Question

In rabbits, a series of multiple alleles controls coat color in the following way: \(C\) is dominant to all other alleles and causes full color. The chinchilla phenotype is due to the \(c^{\mathrm{ch}}\) allele, which is dominant to all alleles other than \(C\). The \(c^{h}\) allele, dominant only to \(c^{a}\) (albino), results in the Himalayan coat color. Thus, the order of dominance is \(C>c^{\mathrm{dh}}>c^{h}>c^{a} .\) For each of the fol- lowing three cases, the phenotypes of the \(\mathrm{P}_{1}\) generations of two crosses are shown, as well as the phenotype of one member of the \(\mathrm{F}_{1}\) generation. For each case, determine the genotypes of the \(P_{1}\) generation and the \(\mathrm{F}_{1}\) offspring, and predict the results of making each indicated cross between \(F_{1}\) individuals.

Short answer

Answer: Case 1 (Chinchilla x Chinchilla): P1 Generation: Genotypes - \(c^{\mathrm{ch}}c^{\mathrm{ch}}\) for both rabbits, Phenotypes - Chinchilla for both rabbits. F1 Generation: Genotypes - \(c^{\mathrm{ch}}c^{\mathrm{ch}}\) for all offspring, Phenotypes - Chinchilla for all offspring. Case 2 (Full color x Himalayan): P1 Generation: Genotypes - \(Cc^{\mathrm{h}}\) for the full-color rabbit and \(c^{\mathrm{h}}c^{\mathrm{h}}\) for the Himalayan rabbit, Phenotypes - Full color for one rabbit, Himalayan for the other rabbit. F1 Generation: Genotypes - \(Cc^{\mathrm{h}}\) for all offspring, Phenotypes - Full color for all offspring. Case 3 (Himalayan x Himalayan): P1 Generation: Genotypes - \(c^{\mathrm{h}}c^{\mathrm{h}}\) for both rabbits, Phenotypes - Himalayan for both rabbits. F1 Generation: Genotypes - \(c^{\mathrm{h}}c^{\mathrm{h}}\) for all offspring, Phenotypes - Himalayan for all offspring.

Step-by-step solution

Case 1: Chinchilla x Chinchilla

The phenotype of both rabbits in the P1 generation is chinchilla, which is due to allele \(c^{\mathrm{ch}}\). Since this allele is dominant to all others except \(C\) and they both show the chinchilla phenotype, their genotypes must be \(c^{\mathrm{ch}}c^{\mathrm{ch}}\) for both rabbits. Given this information, both parents will pass on their \(c^{\mathrm{ch}}\) allele to their offspring, and the F1 generation will be entirely comprised of chinchilla offspring with the same \(c^{\mathrm{ch}}c^{\mathrm{ch}}\) genotype.

Case 2: Full color x Himalayan

In this case, the phenotype of one parent is full color, and the other parent is Himalayan. The full color phenotype is due to the dominant allele \(C\), while the Himalayan phenotype is due to the \(c^{h}\) allele, which is dominant only to \(c^{a}\). Therefore, the genotypes of the parents must be \(Cc^{\mathrm{h}}\) for the full-color rabbit and \(c^{\mathrm{h}}c^{\mathrm{h}}\) for the Himalayan rabbit. For the F1 generation, the offspring will inherit either \(C\) or \(c^{\mathrm{h}}\) from the full-color parent and will inherit \(c^{\mathrm{h}}\) from the Himalayan parent. This results in F1 offspring with a genotype of \(Cc^{\mathrm{h}}\), which displays a full-color phenotype.

Case 3: Himalayan x Himalayan

Here, the phenotype of both rabbits in the P1 generation is Himalayan, which is due to the \(c^{h}\) allele. Since this allele is dominant only to \(c^{a}\) and both parents exhibit the Himalayan phenotype, their genotypes must be \(c^{\mathrm{h}}c^{\mathrm{h}}\) for both rabbits. Given these genotypes, both parents will pass on their \(c^{\mathrm{h}}\) allele to their offspring, and the F1 generation will be comprised entirely of Himalayan offspring with the same \(c^{\mathrm{h}}c^{\mathrm{h}}\) genotype.

Key concepts

Dominance Hierarchy

Genetics often involves understanding how different allele combinations influence the traits of organisms. A key concept in this is the dominance hierarchy of alleles. This hierarchy defines which alleles express their traits over others, forming a ranking order of genetic control. In our rabbit example, **dominance hierarchy** is clearly displayed:

• The allele \( C \) is the most dominant and always expresses its trait of full coat color, regardless of what other alleles it is paired with.
• The \( c^{\mathrm{ch}} \) allele, responsible for the chinchilla phenotype, is second in the hierarchy. It shows chinchilla color unless paired with the \( C \) allele.
• Next is the \( c^{h} \) allele which expresses the Himalayan coat color, but only when \( C \) or \( c^{\mathrm{ch}} \) are not present.
• The least dominant allele is \( c^{a} \), which results in albino coloration and is only expressed when both copies of the allele are present.

Understanding the dominance hierarchy allows us to predict the phenotypes of offspring given the genotype of parents. It demonstrates that not all alleles contribute equally to an organism's traits.

Coat Color Inheritance

In rabbits, coat color inheritance illustrates how genetic traits are passed from parents to offspring. The color an offspring displays depends on its specific genotype, which it receives through allele combinations from each parent.Each rabbit has two alleles for coat color, one inherited from each parent. The phenotype (visible trait) exhibited by the rabbit is determined by the interaction between these two alleles:

• If a rabbit inherits at least one \( C \) allele, it will show a full-color coat.
• If no \( C \) allele is present, but at least one \( c^{\mathrm{ch}} \) allele is inherited, the rabbit will have a chinchilla coat.
• If the rabbit has no \( C \) or \( c^{\mathrm{ch}} \) alleles, but at least one \( c^{h} \) allele, it will display a Himalayan coat.
• Finally, if no alleles other than \( c^{a} \) are present, the rabbit will be albino.

This example highlights the importance of **coat color inheritance** in genetic studies, as it provides a straightforward demonstration of how visible characteristics are inherited based on genetic combinations.

Allele Interactions

The way alleles combine and interact within an organism significantly influences its genetic makeup and, consequently, its physical traits. Understanding **allele interactions** allows students to comprehend how various combinations can lead to different phenotypic expressions.In rabbits, certain interactions dominate others based on the presence of more dominant alleles:

• The \( CC \) genotype results in a full-color coat because \( C \) is the most dominant allele.
• When \( C \) is absent, \( c^{\mathrm{ch}}c^{\mathrm{ch}} \) leads to a chinchilla coat due to \( c^{\mathrm{ch}} \)'s dominance over \( c^{h} \) and \( c^{a} \).
• The \( c^{h}c^{h} \) genotype shows the Himalayan pattern in the absence of more dominant alleles.
• Finally, \( c^{a}c^{a} \) will produce an albino phenotype, as it lacks any dominant alleles to override.

The interaction between these alleles demonstrates how multiple factors work together to determine genetic outcomes. By studying allele interactions, researchers and students can better predict the traits of future generations.