Given that these two gene loci are very closely linked, the phenotypic ratio in the F2 generation should be closest to

Question

A.  1 brown eye, heavy wing veins: 2 red eye, thin wing veins: 1 brown eye, thin wing veins
B.  1 brown eye, thin wing veins: 2 red eye, thin wing veins: 1 red eye, heavy wing veins
C.  3 red eye, thin wing veins: 1 brown eye, heavy wing veins
D.  1 brown eye: 1 red eye: 1 heavy wing veins: 1 thin wing veins

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 genetic linkage?
 
Consider Possibilities
· Consider the different answer options. Which can you rule out?

  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

B.  1 brown eye, thin wing veins: 2 red eye, thin wing veins: 1 red eye, heavy wing veins

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses genetic linkage.
· What type of thinking is required?
        o You need to Analyze the information given, using logic, to dissect the problem and determine the answer.
· What key words does the question contain?
        o This question describes two gene loci (singular is gene locus). Each gene’s abbreviation uses two letters – bw for brown and hv for heavy veins. The genotypic ratio refers to the alleles in the DNA, and the phenotypic ratio refers to the physical phenotypes.
 
Gather Content
· What do you already know about genetic linkage?
        o Genetic linkage refers to genes that lie close to each other on the chromosome.
        o Because their loci are linked, they are not segregated by independent assortment.
 
Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o In the previous question, we determined the genotypic ratio to be 1:2:1.
        o The phenotypes are a reflection of the genotypes, so we should expect phenotypes in the same 1:2:1 ratio.
        o We can rule out the answer with a 1:1:1:1 ratio.
        o The three genotypes produce three distinct phenotypes, so we can also rule out the answer with a 3:1 ratio.

  Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o The cross will produce four categories of offspring in the F2: AA, AB, BA, and BB. But AB and BA are the same genotypically. So the genotypic ratio is 1:2:1 of AA: AB: BB.
        o AA stands for bw hv+/ bw hv+, or brown eyes and thin wing veins.
        o AB stands for bw hv+/ bw+ hv, or red eyes and thin wing veins (the dominant phenotypes).
        o BB stands for bw+ hv/ bw+ hv, or red eyes and heavy wing veins.
        o So we get 1 brown, thin: 2 red, thin: 1 red, heavy.
 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 recognize that you can treat the two tightly linked genes like a single gene?
        o Did you write out the crosses and draw a Punnett square to determine the answer?
        o Did you correctly match the genotypes to the phenotypes?