9.6.1 Epistasis: One Gene Affecting Another

Question 1

Epistasis

Answer 1

• Genes make protein products that interact in the organism. In epistasis, one gene’s product affects another gene.
• A modification in the 9:3:3:1 phenotypic ratio expected from a dihybrid cross—such as 9:3:4 or 12:3:1—indicates that epistasis is occurring.
• In epistasis, two genes assort independently of one another, but a dominant allele for one gene may not be expressed because the product of the second gene masks it.
- “No gene is an island.” Organisms are composed of many complex systems, and each system depends on the interaction of many gene products.
- Genes make protein products that interact in the organism. In epistasis, a gene’s product modifies another gene.

Question 2

note

Answer 2

• In mice, agouti (gray) coat color is dominant to black. A
  pure-breeding gray mouse (BB) is crossed with a
  pure-breeding black mouse (bb). The F1 mice are all agouti (Bb). For a single gene locus, crossing the F1 mice should yield a 3:1 ratio of agouti mice to black mice in the F2 generation. Instead, a ratio of 9:3:4 agouti, black, and albino mice occurs. How can the results be explained?
• A second gene is responsible for whether or not pigment is deposited in the hair. Individuals of the Cc or CC genotype will deposit pigment, resulting in agouti or black coat color. Individuals of the cc genotype will not deposit pigment and will be albino regardless or their genotype at the other locus.
• The genotype of the F1 mice is actually BbCc. The cross between two F1 mice is a dihybrid cross, not a monohybrid cross.
• Review: A dihybrid cross between two heterozygotes yields a 9:3:3:1 ratio of phenotypes in the offspring.
• The results show a 9:3:4 ratio of phenotypes in the offspring: 9/16 are gray, 3/16 are black, and 4/16 are albino. A modification of the 9:3:3:1 ratio indicates that epistasis is
  occurring.

Question 3

Which of the following statements is true?

Answer 3

• Genes code for protein products that interact in an organism.

Question 4

What ratio of phenotypes would you expect to see in the offspring resulting from this cross:
Bbcc × bbCc?
B = Agouti color
b = black color
C = pigment
c = defective (no pigment production)

Answer 4

• 8 albino:4 agouti:4 black

Question 5

In epistasis

Answer 5

• one gene’s expression modifies or suppresses another gene.

Question 6

In the fictitious cactus Cacto verde, two separate gene loci influence the length of the spines. The genotype P-ss results in the phenotype prickly and the cacti have long sharp spines. The genotype pp– results in the phenotype smooth and the cacti lack spines altogether. Heterozygotes of the genotype P–S- are stumpy, and have relatively short spines. Two cacti are crossed and 1/2 of the offspring are smooth, 1/2 are stumpy. Which of the following statements regarding the genotypes and phenotypes in question are correct?

Answer 6

• The genotypes of the parents could be ppSS and PpSs.

Question 7

What would be the color of a rodent with the homozygous dominant genotype?
B = Agouti color
b = black color
C = pigment
c = defective (no pigment production)

Answer 7

• Agouti

Question 8

If the offspring of a pair of mice are only albino and black, which of the following is true concerning the parents?

Answer 8

• One of the parents could be albino, but not both.

Question 9

What would be the color of a rodent with the genotype bbCc?

Answer 9

• Black

Question 10

What would be the color of a mouse with the genotype BbCC?
B = Agouti color
b = black color
C = pigment
c = defective (no pigment production)

Answer 10

• Agouti

Question 11

You purchase two gray chinchillas. After mating them, nine of the offspring are gray, three are black, and four are albino. What are the genotypes of the parents?

Answer 11

• BbCc × BbCc

Question 12

Which of the following phenotypes are not possible in the offspring of an agouti rodent and a black rodent?
B = Agouti color
b = black color
C = pigment
c = defective (no pigment production)

Answer 12

• All three phenotypes are possible.

Question 13

What is one indication that epistasis has occurred?

Answer 13

• Offspring appear in a modified 9:3:3:1 ratio.

Question 14

Feather color of the resplendent quetzel is the result of an interaction between two separate genes. For gene one, orange (O) is dominant to yellow (o). For the second gene, the allele (C) is dominant to (c) and determines whether or not color is deposited. An orange and a yellow bird are crossed. Three-eighths of the offspring are orange, 3/8 of the offspring are yellow, and 2/8 are albino. What are the genotypes of the orange and yellow parents, respectively?

Answer 14

• OoCc × ooCc

Question 15

In chickens certain characteristics of the comb are determined by two distinct gene loci. The four possible phenotypes are rose, pea, walnut, and single comb. Assume that the phenotypes in question are controlled by the alleles R and P. A walnut-combed rooster is mated with a pea-combed hen. They have offspring in the ratio of 3 walnut; 3 pea; 1 rose; and 1 single comb. Which of the following statements is correct concerning the genotypes and phenotypes in question?

Answer 15

• Both parents are heterozygous at the P loci.

Question 16

Two albino rodents are crossed. What are the possible phenotypes of the offspring?

Answer 16

• Albino

Question 17

Which is a description of epistasis?

Answer 17

• One gene’s product modifies another gene’s expression.