In humans, hemophilia is caused by a recessive allele on the X chromosome. Suppose a man with hemophilia has children with a healthy woman whose mother had hemophilia. What is the probability that their second child will have hemophilia? (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

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

Answer 1

50

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 Hemophilia is a disease that interferes with blood clotting. To answer this question, you just need to know that it is a recessive, X-linked disease. That is, it is due to a gene on the X chromosome, and the disease is only expressed when the allele is homozygous (in females) or hemizygous (on the only X, in males).

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 only carry one X chromosome.
        o Since males only carry one X, recessive disease genes on the X are more likely to be expressed. X-linked diseases are more common in males, and females are usually carriers.

Consider Possibilities
• What other information is related to the question? Which information is most useful?
        o In this case, we are told that the mother's mother had hemophilia. She must have been homozygous (since hemophilia is caused by a recessive allele). But her daughter is healthy.
        o Therefore we know that the mother is a carrier of the gene - she is heterozygous.
        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 gametes for the father are XH and Y.
        o The gametes for the mother are XH and X+.
        o There are 4 combinations of offspring : XH / Y, X+ / Y, XH / X+ and XH / XH.
        o So whether boy or girl, in this case the child has a 50% chance of expressing the hemophilia phenotype.
        o Whether it is the first child, the second child, or the tenth child, each child has the same probability.

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 recessive X-linked genes are expressed in males that carry the allele?
        o Did you recognize the information provided by knowing that the maternal grandmother had hemophilia?
        o Did you try drawing a Punnett square to help answer the question?
        o This example is unusual because the father has the disease and the mother is a carrier. So in this case the risk is equal for both males and females. In the population at large, though, we see X-linked diseases more commonly expressed in males. Why?