures an equal level of expression from the sex chromosomes despite a differing number of sex chromosomes in males and females. The MSL complex and roX RNAs are the names of genes that participate in the process in flies.

  Gather Content
· What do you already know about dosage compensation?
        o Dosage compensation is the process that ensures an equal level of expression from the sex chromosomes despite a differing number of sex chromosomes in males and females.
        o If there were no dosage compensation, females would have twice the level of X-linked gene expression as males.
        o This would create problems in the cell, because the products of X-linked genes have to work in the correct proportions with the products of autosomal genes.

  Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o There are few genes on the Y chromosome, and most of those are on the X as well (in the pseudo-autosomal region). So dosage compensation does not affect Y-linked genes.
 
Choose Answer
 · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o So for male fruit flies, would dosage compensation need to increase or decrease gene expression?
        o Flies use the XX/XY system, so male flies have one copy of the X chromosome.
        o Therefore, male flies need to have increased gene expression relative to females.
        o How much more expression do the males need to equal that of females (who have two X chromosomes)? 
        o Twice as much – so dosage compensation doubles the expression of X-linked genes.

  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 Apply your knowledge and understanding about dosage compensation to this unfamiliar situation.
        o Did you read carefully enough to understand that this question asked specifically about male fruit flies?
        o Did you recognize that the function of dosage compensation is to equalize gene expression between the sexes?
        o Did you realize that dosage compensation in flies uses a different mechanism than in mammals? In mammals, the dosage of the X is lowered in females, rather than raised in males.
        o Why do you think the genes were named “male-specific lethals”?