Epistasis problems can follow a similar solution process as traditional Mendelian inheritance problems. Both types of problems will involve segregation of alleles and independent assortment; a Punnett square or branch diagram can be used to help you obtain the genotypes and frequencies for each cross. Completing this portion of the problem will provide you with the basic information—you will then need to interpret the genotypes based on the allele interactions to assign the correct phenotypes.
In this problem, you worked step by step to: identify the relationship of the alleles to one another; determine the gametes from each parent; conduct the cross to obtain the progeny genotypes; assign phenotypes; and determine the proportions of each phenotype. All of this information is combined to provide the final, summary answer of proportions and phenotypes of progeny for each cross.
The type of plumage found in mallard ducks is determined by three alleles at a single locus: MR, which encodes restricted plumage; M, which encodes mallard plumage; and md, which encodes dusky plumage. The restricted phenotype is dominant over mallard and dusky; mallard is dominant over dusky (MR > M > md).
Give the expected phenotypes and proportions of offspring produced by the following crosses:
a. MRM × mdmd | restricted | mallard | dusky |
b. MRmd × Mmd | restricted | mallard | dusky |
c. MRmd × MRM | restricted | mallard | dusky |
d. MRM × Mmd | restricted | mallard | dusky |
Determine the best approach to solving this problem by putting the following steps in order:
1. Assign the correct phenotype to each progeny genotype.
2. Determine the gametes produced by each parent.
3. Predict the offspring genotypes with a Punnett square.
4. Determine the relationship between the alleles with regard to dominance.
5. Identify relevant information from the problem that will help you answer the questions.
6. Review your answer to be sure that you have arrived at a logical solution.
7. Determine the proportions of each progeny phenotype.
A. |
B. |
C. |
D. |
It is important to first identify the key information from the problem. Even though this problem involves epistasis, the approach will be the same as solving typical Mendelian inheritance problems. It will be very important to understand the dominance relationship between the alleles before carrying out the crosses.
1. Match the allele with the correct phenotype.
dusky
mallard
restricted
Review the Problem Statement to help you identify the allele designation that is used for each phenotype.
1. Use the dominance relationship between the alleles to identify the phenotype associated with each genotype.
M R M R
Mm d
M R m
m d m d
M R m d
MM
Review the Problem Statement to help you identify the dominance relationships between the alleles.
1. For cross “a,” determine the types of gametes produced by each parent.
Gametes from MRM parent: MR | M | md
Gametes from mdmd parent: MR | M | md
The alleles for plumage type are encoded by a single locus. The parental alleles will segregate in meiosis so that only one allele would be found in each gamete. Even though there are three alleles for this locus, an individual can only carry two alleles.
1. For cross “a,” combine the gametes of the two parents with the use of a Punnett square. Indicate the correct gametes from the first parent along the top of the table and the gametes from the second parent along the left side of the table. The gamete order should be maintained as indicated (the first gamete for each parent would be listed first in the row or column). Select the correct genotype for each combination of gametes which would appear in the labeled cells of the Punnett square.
Please note that that the dropdown menus are not able to display the proper notation for the alleles (italics and superscripts).
The Punnett square provides an easy way to determine the genotypes of the progeny for this cross. Remember that the gametes from the first parent should be listed across the top of the table and the gametes from the second parent should be listed along the left side of the table.
1. For cross “a,” determine the phenotype of each offspring.
Progeny genotype: MRmd. Phenotype:
Progeny genotype: Mmd. Phenotype:
The phenotypes of the progeny can be determined by looking at the dominance relationships of the alleles in the progeny’s genotype.
1. For cross “a,” (MRM × mdmd) determine the proportion of each progeny phenotype. Any phenotypes that do not appear with this cross should be indicated as “0.”
Restricted:
Mallard:
Dusky:
The proportion should be expressed as a fraction and the values should add up to 1. This value can be determined by using the information in the Punnett square.
1. Give the expected phenotypes and proportions of offspring produced by the following cross:
MRM × mdmd | restricted | mallard | dusky |
Review your answers provided in the earlier steps of this tutorial.