Question

Horses can be cremello (a light cream color), chestnut (a brownish color), or palomino (a golden color with white in the horse's tail and mane). Of these phenotypes, only palominos never breed true. \(\begin{array}{ll}\text { cremello } \times \text { palomino } & \longrightarrow \begin{array}{l}1 / 2 \text { cremello } \\ 1 / 2 \text { palomino }\end{array} \\ \text { chestnut } \times \text { palomino } \longrightarrow & \begin{array}{l}1 / 2 \text { chestnut } \\ 1 / 2 \text { palomino }\end{array} \\ \text { palomino } \times \text { palomino } \longrightarrow & \begin{array}{l}1 / 4 \text { chestnut } \\ 1 / 2 \text { palomino }\end{array} \\ & 1 / 4 \text { cremello }\end{array}\) (a) From the results given above, 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 mode of inheritance is controlled by a single gene "C" with two alleles "C" for chestnut and "c" for cremello, inherited in a dominant/recessive manner. In the F2 generation, the phenotype ratio is 1 chestnut : 2 palomino : 1 cremello.

Step-by-step solution

Assign gene symbols

Based on the given data, let's use the letter C for the gene that controls the coat color in horses. Cremello will be represented by "cc," chestnut will be represented by "CC," and the heterozygous condition, or palomino, will be represented by "Cc." #Step 2: Observe the phenotypes and genotypes#

Review the three cross results

From the given results, we know that: 1. Cremello (cc) x Palomino (Cc) → 1/2 Cremello (cc) + 1/2 Palomino (Cc) 2. Chestnut (CC) x Palomino (Cc) → 1/2 Chestnut (CC) + 1/2 Palomino (Cc) 3. Palomino (Cc) x Palomino (Cc) → 1/4 Chestnut (CC) + 1/2 Palomino (Cc) + 1/4 Cremello (cc) #Step 3: Predicting F1 and F2 results#

The initial crossing of cremello (cc) x chestnut (CC)

In the initial cross (P generation) between a cremello (cc) and a chestnut (CC) horse, all offspring will have the heterozygous genotype (Cc), which corresponds to the palomino phenotype. Therefore, the F1 generation will have only palomino horses.

F1 cross and F2 generations

Now, let's consider the cross between the F1 palomino horses (Cc x Cc). From the given data in Step 2, we already know the ratio of the resulting phenotypes: Palomino (Cc) x Palomino (Cc) → 1/4 Chestnut (CC) + 1/2 Palomino (Cc) + 1/4 Cremello (cc) So, the F2 generation will have a ratio of 1 chestnut : 2 palomino : 1 cremello. To summarize: (a) Mode of inheritance is controlled by a single gene "C" with two alleles "C" for chestnut and "c" for cremello. The gene is inherited in a dominant/recessive manner, with chestnut being dominant and cremello being recessive. The heterozygous condition, "Cc," results in the palomino phenotype. (b) From initial matings between cremello and chestnut horses, the F1 generation will all be palomino. In the F2 generation, the phenotype ratio will be 1 chestnut : 2 palomino : 1 cremello.

Key concepts

Genotype and Phenotype

Understanding the concepts of 'genotype' and 'phenotype' is crucial in genetics. Genotype refers to the specific genetic makeup of an organism, encompassing all the genes and the various alleles for each gene. For example, in horses, the coat color gene might have two alleles: one for the chestnut color (C) and the other for the cremello color (c). A chestnut horse has a genotype of CC, a cremello horse is cc, and a palomino, with a mix of chestnut and cremello, is Cc.

On the other hand, 'phenotype' is the term used for the observable characteristics or traits expressed as a result of the genotype interacting with the environment. In our case, the color of the horse's coat—chestnut, cremello, or palomino—is the phenotype that we see. The genotype determines the potential for certain traits, while the phenotype is the actual manifestation of those traits.

The relationship between genotype and phenotype is one where the genetic code provides the potential for physical traits, but the actual outcome can be influenced by various factors. For example, the genotype of a palomino horse (Cc) allows for the possibility of both cremello and chestnut offspring, manifesting different phenotypes based on their genetic combination.

Dominant and Recessive Alleles

Alleles are different versions of the same gene. They can be dominant or recessive, and this difference plays a key role in determining an organism's traits. Dominant alleles, such as the allele for chestnut color in horses (C), are expressed in the phenotype even when only one copy is present. This means that a horse with a genotype of CC or Cc will exhibit the chestnut phenotype, as the dominant allele 'overrides' the effect of the recessive allele.

In contrast, recessive alleles like the allele for cremello color (c) are only expressed in the phenotype when two copies are present, as in the genotype cc. A single recessive allele is masked by the presence of a dominant allele and does not affect the phenotype. Thus, a palomino horse (Cc) shows a mixed phenotype because it carries one allele for chestnut (C) and one for cremello (c), with the dominant chestnut allele being expressed in the phenotype.

A greater understanding of these principles helps make sense of the breeding patterns and the appearance of certain traits in offspring. It's important to note that for the palomino horses to not 'breed true', meaning not always produce palomino offspring, implies that the palomino characteristic is not a dominant trait, but rather a result of the interaction between a dominant and a recessive allele.

Mendelian Inheritance

The principles of Mendelian inheritance are rooted in the work of Gregor Mendel, who discovered the fundamental laws of genetics through his experiments with pea plants. Mendelian inheritance explains how traits are passed down from parents to offspring through the distribution of alleles.

According to Mendelian genetics, for a gene with two alleles, such as the horse coat color gene (C), each parent contributes one allele to their offspring. When considering a monohybrid cross, the combination of alleles in the parents' genotypes determine the potential genotypes and phenotypes in the offspring. For example, the initial matings of cremello (cc) and chestnut (CC) horses will always result in an F1 generation of palomino horses (Cc), due to each parent contributing one allele to the offspring.

The F2 generation, resulting from F1 individuals breeding, can reveal interesting ratios that exemplify Mendel's law of segregation. The alleles for each gene segregate during gamete formation, leading to the observed phenotypic ratio of 1 chestnut : 2 palomino : 1 cremello, as shown in the exercise. This ratio aligns with the Mendelian inheritance patterns – each genotype (CC, Cc, cc) has a predictable phenotypic consequence, which is governed by the interaction between the dominant and recessive alleles.