In watermelons, the gene for green color is dominant over its allele for striped color. At another gene locus, the gene for short shape is dominant over its allele for long shape. These two allele pairs assort independently. If a plant with long, striped fruit is crossed with a plant that is heterozygous for both of these traits, what percentage of the offspring do you expect will be short and green? ( 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 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

25

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 recessive for both genes.
· 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, and is typically written with a capital letter. The recessive allele will exhibit its phenotype only when homozygous, and is written with a lower-case letter. You might call the alleles G (green), g (striped), S (short) and s (long). 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 G allele, it could also get either an S or an s.

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.

  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 only gametes for the striped, long (gg ss) fruit are gs. The gametes for the heterozygous fruit (Gg Ss) are GS, Gs, gS, and gs.
        o This creates 4 categories of offspring: Gg Ss, Gg ss, gg Ss, and gg ss. Only the first category Gg Ss (1/4) is both green and short.
        o The green and short phenotypes are determined by the contribution from the heterozygous parent. Since they assort randomly, there is a 50% chance of getting green or short, but only a 25% chance of getting both.

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 come up with gene abbreviations for the two genes, and match them to the alleles correctly, with green and short dominant?
        o Did you diagram the categories of offspring with a Punnett square?