Let P = purple flowers and p = white, and T = tall plants and t = dwarf. Of the 16 possible gamete combinations in the dihybrid cross between 2 double heterozygotes, how many would produce the phenotype white, tall?

Question

A.  none
B.  1
C.  3
D.  9
E.  16

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 dihybrid crosses? How does it relate to the question?

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

C.  3

Clarify Question
· What is the key concept addressed by the question?
        o This question requires you to work out the results of a dihybrid cross, using the Punnett square approach. Jotting on scrap paper will help.
· What type of thinking is required?
        o You must Analyze to determine the 16 gamete combinations produced by the cross, and then count how many of them have a white, tall phenotype.
· What key words does the question contain?
        o A dihybrid cross is a cross between two dihybrids, or double heterozygotes – organisms that are heterozygous for two different genes. So a dihybrid cross is Aa Bb x Aa Bb. Or in this case, Pp Tt x Pp Tt.

Gather Content
· What do you already know about dihybrid crosses? How does it relate to the question?
        o You may remember that Punnett squares are a good way to work out the results of a dihybrid cross. A pen and paper will help.

Consider Possibilities
· What other information is related to the question? Which information is most useful?
        o You may decide there are only two genotypes that produce white, tall plants – TT pp and Tt pp. This is correct. But careful reading of the question shows that it asks for the proportion of the 16 gamete combinations. These 16 combinations are the results of drawing the Punnett square, which differentiates between the T (maternal) t (paternal) pp combination and the t (maternal) T (paternal) pp combination.

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o You could do this in your head, but drawing a Punnett square on scrap paper is an easier approach. Write the possible parental gametes along both the top and side: PT, Pt, pT, and pp. Fill in the 16 combinations, then circle the ones that have a white, tall phenotype.

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 Some questions may be deceptively short, but require some time to work out the correct answer. Don’t be shy about using pen and paper to help you by diagramming the cross– working geneticists do it all the time!