In white tigers, the absence of fur pigmentation is caused by a recessive allele that also causes the tigers to be cross-eyed. If two tigers heterozygous for this allele mate, what do you expect to see among the offspring?

Question

A.  1/4 will be both white and cross-eyed, 3/4 will be orange with normal eyes.
B.  1/4 will be white with normal eyes, 1/4 will be white and cross-eyed, 1/4 will be orange with normal eyes, and 1/4 will be orange and cross-eyed.
C.  1/16 will be white and cross-eyed, 15/16 will be orange with normal eyes.
D.  1/3 will be both white and cross-eyed, 2/3 will be orange with normal eyes.
E.  1/4 will be orange with normal eyes, 3/4 will be both white and cross-eyed.

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?

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

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

A.  1/4 will be both white and cross-eyed, 3/4 will be orange with normal eyes.

Clarify Question
· What is the key concept addressed by the question?
        o This question asks about the phenotypic ratio that results from a cross of two heterozygotes. This is similar to the monohybrid cross we studied in the example of Mendel’s purple and white pea flowers.
· What type of thinking is required?
        o You need to Analyze the information to determine the phenotype ratio.
· What key words does the question contain?
        o A recessive allele is one that is only exhibited in the phenotype of a homozygote. The parents in this cross are heterozygous – they carry two different alleles.

Gather Content
· What do you already know about monohybrid crosses?
        o Recall that we saw a monohybrid cross like this for purple and white flower color in peas.

Consider Possibilities
· What other information is related to the question?
        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 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. Just make up your own abbreviation – say, W (orange) and w (white). Fill in the 4 combinations.
        o You should get WW, Ww, wW and ww. Which are the phenotypes of each?
        o Remember, the w allele is recessive for both white fur and crossed-eyes. So only ww are white and cross-eyed. The other three categories are orange with normal eyes.

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 For questions like this, try drawing a Punnett square.
        o Remember that even though there are three genotypes – WW, Ww, and ww – they are not in equal proportions. There are two categories of Ww offspring, giving a 1:2:1 genotype ratio, and a 3:1 phenotype ratio. So only 1/4 of offspring are the white, cross-eyed ww tigers.