Two parents each have blood type AB, and their first child is blood type AB. What is the probability that their second child will have blood type AB? ( Express 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 monohybrid crosses, probability, and co-dominance?

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

50

Clarify Question
· What is the key concept addressed by the question?
        o This question asks about the result of a cross between two heterozygotes for one gene – a monohybrid cross. The gene for blood type has co-dominant alleles A and B.
· What type of thinking is required?
        o You need to Apply your understanding of monohybrid crosses, probability, and co-dominance to answer this question.
· What key words does the question contain?
        o Blood type AB is the phenotype of heterozygotes who carry both the HbA allele and the HbB allele. Those alleles are co-dominant – they are both exhibited in the heterozygote.

Gather Content
· What do you already know about monohybrid crosses, probability, and co-dominance?
        o You may recall that monohybrid crosses produce a 1:2:1 genotype ratio. In the case of co-dominant alleles like IA and IB, the phenotype ratio is also 1:2:1.
        o This question asks about the likelihood that the second child will have blood type AB – what genotype is this? (IA / IB).
        o Will the genotype of the first child have any bearing on the result for the second child? No! They are independent events.

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 What is the genotype of the parents?(IA / IB for both).

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 both parents are IA and IB.
        o This creates 4 categories of offspring: IA / IA , IA / IB , IB / IA , and IB / IB.
        o Two of the four categories will be blood type AB . So there is a 50% probability that any given child will have blood type AB.

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 IA and IB are codominant alleles?
        o Did you realize that the result for the first child does not affect the result for the second?
        o Did you write out the cross and the Punnett square to help answer the question?
        o Did you recognize that even though the two IA / IB categories have the same genotype, they contribute twice to the ratio? One category receives the A allele from the mother, and the B from the father, and vice versa. This is where a Punnett square can be particularly helpful.
        o Did you read carefully enough? Would the answer have been different if the question had said “What is the likelihood that both their first and second child will both have blood type AB?”