In rabbits, there is a gene that controls ear length, with a dominant allele "T" for long ears and a recessive allele "t" for short ears. At another gene locus, there are alleles “B” for black coat and “W” for white coat. Neither the B or W allele is dominant, and BW produces a gray coat. These two allele pairs assort independently. If a gray rabbit that is heterozygous at the gene locus controlling ear length is mated with a white rabbit that is also heterozygous at the gene locus controlling ear length, what is the probability that their first offspring will be gray with long ears?

Question

A.  1/16
B.  3/16
C.  1/8
D.  3/8
E.  1/2

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 two-gene crosses and independent assortment?

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

D.  3/8

Clarify Question
· What is the key concept addressed by the question?
        o This question asks about a cross involving the independent assortment of two genes. This is similar to Mendel’s dihybrid cross, although in this case one parent is homozygous for one of the genes. Also, one gene has dominant and recessive alleles, but the other gene has an incompletely dominant allele: when heterozygous, the “black” allele (B) only turns the fur gray.
· What type of thinking is required?
        o This question requires you to Analyze the information to determine the phenotype ratio of the offspring.
· What key words does the question contain?
        o Pay close attention to the description of the alleles. The dominant allele will exhibit its phenotype in the hybrid, but the recessive allele will exhibit its phenotype only when homozygous. The alleles of the fur color gene are incompletely dominant. In the heterozygote, fur is gray – an intermediate phenotype. The two genes assort independently – this tells you that they are either on separate chromosomes or far apart on a chromosome, and the alleles end up in the gametes randomly. If a gamete gets a T allele, it could also get either a B or a W from the other gene.
        o Don’t be confused by the gene abbreviations. The abbreviation for the ear length gene is T or t. For the fur gene, however, the abbreviations are different letters: B or W. From the question, though, we know that these are alleles of the same gene. So there are two genes in this scenario: the ear length gene T/t, and the fur color gene B/W.

Gather Content
· What do you already know about two-gene crosses and independent assortment?
        o Recall the dihybrid cross from the text. A two-gene cross can produce as many as 16 categories of offspring. This is not a problem to do in your head!

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 gray rabbit heterozygous for ear length? (BW Tt)
        o What is the genotype of the white rabbit heterozygous for ear length? (WW Tt)
        o What is the genotype of the offspring in question, gray with long ears? (BW TT or BW Tt)

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 gray heterozygous (BW Tt) rabbit are B T, B t, W T, and W t.Gametes for the white heterozygous (WW Tt) rabbit are W T and W t.
        o This creates 8 categories of offspring. Of these 8 categories, 3 are gray with long ears: BW TT, BW Tt, and BW tT. So the phenotype ratio is 3/8, and the likelihood that any individual rabbit will be gray with long ears is 3/8.

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 Did you use a pen and paper to help with this question?
        o Did you diagram the categories of offspring with a Punnett square?
        o Did you recognize that there are two different genotypes that correspond to “gray with long ears”? (BW TT and BW Tt)
        o Did you recognize that even though the two BW Tt categories have the same genotype, they contribute twice to the ratio? One category receives the T allele from the mother, the other category from the father. This is where a Punnett square can be particularly helpful.