In 1910, Morgan did a series of experiments with the fruit fly Drosophila, an organism where females are XX and males are XY. When a mutant male fly with white eyes was crossed with a wild type female with red eyes, none of the F1 progeny had white eyes but 18% of the F2 progeny had white eyes. Unexpectedly, all of these white-eyed F2 flies were males. Based on these results, Morgan concluded that white eyes is caused by a recessive X-linked allele. Suppose Morgan had found that half of the F1 progeny had white eyes but all of these white-eyed F1 flies were females. In this case, a valid hypothesis would be

Question

A.  White eyes is caused by a recessive Y-linked allele.
B.  White eyes is caused by a dominant Y-linked allele.
C.  White eyes is caused by a dominant X-linked allele.
D.  White eyes is caused by a dominant autosomal allele.

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 X-linked inheritance?

  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

C.  White eyes is caused by a dominant X-linked allele.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses X-linked inheritance.
· 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 Dominant alleles are expressed when heterozygous, but recessive alleles are expressed only when homozygous or on the only X chromosome in males (hemizygous).
 
Gather Content
· What do you already know about X-linked inheritance?
        o X-linked genes have different patterns of gene expression because males have only the one X chromosome.
        o Since females have two X chromosomes, they must always receive the one X chromosome from their father. Males always receive one of their mother’s X chromosomes, and the Y from their father.

  Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Since the phenotype is only seen in one sex of the progeny, we can rule out the gene sitting on an autosome. If it was, both sexes would receive it equally.

  Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Write out the imaginary cross (“Suppose...”) to answer this question. Call the white-eyed allele w-, and the red allele w+.
        o The original male is w- / ?, and the female is w+/ w+.
        o In this imaginary scenario, all the FEMALES of the F1 are white-eyed. What would this mean?
        o They would have received the mutant allele from the white-eyed father, so it would have to be on the X. Females never receive a Y – or they would become males!
        o If the w- / w+ females exhibited the phenotype, that would mean that w- was dominant over w+.
        o Thus, a dominant X-linked allele.

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 females never inherit the Y chromosome?
        o Did you try writing out the cross to help answer the question?