Kaplan Bio Subject Test 1 Question 30

[Axis8Mundi]

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I have a concern with the following problem:

Red is dominant over white in a certain flower. Which of the following is a valid way to test whether a red offspring is homozygous or heterozygous in this flower?

I. Cross it with a red plant that gad a white parent

II. Cross it with a red plant that had two red parents

III. Cross it with a white plant

A. I only
B. II only
C. I and III only
D. II and III only
E. I, II, and III

Now, the answer that kaplan provides is C, I and III only. The explanation states that if the cross in II were performed, crossing R_ with RR, you would not be able to determine the genotype. If the unknown allele was recessive, it would be masked by the homozygous dominant red genotype.

My concern here is that the Option II only states that the parents are red, not that they are homozygous. Now, isn't it possible that the parents are red and also heterozyous since Red is dominant over white? In that case, the cross would be between R_ and Rr, which could produce a white genotype if the flower in questions is also Rr.

 

[Jepstein30]

I have a concern with the following problem:

Red is dominant over white in a certain flower. Which of the following is a valid way to test whether a red offspring is homozygous or heterozygous in this flower?

I. Cross it with a red plant that gad a white parent

II. Cross it with a red plant that had two red parents

III. Cross it with a white plant

A. I only
B. II only
C. I and III only
D. II and III only
E. I, II, and III

Now, the answer that kaplan provides is C, I and III only. The explanation states that if the cross in II were performed, crossing R_ with RR, you would not be able to determine the genotype. If the unknown allele was recessive, it would be masked by the homozygous dominant red genotype.

My concern here is that the Option II only states that the parents are red, not that they are homozygous. Now, isn't it possible that the parents are red and also heterozyous since Red is dominant over white? In that case, the cross would be between R_ and Rr, which could produce a white genotype if the flower in questions is also Rr.

RR x __
Rr x __

I. RR x Rr and Rr x Rr
the first cross will produce only red flowers
the second cross will produce 75% red and 25% white

II. RR x RR and Rr x RR
OR
RR x Rr and Rr x Rr

first pair of crosses would produce all red flowers (no difference)
second pair of crossing would have all red vs. 75% red and 25% white

III. RR x rr and Rr x rr
first cross would be all red
second cross would 50/50

yea, you're right. they are not considering that one of II's parents could be heterozygous and pass on the recessive gene to its offspring.. which would still have red flowers if it got the dominant gene from its other red parent.

 

[Axis8Mundi]

Thank you, Jepstein30! I really appreciate you taking the time to reply to my question.