Chapter 4, Problem 1NST

In the guinea pig, one locus involved in the control of coat color may be occupied by any of four alleles: C (full color), c^k (sepia), c^d (cream), or c^a (albino), with an order of dominance of: C > c^k > c^d > c^a. (C is dominant to all others, c^k is dominant to c^d and c^a, but not C, etc.) In the following crosses, determine the parental genotypes and predict the phenotypic ratios that would result:

1. (a) sepia × cream, where both guinea pigs had an albino parent
2. (b) sepia × cream, where the sepia guinea pig had an albino parent and the cream guinea pig had two sepia parents
3. (c) sepia × cream, where the sepia guinea pig had two full-color parents and the cream guinea pig had two sepia parents
4. (d) sepia × cream, where the sepia guinea pig had a full-color parent and an albino parent and thecream guinea pig had two full-color parents

Summary Introduction

To determine: The parental genotype of sepia and cream coated pigs that had an albino parent and the phenotypic ratio of the cross between sepia and cream pigs.

Introduction: In guinea pig, one locus that controls the coat color may have four alleles: C, c^k, c^d, or c^a. c^a is the least dominant allele, and C is dominant to all other alleles.

Explanation of Solution

If sepia and cream coated pigs have albino parent, the genotype of sepia and cream pigs will be c^kc^a and c^dc^a, respectively. The cross between c^kc^a and c^dc^a results in $\left(\frac{2}{4}\right)$ sepia, $\left(\frac{1}{4}\right)$ cream, and $\left(\frac{1}{4}\right)$ albino pigs. Therefore, 2:1:1 is the phenotypic ratio.

Thus, the genotype of sepia pig is c^kc^a, and cream pig is c^dc^a, and the phenotypic ratio of the cross $c^k{c}^a\times c^d{c}^a$ is 2:1:1.

Summary Introduction

To determine: The parental genotype of sepia and cream coated pigs that had an albino parent and two sepia parents respectively and the phenotypic ratio of their cross.

Introduction: The cream guinea pig could be formed by the cross between c^kc^d and c^kc^d or the cross between c^kc^d and c^kc^a.

Explanation of Solution

If sepia had albino parent, the genotype of sepia guinea pig would be c^kc^a. The genotype of cream guinea pig could be c^dc^d or c^dc^a.

The cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^d results in $\left(\frac{1}{2}\right)$ sepia pig and $\left(\frac{1}{2}\right)$ cream pig. Therefore the phenotypic ratio is 1:1.

The cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^a results in $\left(\frac{1}{2}\right)$ sepia pig, $\left(\frac{1}{4}\right)$ cream pig, and $\left(\frac{1}{4}\right)$ albino pig. Therefore 2:1:1 is the phenotypic ratio.

Thus, the genotype of sepia pig is c^kc^a, and cream pig could be c^dc^d or c^dc^a. The phenotypic ratio of the cross $c^k{c}^a\times c^d{c}^a$ is 2:1:1 and $c^k{c}^a\times c^d{c}^d$ is 1:1.

Summary Introduction

To determine: The parental genotype of sepia and cream coated pigs that had two full-color parents and two sepia parents respectively and the phenotypic ratio of their cross.

Introduction: The genotype of sepia guinea pig parent could be Cc^k, Cc^d, or Cc^a and not CC because sepia could not be produced.

Explanation of Solution

The genotype of sepia pig could be c^kc^k, c^kc^d, or c^kc^a. Cream pig could be formed by the cross between c^kc^d and c^kc^d or the cross between c^kc^d and c^kc^a. The genotype of cream guinea pig could be c^dc^d or c^dc^a.

The cross between sepia guinea pig with genotype c^kc^k and cream guinea pig with genotype c^dc^d forms all sepia pigs. The cross between sepia guinea pig with genotype c^kc^k and cream guinea pig with genotype c^dc^a also forms all sepia pigs.

The cross between sepia guinea pig with genotype c^kc^d and cream guinea pig with genotype c^dc^d forms $\left(\frac{1}{2}\right)$ sepia pig and $\left(\frac{1}{2}\right)$ cream pig. The cross between sepia guinea pig with genotype c^kc^d and cream guinea pig with genotype c^dc^a also forms $\left(\frac{1}{2}\right)$ sepia pig and $\left(\frac{1}{2}\right)$ cream pig. Similarly, the cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^d forms $\left(\frac{1}{2}\right)$ sepia pig and $\left(\frac{1}{2}\right)$ cream pig.

The cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^a  forms $\left(\frac{1}{2}\right)$ sepia pig, $\left(\frac{1}{4}\right)$ albino pig, and $\left(\frac{1}{4}\right)$ cream pig.

Thus, the genotype of sepia pig could be c^kc^k, c^kc^d, or c^kc^a, and cream guinea pig could be c^dc^d or c^dc^a. The crosses $c^k{c}^k\times c^d{c}^a$ and $c^k{c}^k\times c^d{c}^d$ produces all sepia pigs. The phenotypic ratio of the crosses $c^k{c}^d\times c^d{c}^d$, $c^k{c}^d\times c^d{c}^a$, $c^k{c}^a\times c^d{c}^d$ is 1:1. The phenotypic ratio of cross $c^k{c}^a\times c^d{c}^a$ is 2:1:1.

Summary Introduction

To determine: The parental genotype of sepia and cream coated pigs that had full-color parent, and albino parent and two full-color parents respectively and the phenotypic ratio of their cross.

Introduction: Sepia guinea pig is formed by the cross between Cc^k and c^ac^a. The cream pig had two full-color parents that could be Cc^d or Cc^a.

Explanation of Solution

The genotype of the sepia pig must be c^kc^a. The cream pig had two full-color parents that could be Cc^d or Cc^a. The genotype of a cream pig could be c^dc^d or c^dc^a.

The cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^d forms $\left(\frac{1}{2}\right)$ sepia pig and $\left(\frac{1}{2}\right)$ cream pig. The cross between sepia guinea pig with genotype c^kc^a and cream guinea pig with genotype c^dc^a  forms $\left(\frac{1}{2}\right)$ sepia pig, $\left(\frac{1}{4}\right)$ albino pig, and $\left(\frac{1}{4}\right)$ cream pig.

Thus, the genotype of sepia pig must be c^kc^a, and cream guinea pig could be c^dc^d or c^dc^a. The phenotypic ratio of cross $c^k{c}^a\times c^d{c}^d$ is 1:1 and $c^k{c}^a\times c^d{c}^a$ is 2:1:1.