A scientist has an uncharacterized pea plant. She wants to determine the plant’s genotype for seed color. In order to be certain that she will get conclusive results from a single cross, with what plant could she cross it? Check all that apply.

Question

_____  a green-seeded pea plant (yy)
_____  a yellow-seeded pea plant (YY)
_____  itself

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 testcrosses?
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

__X__  a green-seeded pea plant (yy)
_____  a yellow-seeded pea plant (YY)
__X__  itself

Clarify Question
· What is the key concept addressed by the question?
        o This question asks you to consider the results of various crosses, and indicate which would serve as a testcross to determine the parental genotype.
· What type of thinking is required?
        o You must Analyze the information to determine which plant(s) would work.
· What key words does the question contain?
        o The scientist wants to determine the plant’s genotype – the alleles it carries. Is it YY, Yy, or yy?

Gather Content
· What do you already know about testcrosses?
        o We learned about Mendel’s use of the testcross. He used homozygous recessive plants to cross to the uncharacterized plant. Because they carry two recessive alleles, only the traits from the tested individual contribute to the phenotype.
        o But is this the only possibility for this example?

Consider Possibilities
· What other information is related to the question? Which information is most useful?
        o What strategy can you use to determine which cross(es) will provide the needed information?
        o What about considering the results of each possibility in turn?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o With a pen and paper, diagram each of the three possible answer choices.
        o First, consider that the uncharacterized plant may be YY, Yy, or yy. Our goal is to distinguish between these possibilities. Which crosses will differentiate them?
        o If you cross the unknown plant to a green-seeded plant (yy), what will result? If the unknown plant is YY, then YY x yy ® 100% Yy yellow seeds. If it is Yy, then Yy x yy ® 50% Yy yellow seeds, 50% yy green seeds. If it is yy, then yy x yy ®100% yy green seeds.  The three possibilities distinguish the genotype. This is the traditional testcross.
        o If you cross the unknown plant to a yellow-seeded plant (YY), what will result? If the unknown plant is YY, then YY x YY ® 100% YY yellow seeds. If it is Yy, then Yy x YY ® 50% YY yellow seeds, 50% Yy yellow seeds. If it is yy, then yy x YY ®100% Yy yellow seeds.  Although the genotypes vary, the phenotypes are ALL yellow seeds. This cross provides no information at all.
        o What about selfing the mystery plant? If the unknown plant is YY, then YY x YY ® 100% YY yellow seeds. If it is Yy, then Yy x Yy ® 25% YY yellow seeds , 50% Yy yellow seeds, and 25% yy green seeds. If it is yy, then yy x yy ®100% yy green seeds. Three different phenotype ratios distinguish the possibilities. This also works!

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 So although self-fertilization is not the traditional testcross, it does also work to determine the plant’s genotype. However, the recessive testcross is a more foolproof method – can you imagine why?
        o To answer this question, did you try using pen and paper to write out the crosses? With three possible mystery plant genotypes, and three possible tester plants, that’s 9 different crosses to consider. So you can see that writing it out will help!