In fruit flies (Drosophila melanogaster) there is a dominant allele for red eyes and a recessive allele for white eyes. These alleles are located on the X chromosome. If a heterozygous red-eyed female is mated with a white-eyed male, what percentage of the offspring are expected to be white-eyed females? (Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5)

Question

Fill in the blank(s) with the appropriate word(s).

Answer 1

25

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 Although you may not have seen this example before, you know enough about the concept to Apply your knowledge and understanding to this unfamiliar situation.
• What key words does the question contain?
        o Dominant alleles are expressed in the heterozygote; recessive alleles are only expressed in the homozygote.

Gather Content
• What do you already know about X-linked inheritance?
        o X-linked inheritance works differently than autosomal inheritance. For females, genes act similarly. But males carry just one X chromosome and the other paired chromosome is the Y chromosome. 

Consider Possibilities
• What other information is related to the question? Which information is most useful?
        o Punnett squares are a useful way to determine both genotype and phenotype ratios.

Choose Answer
• Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Try drawing a Punnett square. Write the possible gametes on the top and the side.
        o The female gametes are w+ and w-.
        o The male gametes are w- and Y.
        o There are four combinations of offspring : w+/ w-, w-/ w-, w+/ Y and w-/ Y in equal ratios.
        o So 25% of the offspring will be white-eyed females: w-/ w-.

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 to this unfamiliar situation.
        o Did you remember that the X chromosome is paired with the Y chromosome?
        o Did you try drawing a Punnett square to help answer the question?