Question

Consider that coat color in rabbits is controlled by a complex locus with a series of four (4) alleles. The alleles arranged in order of their dominance are: \(c^{+}=\) agouti, \(c^{\mathrm{ch}}=\) chinchilla, \(c^{h}=\) Himalayan and \(c=\) albino. Predict for the following crosses the phenotypes of the progeny and their expected proportions: (a) agouti \(\% \times\) Himalayan 0 \(c^{+} c^{c h}\) \(c^{\mathrm{ch}} \mathrm{c}\) (b) agouti \(q \times\) chinchilla

Short answer

The phenotypic ratios of the progeny for the given crosses are: (a) Agouti $c^+ c^h \times$ Himalayan $c^{ch} c$: - 2 Agouti : 1 Chinchilla : 1 Himalayan (b) Agouti $q \times$ chinchilla: 1. If the agouti genotype is \(c^+ c^+\): 100% Agouti 2. If the agouti genotype is \(c^+ c^{ch}\): 50% Agouti : 50% Chinchilla

Step-by-step solution

Organize Genotypes

Before performing the cross, it's essential to understand the dominance order and the phenotypes of the given genotypes. As mentioned earlier: - \(c^+\) is dominant and produces the agouti phenotype. - \(c^{ch}\) is second dominant and produces the chinchilla phenotype. - \(c^h\), followed by \(c^{ch}\), gives the Himalayan phenotype. - \(c\) is recessive and produces the albino phenotype. Rabbits involved in this cross have the genotypes \(c^+ c^h\) and \(c^{ch} c\). Notice that both genotypes have two different alleles each, so it is considered a cross between heterozygotes.

Crossed Genotypes

Now perform the cross to find possible genotypes of the progeny. The Punnett square is an excellent tool for this step: | | \(c^+\) | \(c^h\) | | --- | --- | --- | | \(c^{ch}\) | \(c^+ c^{ch}\) | \(c^h c^{ch}\) | | \(c\) | \(c^+ c\) | \(c^h c\) | As you can see, there will be 4 different genotypes among the progeny: 1. \(c^+ c^{ch}\) 2. \(c^h c^{ch}\) 3. \(c^+ c\) 4. \(c^h c\)

Progeny Phenotypes

We can now determine the phenotypes of each genotype based on the dominance order we received earlier. This would be as follows: 1. \(c^+ c^{ch}\) will be agouti (since \(c^+\) is dominant over all other alleles), 2. \(c^h c^{ch}\) will be chinchilla (since \(c^{ch}\) is dominant over \(c^h\)), 3. \(c^+ c\) will be agouti (since \(c^+\) is dominant over all other alleles), 4. \(c^h c\) will be Himalayan (since \(c^h\) is dominant over \(c\)).

Progeny Proportions

As the cross resulted in two agouti, one chinchilla, and one Himalayan, the phenotypic ratio of the progeny will be: - 2 Agouti : 1 Chinchilla : 1 Himalayan (b) agouti (\(q\)) x chinchilla

Determine \(q\)

In this question, only the chinchilla genotype is provided. The agouti genotype is represented by \(q\). As agouti is \(c^+\), we have two main possibilities: \(c^+ c^+\) or \(c^+ c^{ch}\). We will perform crosses for both potential genotypes to be sure about the result.

Crossed Genotypes

Now perform both potential crosses: 1. \(c^+ c^+\) (agouti) x \(c^{ch} c\) (chinchilla) 2. \(c^+ c^{ch}\) (agouti) x \(c^{ch} c\) (chinchilla) To simplify this explanation, we will focus on the ratios for each cross: Cross 1: Agouti: 100% Cross 2: Agouti: 50% (genotypes \(c^+ c^{ch}\) and \(c^+ c\)) Chinchilla: 50% (genotypes \(c^{ch} c^{ch}\) and \(c^{ch} c\))

Conclusion

The phenotypic ratios of the progeny will depend on the agouti genotype: 1. If the agouti genotype is \(c^+ c^+\): 100% Agouti 2. If the agouti genotype is \(c^+ c^{ch}\): 50% Agouti : 50% Chinchilla

Key concepts

Alleles

In genetics, **alleles** refer to the different forms of a gene that exist at a specific locus on a chromosome. These variants can influence a variety of traits, such as coat color, eye color, or blood type. Each individual inherits two alleles for each gene, one from each parent. These alleles can be either identical or different, leading to the following possible scenarios:

• **Homozygous**: An individual has two identical alleles (e.g., AA or aa).
• **Heterozygous**: An individual has two different alleles (e.g., Aa).

In the context of the rabbit coat color, the alleles involved are represented as:

• \(c^+\): Agouti
• \(c^{ch}\): Chinchilla
• \(c^h\): Himalayan
• \(c\): Albino

These alleles follow a specific **dominance**: order from dominant to least dominant is \(c^+ > c^{ch} > c^h > c\). This hierarchy affects the phenotypic expression seen in the rabbits.

Phenotypic Ratios

**Phenotypic ratios** describe the relative number of offspring manifesting particular traits. In genetics, these ratios help predict the outcomes of crosses based on the alleles involved.
For the rabbit coat color example, different genotypes yield different phenotypes based on allele dominance. The calculation of phenotypic ratios involves determining all possible genotype combinations from a genetic cross and applying the dominance rules. Consider a cross involving:

• \(c^+ c^h\) (agouti x Himalayan)
• \(c^{ch} c\) (chinchilla x albino)

The possible phenotypes would be divided as follows:

• \(c^+ c^{ch}\) and \(c^+ c\) both result in agouti, since \(c^+\) is dominant.
• \(c^{ch} c^h\) results in chinchilla due to \(c^{ch}\) dominance over \(c^h\).
• \(c^h c\) results in Himalayan since \(c^h\) is dominant over \(c\).

The ratio concluded from this cross results in 2 Agouti: 1 Chinchilla: 1 Himalayan.

Punnett Square

The **Punnett square** is a valuable tool used in genetics to predict the genetic outcomes of a cross between individuals.
It provides a visual representation of all possible allelic combinations and their resulting genotypes. In our example, the Punnett square would organize the alleles of the parental rabbits:
One parent (\(c^+ c^h\)) contributes either the \(c^+\) or \(c^h\) allele, while the other parent (\(c^{ch} c\)) contributes either the \(c^{ch}\) or \(c\) allele.
Filling out the Punnett square offers the potential combinations:

• \(c^+ c^{ch}\)
• \(c^h c^{ch}\)
• \(c^+ c\)
• \(c^h c\)

This method allows for an organized approach to predict the genotypes and phenotypes of offspring, simplifying complex genetic crosses into understandable outcomes.

Dominance Hierarchy

In genetic terms, a **dominance hierarchy** refers to the order in which alleles express themselves over others within heterozygous pairs. Understanding this hierarchy is crucial for predicting phenotypes.
In the rabbit module, the alleles for coat color follow a clear dominance hierarchy:

• \(c^+\) is dominant over all others, determining the agouti phenotype when present.
• \(c^{ch}\) ranks second, expressing chinchilla traits when paired with lesser alleles \((c^h, c)\).
• \(c^h\) is dominant over only \(c\), producing the Himalayan coat when present.
• \(c\) is recessive and only shows the albino phenotype in homozygous forms.

By respecting this hierarchy, one can predict the visible characteristics (phenotype) in offspring based on their genetic makeup (genotype). This understanding aids in tasks such as calculating phenotypic ratios or determining potential breed outcomes.