The coat color in mink is controlled by two codominant alleles at a single locus. Red coat color is produced by the genotype R₁R₁, silver coat by the genotype R₁R₂, and platinum color by R₂R₂. White spotting of the coat is a recessive trait found with the genotype ss. Solid coat color is found with the S– genotype.


If the cross SsR₁R₂ x SsR₁R₁ is made, what are the progeny phenotypes, and in what proportions are they expected to occur?

Verified step by step guidance

1. Step 1: Identify the genotypes of the parents. The first parent has the genotype SsR₁R₂, and the second parent has the genotype SsR₁R₁. The 'S' and 's' alleles determine whether the coat is solid (S–) or has white spotting (ss), while the 'R₁' and 'R₂' alleles determine the coat color (red, silver, or platinum).
2. Step 2: Determine the possible gametes for each parent. For the first parent (SsR₁R₂), the possible gametes are SR₁, SR₂, sR₁, and sR₂. For the second parent (SsR₁R₁), the possible gametes are SR₁ and sR₁.
3. Step 3: Set up a Punnett square to combine the gametes from both parents. The rows will represent the gametes from one parent (e.g., SR₁, SR₂, sR₁, sR₂), and the columns will represent the gametes from the other parent (e.g., SR₁, sR₁). Fill in the squares by combining the alleles from the corresponding row and column.
4. Step 4: Analyze the genotypes in the Punnett square to determine the phenotypes. For coat color, R₁R₁ produces red, R₁R₂ produces silver, and R₂R₂ produces platinum. For coat pattern, S– produces solid color, and ss produces white spotting. Combine these traits to determine the phenotype for each genotype.
5. Step 5: Count the occurrences of each phenotype in the Punnett square and calculate the proportions. Divide the number of occurrences of each phenotype by the total number of squares in the Punnett square to determine the expected proportions of the progeny phenotypes.

Key Concepts

Codominance

Codominance occurs when two different alleles at a locus are both expressed in the phenotype of a heterozygote. In the case of mink coat color, the alleles R₁ and R₂ are codominant, resulting in the silver coat color when both alleles are present (R₁R₂). Understanding codominance is crucial for predicting the phenotypic outcomes of genetic crosses involving these alleles.

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable physical or biochemical characteristics resulting from the genotype. In the given question, the genotypes (R₁R₁, R₁R₂, R₂R₂, ss, and S–) determine the phenotypes (red, silver, platinum, and solid coat colors) of the mink. Analyzing the genotypes helps in predicting the expected phenotypic ratios in the offspring.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. By organizing the possible gametes from each parent, it allows for a visual representation of the potential genetic combinations. In this case, constructing a Punnett square for the cross SsR₁R₂ x SsR₁R₁ will help determine the expected proportions of the various coat colors in the progeny.