At an autosomal gene locus in humans, the allele for brown eyes is dominant over the allele for blue eyes. At another gene locus, located on the X chromosome, a recessive allele produces colorblindness while the dominant allele produces normal color vision. A heterozygous brown-eyed woman who is a carrier of colorblindness has a child with a blue-eyed man who is not colorblind. An ultrasound test shows that the child is a girl. What is the probability that she will be colorblind? (Enter the probability as a percent. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5)

Question

What will be an ideal response?

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 and autosomal inheritance?

  Consider Possibilities
· What other information is related to the question? Which information is most useful?

  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

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Clarify Question
· What is the key concept addressed by the question?
        o This question addresses both X-linked and autosomal inheritance.
· What type of thinking is required?
        o You need to Analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain?
        o Autosomal refers to genes on a non-sex chromosome. X-linked genes are on the X chromosome, like the colorblindness gene here. 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 and autosomal inheritance?
        o X-linked genes have different patterns of gene expression because males have only the one X chromosome.
        o Since girls have two X chromosomes, they must always receive the one X chromosome from their father.

  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.
        o Do you need to worry about the eye color gene to answer this question? No!

  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. You will have to create your own abbreviations for the gene alleles.
        o Let’s call the colorblindness gene C. To answer this question, we can ignore the eye color gene.
        o The mother is a carrier for colorblindness (C/c). Her gametes are C and c.
        o The father is not colorblind (C/Y) . His gametes are C and Y.
        o This creates four combinations in the offspring.
        o Consider just the offspring which are female (since we know the child is a female). The females are CC or Cc.
        o Since the colorblindness allele is recessive, both categories of females will have normal vision. There is a 0% chance of a daughter being colorblind.

  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 Analyze the information given, using logic, to dissect the problem and determine the answer.
        o Did you recognize that a daughter will always receive the X from her father?
        o Did you try drawing a Punnett square, or at least writing out the cross, to help answer this question?