what proportion of the long-eared offspring will be homozygous for the long-eared allele?

Question

A.  1/8
B.  1/4
C.  1/3
D.  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 monohybrid 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

C.  1/3

Clarify Question
· What is the key concept addressed by the question?
        o This question seems to be asking about a cross involving two genes, similar to Mendel’s dihybrid cross. But in fact, does the coat color gene need to be considered to answer the question? Since the question addresses only the ear phenotype, you can ignore the coat color gene entirely. Remember, for unlinked genes one gene does not affect the assortment of the other. So you can treat this as a cross between two heterozygotes for the ear length gene – a monohybrid cross.
· What type of thinking is required?
        o This question requires you to Analyze the information to determine the genotype 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 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. Since the question only asks about the long-ear genotype, you can ignore the B/W gene.

Gather Content
· What do you already know about monohybrid crosses?
        o You may recall that monohybrid crosses produce a 3:1 phenotype ratio, and a 1:2:1 genotype ratio. What does this question ask you to determine? (genotype ratio)

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 parental rabbits heterozygous for ear length? (Tt)
        o What is the genotype of the offspring in question with long ears? (TT or 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 both heterozygous parents are T and t. (Remember, we can ignore the fur color gene to answer this question.)
        o This creates 4 categories of offspring: TT, Tt, tT, and tt. Even though Tt and tT are the same genotype, they constitute 2 of the 4 categories, because one is T (maternal) t (paternal) and the other is t (maternal) T (paternal).
        o Of the four categories, which have long ears? (TT, Tt, and tT)
        o So, considering JUST the long-eared offspring, what proportion of them will be homozygous TT? One of the three categories. So 1/3 of long-eared offspring are homozygous.

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 you could ignore fur color to answer the question?
        o Did you recognize that even though the two 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.
        o Did you realize that you should answer the proportion of just the long-eared offspring, not all offspring? You can still use the Punnett square ratios – just ignore the short-eared tt category, and determine how many of the three remaining categories are homozygous (one).