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Chapter 5: Problem 13

Palomino horses have a golden yellow coat, chestnut horses have a brown coat, and cremello horses have a coat that is almost white. A series of crosses between the three different types of horses produced the following offspring: $$ \begin{array}{ll} {\text { Cross }} & {\text { Offspring }} \\ \hline \text { palomino } \times & 13 \text { palomino, } 6 \text { chestnut, } 5 \\ \text { palomino } & \text { cremello } \\ \text { chestnut } \times \text { chestnut } & 16 \text { chestnut }\\\ \text { cremello } \times \text { cremello } & 13 \text { cremello }\\\ \text { palomino } \times \text { chestnut } & 8 \text { palomino, } 9 \text { chestnut }\\\ \text { palomino } \times \text { cremello } & 11 \text { palomino, } 11 \text { cremello }\\\ \text { chestnut } \times \text { cremello } & 23 \text { palomino }\\\ \end{array} $$ a. Explain the inheritance of the palomino, chestnut, and cremello phenotypes in horses. b. Assign symbols for the alleles that determine these phenotypes, and list the genotypes of all parents and offspring given in the preceding table.

Short Answer

Expert verified
Palomino is heterozygous (Cc), chestnut is homozygous dominant (CC), and cremello is homozygous recessive (cc). Inheritance follows incomplete dominance.

Step by step solution

01

Analyze the Crosses

First, we analyze the set of crosses and their results. Notice that **palomino** crossed with **chestnut** produces **palomino** and **chestnut**, suggesting these might have incomplete dominance. Similarly, **palomino** crossed with **cremello** produces **palomino** and **cremello**, reinforcing the theory of incomplete dominance.
02

Identification of Genotypes for Palomino

Let's identify the possible genotypes. - Since a **chestnut** crossed with **chestnut** only results in **chestnut**, **chestnut** must be homozygous. - **Cremello** crossed with **cremello** results only in **cremello**, therefore **cremello** is also homozygous. - **Palomino** results from crosses including both **palomino** and **cremello**, suggesting it's a heterozygous phenotype.
03

Assign Symbols for Alleles

We assign symbols: - Let **C** be the allele for **chestnut**. - Let **c** be the allele for **cremello**. The genotype for:- **Chestnut** is \( CC \).- **Cremello** is \( cc \).- **Palomino** being a mix (heterozygous form) will be \( Cc \).
04

Analyze Each Cross

Using the symbols and genotypes established:- Cross 1: \( Cc \times Cc \) gives 1 \( CC \) : 2 \( Cc \) : 1 \( cc \). Observed results match closely with this expected ratio (13 palomino, 6 chestnut, 5 cremello).- Cross 2: \( CC \times CC \) yields only \( CC \) (chestnut), and this is observed.- Cross 3: \( cc \times cc \) yields only \( cc \) (cremello), and this is observed.- Cross 4: \( Cc \times CC \) yields 1 \( CC \) : 1 \( Cc \) (observed as 9 chestnut, 8 palomino).- Cross 5: \( Cc \times cc \) yields 1 \( Cc \) : 1 \( cc \) (observed as 11 palomino, 11 cremello).- Cross 6: \( CC \times cc \) yields only \( Cc \) (palomino), and is observed.
05

Conclude the Inheritance Pattern

The inheritance pattern follows incomplete dominance where the heterozygous (\(Cc\)) results in a different phenotype (palomino) than either of the homozygous options (\( CC \) is chestnut, \( cc \) is cremello).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Incomplete Dominance
In genetics, incomplete dominance occurs when the allele for a specific trait is not fully dominant over its counterpart. This results in a heterozygous phenotype that is distinct from both homozygous forms. In the case of palomino horses, the golden yellow coat color is an example of incomplete dominance between the chestnut brown coat and the almost white cremello coat. When a horse inherits one allele for chestnut and one for cremello, the resulting color is palomino.

This blending creates a phenotype that is an intermediate shade between the chestnut and cremello coats. Hence, palomino horses become a showcase of incomplete dominance in real-world genetics.
Genotype
The term genotype refers to the genetic makeup of an organism concerning a particular trait. It is represented by the combination of alleles inherited from the parents. For the horses described in the exercise, each phenotype (coat color) corresponds to specific genotypes:
  • Chestnut horses have the genotype \( CC \).
  • Cremello horses carry the genotype \( cc \).
  • Palomino horses, exhibiting the intermediate color, possess the genotype \( Cc \).
Understanding the genotype of an individual is crucial as it determines the traits they exhibit and allows for predictions about the offspring they might produce.
Phenotype Inheritance
Phenotype refers to the observable characteristics of an organism, which result from both its genotype and its environment. In the inheritance of horse coat colors in our exercise, the different phenotypes of palomino, chestnut, and cremello coats are determined by the combinations of alleles passed down from parent horses.

Each cross between different parental genotypes results in offspring with specific phenotypes, according to the rules of dominance and incomplete dominance. Understanding phenotype inheritance helps us predict the appearance of future generations based on the genetic makeup of the parents. This is evident in the proportions of different colored horses observed in various genetic crosses detailed in the exercise.
Alleles
Alleles are different forms of the same gene. In our horse coat color example, the alleles dictate the color of the horse's coat, with each horse having two alleles per gene 鈥� one inherited from each parent. We have identified two main alleles here:
  • **C**: The allele associated with the chestnut coat color.
  • **c**: The allele associated with the cremello coat color.
The combination of these alleles determines the horse's coat color or phenotype.

Understanding alleles and their roles in conveying certain traits is critical to genetics. They provide the foundational blueprint for how traits are expressed and inherited across generations, demonstrating the complexity and variation within even a single gene.

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Most popular questions from this chapter

When a Chinese hamster with white spots is crossed with another hamster that has no spots, approximately \(1 / 2\) of the offspring have white spots and \(1 / 2\) have no spots. When two hamsters with white spots are crossed, \(2 / 3\) of the offspring possess white spots and \(1 / 3\) have no spots. a. What is the genetic basis of white spotting in Chinese hamsters? b. How might you go about producing Chinese hamsters that breed true for white spotting?

A summer-squash plant that produces disc-shaped fruit is crossed with a summer-squash plant that produces long fruit. All the \(\mathrm{F}_{1}\) have disc-shaped fruit. When the \(\mathrm{F}_{1}\) are intercrossed, \(\mathrm{F}_{2}\) progeny are produced in the following ratio: \(9 / 16\) disc- shaped fruit : \(6 / 16\) spherical fruit \(: 1 / 16\) long fruit. Give the genotypes of the \(\mathrm{F}_{2}\) progeny.

What is gene interaction? What is the difference between an epistatic gene and a hypostatic gene?

In chickens, comb shape is determined by alleles at two loci \((R, r\) and \(P, p)\). A walnut comb is produced when at least one dominant allele \(R\) is present at one locus and at least one dominant allele \(P\) is present at a second locus (genotype \(\left.R_{-} P_{-}\right) .\) A rose comb is produced when at least one dominant allele is present at the first locus and two recessive alleles are present at the second locus (genotype \(\left.R_{-} p p\right)\). A pea comb is produced when two recessive alleles are present at the first locus and at least one dominant allele is present at the second (genotype \(r r P_{-}\) ). If two recessive alleles are present at the first and at the second loci \((r r p p)\), a single comb is produced. Progeny with what types of combs and in what proportions will result from the following crosses? a. \(R R \space P P \times \operatorname{rr}\space p p\) b. \(\operatorname{Rr} P p \times \operatorname{rrpp}\) c. \(\operatorname{Rr} P p \times \operatorname{Rr} P p\) d. \(\operatorname{Rr} p p \times \operatorname{Rr} p p\) e. \(\operatorname{Rr} p p \times \operatorname{rr} \mathrm{Pp}\) f. \(\operatorname{Rr} p p \times \operatorname{rrpp}\)

Long ears in some dogs are an autosomal dominant trait. Two dogs mate and produce a litter in which \(75 \%\) of the puppies have long ears. Of the dogs with long ears in this litter, \(1 / 3\) are known to be phenocopies. What are the most likely genotypes of the two parents of this litter?
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