Question

Horses can be cremello (a light cream color), chestnut (a reddish brown color), or palomino (a golden color with white in the horse's tail and mane).Of these phenotypes, only palominos never breed true. The following results have been observed: (a) From these results, determine the mode of inheritance by assigning gene symbols and indicating which genotypes yield which phenotypes. (b) Predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of many initial matings between cremello and chestnut horses.

Short answer

Answer: The genotypes for the horse coat color phenotypes are Cremello (CC), Chestnut (cc), and Palomino (Cc). When a cremello horse (CC) mates with a chestnut horse (cc), the F1 generation will be all Palomino (Cc). When the F1 Palomino horses mate, the F2 generation will have 1/4 Cremello (CC), 1/2 Palomino (Cc), and 1/4 Chestnut (cc).

Step-by-step solution

1. Assigning gene symbols to different horse color phenotypes

We can use the letter C for the gene that controls the horse coat color. Let's represent the alleles responsible for cremello, chestnut, and palomino coat colors as CC, cc, and Cc, respectively.

2. Finding the genotypes and inheritance patterns from given results

We can analyze each possible cross to understand the gene inheritance pattern: - Cremello × Palomino: (CC) × (Cc) → 1/2 Cremello (CC) + 1/2 Palomino (Cc) - Chestnut × Palomino: (cc) × (Cc) → 1/2 Chestnut (cc) + 1/2 Palomino (Cc) - Palomino × Palomino: (Cc) × (Cc) → 1/4 Chestnut (cc) + 1/2 Palomino (Cc) + 1/4 Cremello (CC) From the above crosses, we can say that the genotype of the Cremello is homozygous dominant (CC), Chestnut is homozygous recessive (cc), and Palomino is heterozygous (Cc). (b)

1. Predicting F1 generation

To predict the F1 generation from the initial matings between cremello (CC) and chestnut (cc) horses, we can perform a Punnett square between CC and cc: | | C | C | |---|---|---| | c | Cc | Cc | | c | Cc | Cc | All F1 offspring will be heterozygous (Cc) and exhibit the palomino phenotype.

2. Predicting F2 generation

From the result in the F1 prediction, we know the F1 offspring have a Cc genotype. To predict the F2 generation, we can perform a Punnett square of the F1 offspring between Cc and Cc: | | C | c | |---|---|---| | C | CC | Cc | | c | Cc | cc | The F2 generation will have the following distribution of genotypes and phenotypes: - 1/4 Cremello (CC) - 1/2 Palomino (Cc) - 1/4 Chestnut (cc)

Key concepts

Phenotype

The term "phenotype" refers to the visible or observable characteristics of an organism. These traits result from the interaction between an organism's genes and the environment. In horses, phenotypes include the colors such as cremello, chestnut, and palomino. These coloration traits are the physical manifestations of the horse's genetic information.
The color of a horse is an example of a phenotype as it is something you can visibly identify in the animal. For instance, a palomino horse is recognized by its golden coat and white mane. Such phenotypes are influenced by the specific combination of alleles an organism possesses. Phenotypes allow us to categorize horses into groups based on colors and help breeders understand the patterns of inheritance.

Genotype

A genotype is the genetic makeup of an organism. It is the set of alleles that an individual inherits from its parents. The genotype is crucial in determining an organism's phenotype, although not all traits may be expressed visually.
In the case of horses discussed, the genotypes were represented as pairings of alleles. For example, a cremello horse would have the genotype CC, meaning it has two copies of the dominant allele. On the other hand, chestnut horses present the genotype cc, having two copies of the recessive allele. The palomino horse carries a heterozygous genotype Cc, combining one dominant and one recessive allele.
Understanding genotypes helps us predict the potential offspring outcomes when two horses breed and explains variations in the coat colors viewed in the phenotypes.

Punnett Square

The Punnett square is a diagram that is used to predict the genotypes of offspring from a particular cross or breeding experiment. Named after geneticist Reginald C. Punnett, this tool simplifies the understanding of how alleles segregate and combine.
When used in breeding horses, a Punnett square allows breeders to determine the probability of obtaining offspring with specific genotypes and phenotypes. For example, when a cremello horse with genotype CC is crossed with a chestnut horse with genotype cc, a Punnett square shows that all F1 offspring would have the genotype Cc, resulting in palomino phenotypes.

• It visually represents how gametes from each parent can combine.
• It helps in predicting genetic diversity and understanding inheritance patterns.

The Punnett square not only assists in academic understanding but is also a practical tool for breeders.

Alleles

Alleles are variant forms of a gene that arise by mutation and exist at the same place on a chromosome. Each parent contributes one allele for every gene, and these alleles determine specific traits in an offspring.
In the case of horse coat color, we have seen that alleles dictate the shades seen in phenotypes. If we denote the coat color gene by C, then C and c are the alleles involved. The dominant allele (C) could lead to a cremello coat if homozygous, whereas the recessive allele (c) could result in a chestnut coat if homozygous.

• Alleles can be either dominant or recessive.
• Dominant alleles mask the presence of recessive alleles in heterozygous combinations.

Understanding alleles and their combinations is fundamental to predicting inheritance patterns and explaining variances in offspring phenotypes.