Let Y = yellow and y = green, and R = round and r = wrinkled. You cross YYRR peas with yyrr peas. All of the F1 individuals are yellow and round with a genotype of YyRr. You then perform an F2 cross and get the expected 9:3:3:1 phenotypic ratio. What proportion of the F2 plants are expected to be heterozygous for both traits?

Question

  Let Cy = curly wings and Cy+ = wild type. In Drosophila, the Cy allele behaves as a dominant mutation that produces curly wings in the heterozygous condition (Cy/Cy+), but also behaves as a recessive lethal mutation. Flies homozygous for the Cy allele die before reaching adulthood.

A.  1/16
B.  1/4
C.  1/2
D.  9/16

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?

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

B.  1/4

Clarify Question
· What is the key concept addressed by the question?
        o You may recognize that this is a dihybrid cross, like the example in the text.
· What type of thinking is required?
        o You need to Analyze the information to determine the genotype ratios.
· What key words does the question contain?
        o The genotype refers to the alleles present, while the phenotype is the visible trait, like “round” or “wrinkled”. F1 and F2 are the generations of crosses. The plants we are looking for are heterozygous for both traits. In other words, they have two different alleles for each trait, or Yy and Rr.

Gather Content
· What do you already know about dihybrid crosses?
        o Dihybrid crosses produce numerous categories of offspring. It can be hard to keep track of them all without a method of diagramming the cross. What method have you learn for keeping track of genotypes and phenotypes from a cross?

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 different gametes for a YyRr parent are: YR, Yr, yR, and yr.
        o There are 16 combinations of offspring .
        o Which offspring are heterozygous for both traits , or YyRr? (Four of the sixteen categories have this genotype, or 1/4.)

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 Writing down the cross is helpful with questions like these. Even better, draw the Punnett square.