Epistasis Practice
Answer the following questions on a SEPARATE sheet of paper. Be neat and make sure you distinguish
between letters!
1.
Explain how the following phenomena can disrupt Mendelian phenotypic ratios.
a. Lethal alleles
d. Codominance
b. Multiple alleles
e. Epistasis
c. Incomplete dominance
2. How does the relationship between dominant and recessive alleles differ in epistasis?
3. In sweet peas, the two allelic pairs C, c and P, p are known to effect pigment formation in the flowers.
The dominants, C&P, are both necessary for colored flowers. Absence of either results in white flowers.
A dihybrid plant is crossed to a white one which is heterozygous at the “C” locus.
a. What is the genotype of the dihybrid plant?
b. What is the genotype of the white plant?
c. What kinds of flowers, colored or white, are to be expected from the cross above? Include the
ratio.
4. Assume that another allelic pair in sweet peas also affects pigment formation in addition to the genes
mentioned in question 3. The presence of the dominant, R, is required for red flowers. It’s recessive
allele, r, produces yellow flowers. What would be the phenotypes of the following plants in relation to
flower color?
a. CcPpRr
b. CcppRR
c. CcPPrr
d. ccPPRR
5. In a certain breed of dog, the dominant, B, is required for black fur; it’s recessive, b, produces brown fur.
However, the dominant, I is the epistatic to the color locus and can inhibit pigment formation. The recessive
allele, i, on the other hand, permits pigment deposition in the fur. What would be the phenotypes of the
following sets of parents and what would be the results of the mating?
a. bbii x BbIi
b. bbii x Bbii
c. bbIi x BBIi
6. What type of epistasis is being demonstrated in question #4?
7. What type of epistasis is being demonstrated in question #5?
8. In poultry, the allele, B, can result in black feathers. Its allele, b, can produce white splashed feathers. The
heterozygous results in blue feathers. The factors O and C are both required for any color at all in the feathers.
Homozygous recessiveness at either locus results in white feathers. In each of the following 3 cases give the
genotypes of the parental birds and their offspring.
a. Two completely white birds which produce a F1 which is black.
b. Two black birds which produce 54 black birds and 42 white birds.
c. Two blue birds which produce 16 black, 47 blue, and 14 white slashed birds.
9. Two pairs of alleles are involved in cyanide production in white clover. Some strains have high cyanide
content, others a low one. When high Strain A was crossed to low Strain B, the F1 all had high cyanide content.
When F1 plants were crossed among themselves, the following resulted:
High cyanide content: 450 plants
Low cyanide content: 350 plants
Offer an explanation for these results.