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

Question

In Drosophila, the allele red eyes (bw+) is dominant to the allele for brown eyes (bw). At another gene locus on the same chromosome, the allele for thin wing veins (hv+) is dominant to the allele for heavy wing veins (hv). Flies homozygous for bw and hv+ are crossed to flies homozygous for bw+ and hv to obtain doubly heterozygous F1 progeny.

A.  1:2:1
B.  1:1:1:1
C.  9:3:3:1
D.  3:1

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

A.  1:2:1

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, not 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 Since the genes are linked, they do NOT segregate like typical unlinked genes. Therefore we would NOT expect the 9:3:3:1 answer which would be typical for two independent, unlinked genes.
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Since the genes are tightly linked, we can approximate them as one gene. Let’s call the bw hv+ combination A, and the bw+ hv combination B.
        o So the P cross is AA x BB...
        o Creating the F1 double heterozygotes who cross together, AB x AB.
        o This 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.

   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?