Question

Coat color in horses shows incomplete dominance. \(H^{\mathrm{C}} H^{\mathrm{C}}\) individuals have a chestnut color, \(H^{\mathrm{c}} H^{\mathrm{cr}}\) individuals have a palomino color, and \(H^{\mathrm{cr}} H^{\mathrm{cr}}\) individuals have a cremello (white) color (see Figure 10.9). What is the predicted phenotypic ratio of chestnut to palomino to cremello if \(H^{\mathrm{c}} H^{\mathrm{er}}\) individuals are crossed with other \(H^{\mathrm{c}} H^{\mathrm{cr}}\) individuals? a. \(3: 1\) b. \(2: 1: 1\) c. 9:3:1 d. \(1: 2: 1\)

Short answer

1:2:1

Step-by-step solution

- Determine Genotypes

Identify the genotypes of the parents. Each parent is heterozygous, represented as \(H^{\text{c}} H^{\text{cr}}\).

- Set Up Punnett Square

Draw a Punnett square with the genotypes of both parents along each side. The square will be 2x2 since there are two alleles from each parent.

- Fill in Punnett Square

Combine the alleles from each parent to fill in the Punnett square: \( H^{\text{c}} H^{\text{c}} \) \( H^{\text{c}} H^{\text{cr}} \) \( H^{\text{cr}} H^{\text{c}} \) \( H^{\text{cr}} H^{\text{cr}} \)

- Identify Phenotypes

Determine the phenotype for each genotype: \( H^{\text{c}} H^{\text{c}} \) = Chestnut \( H^{\text{c}} H^{\text{cr}} \) = Palomino \( H^{\text{cr}} H^{\text{cr}} \) = Cremello

- Calculate Ratios

Count the frequency of each phenotype and list the predicted ratios: \( H^{\text{c}} H^{\text{c}} \) : \( H^{\text{c}} H^{\text{cr}} \) : \( H^{\text{cr}} H^{\text{cr}} \) = 1 : 2 : 1

Key concepts

Incomplete Dominance

Incomplete dominance is an important concept in genetics where neither allele completely masks the other. Instead, the result is a blend or intermediate phenotype. For instance, when a chestnut horse (with the genotype \(H^{\text{c}}H^{\text{c}}\)) is crossed with a cremello horse (with the genotype \(H^{\text{cr}}H^{\text{cr}}\)), the offspring are palomino (with the genotype \(H^{\text{c}}H^{\text{cr}}\)). This blending due to incomplete dominance is distinct from complete dominance, where the dominant allele completely masks the presence of the recessive allele. Understanding incomplete dominance helps explain the diversity of phenotypes observed in nature.

Horse Coat Color Inheritance

The coat color of horses is an excellent example of incomplete dominance. Three possible genotypes lead to three different phenotypes:

• \(H^{\text{c}}H^{\text{c}}\) results in a chestnut coat.
• \(H^{\text{c}}H^{\text{cr}}\) results in a palomino coat.
• \(H^{\text{cr}}H^{\text{cr}}\) results in a cremello coat.

When breeding horses, predicting their offspring's coat color involves understanding how these alleles interact. In the given exercise, two palomino horses (with \(H^{\text{c}}H^{\text{cr}}\) genotype) are crossed. Using a Punnett square, you can visualize the possible outcomes for their offspring. Combining the alleles of each parent allows us to predict the coat colors of the next generation.

Phenotypic Ratio

In genetics, the phenotypic ratio represents the relative number of offspring manifesting particular traits. For the given exercise, a cross between two palomino horses produces a phenotypic ratio of 1:2:1 for chestnut, palomino, and cremello coat colors respectively. Here is how those ratios arise:

• \(H^{\text{c}}H^{\text{c}}\) leading to chestnut appears once in the Punnett square.
• \(H^{\text{c}}H^{\text{cr}}\) leading to palomino appears twice.
• \(H^{\text{cr}}H^{\text{cr}}\) leading to cremello appears once.

This ratio indicates that among the offspring, for every one chestnut horse and one cremello horse, there will be two palomino horses. Understanding phenotypic ratios helps in predicting the likelihood of certain traits appearing in the next generation, which is a crucial aspect of genetic inheritance studies.