Question 1

In a hypothetical plant pigmentation pathway, three genes (A, B, and C) each encode enzymes that catalyze sequential steps from a colorless precursor to a brown product. If an individual has the genotype aaBbCc, what will be the phenotype of this plant?

Accepted Answer

Colorless

Question 2

You perform a trihybrid cross of two AaBbCc heterozygotes in the plant pigmentation pathway described above. What proportion of the progeny will be red? (Assume red phenotype requires $$A\_B\_cc$$ genotype.)

Accepted Answer

$$\frac{9}{64}$$

Question 3

In the same pathway, you cross two AaBb x AaBb heterozygotes. You select one colorless offspring and backcross to a heterozygous parent. What is the probability that the first F2 is orange? (Assume orange requires $$A\_bb$$ genotype.)

Accepted Answer

$$\frac{1}{4}$$

Question 4

Which of the following statements about genetic interactions and epistasis is correct?

Accepted Answer

Genotype ratios are not affected by genetic interaction; only phenotype ratios change.

Question 5

A plant with genotype AAbbCC is crossed with a plant with genotype aaBBcc. What is the expected phenotype of the F1 progeny, assuming all genes are unlinked and dominant alleles are required for each step in the pathway?

Accepted Answer

Orange intermediate

Question 6

In a four-gene pathway (A, B, C, D), all wild-type alleles are required for brown pigment. If you cross two BbDd individuals (all other loci are homozygous wild-type), what is the probability that their offspring will be colorless?

Accepted Answer

$$\frac{1}{16}$$

Question 7

You cross two BbDd individuals in the four-gene pathway. You then select one brown and one orange F1. What is the probability their first offspring is orange? (Assume all other loci are wild-type.)

Accepted Answer

$$\frac{1}{4}$$

Question 8

In snapdragon flowers, loss of function in gene b (bb) results in white flowers, regardless of the genotype at gene a. What type of genetic interaction is this an example of?

Accepted Answer

Recessive epistasis

Question 9

Which of the following best describes how the order of gene action in a pathway can be inferred from genetic data?

Accepted Answer

By identifying which gene's mutation is epistatic to others and the presence of intermediate phenotypes

Question 10

A double mutant in snapdragon flowers (aabb) is white. The single mutant aa is light red, and bb is white. What does this indicate about the relationship between genes a and b?

Accepted Answer

Gene b is epistatic to gene a

Question 11

In a biosynthetic pathway, which gene is considered epistatic when a mutation in that gene masks the effect of mutations in downstream genes?

Accepted Answer

The upstream (earlier acting) gene

Question 12

If you observe a 9:7 ratio in the F2 generation of a dihybrid cross, what does this suggest about the genetic interaction between the two genes?

Accepted Answer

The two genes are in the same pathway and both are required for the phenotype

Question 13

A researcher is studying a pigment pathway and finds that double mutants for genes A and C (aa cc) have the same phenotype as the single mutant cc. What does this indicate?

Accepted Answer

Gene C is epistatic to gene A

Question 14

Which of the following is a recommended approach to solving epistasis problems when the pathway is known?

Accepted Answer

Identify the genotype(s) associated with the phenotype(s) you are solving for, draw out the cross, and label expected genotype and phenotype ratios.

Question 15

In a plant pigment pathway, a mutation in gene D blocks the conversion of the colorless precursor to the orange intermediate. What phenotype would you expect for a plant with genotype ddAABBCC?

Accepted Answer

Colorless

Question 16

A student claims that in a trihybrid cross AaBbCc x AaBbCc, the genotype ratio for each gene is affected by epistasis. Is this correct?

Accepted Answer

No, genotype ratios are not affected by epistasis; only phenotype ratios are.

Question 17

In a pathway with three genes (A, B, C), which genotype would result in the accumulation of the orange intermediate?

Accepted Answer

A\_bbC\_

Question 18

Which of the following real-world scenarios best illustrates the use of epistasis to determine gene order in a pathway?

Accepted Answer

Analyzing double mutants in a metabolic pathway to see which mutation masks the other

Question 19

In snapdragons, which genotype combination would result in a dark red flower?

Accepted Answer

A\_B\_dd

Question 20

A pathway is constructed such that gene A acts upstream of gene B. If both genes are mutated (aa bb), which gene's phenotype will be observed?

Accepted Answer

Gene A's mutant phenotype

Genetic Interactions and Pathway Analysis: Epistasis in Pigmentation Pathways

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