In mice, the allele for black fur is dominant to the allele for brown fur. If a male heterozygous black mouse is crossed with a female brown mouse, what percentage of the F1 offspring do you predict will be homozygous? ( Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5 )

Question

What will be an ideal response?

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 crosses and genotype ratios?
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

50

Clarify Question
· What is the key concept addressed by the question?
        o This question asks you to determine the genotypic ratio from a cross.
· What type of thinking is required?
        o You need to Apply your understanding of crosses to this example of black and brown mice.
· What key words does the question contain?
        o An allele is a form of the gene. In this case the alleles specify black fur and brown fur in mice. The black allele is dominant over brown – its phenotype is exhibited in the heterozygote. Of the F1 offspring (the offspring from this cross we’re asking about), what percent are homozygous? In other words, what percent have two of the same allele for fur color?

Gather Content
· What do you already know about crosses and genotype ratios?
        o To analyze a cross, you will want to start by writing out the genotypes of the parents.
        o The male heterozygous black mouse must be Bb. Since black is dominant, the female brown mouse must be bb.
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 Bb male makes B gametes and b gametes. The bb female makes only b gametes.
        o So there are only two combinations, in equal proportions: Bb and bb.
        o The bb offspring are homozygous – so they constitute 50% of the offspring.

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 Apply your knowledge and understanding to this unfamiliar situation.
        o When in doubt, write it out. Don’t be afraid to use pen and paper to help solve genetics problems.
        o Also don’t be afraid to make up your own gene abbreviations. In this case, we used “B” and “b” for the dominant and recessive alleles of the black/brown fur color gene.