Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color: A third gene pair on a separate autosome determines whether or not any color will be produced. The \(C C\) and \(C c\) genotypes allow color according to the expression of the \(A\) and \(B\) alleles. However, the \(c c\) genotype results in albino rats regardless of the \(A\) and \(B\) alleles present. Determine the \(F_{1}\) phenotypic ratio of the following crosses: (a) \(A A b b C C \quad \times \quad\) aaBBcc (b) \(A a B B C C \quad \times \quad A A B b c c\) (c) \(A a B b C c \quad \times \quad\) AaBbcc (d) \(A a B B C c \quad \times \quad A a B B C c\) (e) \(A A B b C c \quad \times \quad A A B b c c\)

Short answer

Answer: The phenotypic ratio of this cross is 1:0, with all progenies having the same colored phenotype.

Step-by-step solution

(a) Cross: \(A A b b C C \times aaBBcc\)

First, let's perform a Punnett square with the given genotypes of the two rats. Since both of the rats' genotypes for Gene A are homozygous (AA and aa), the progeny will maintain their respective genotype. For Gene B, both rats are homozygous but different (bb x BB), and Gene C, one rat is homozygous dominant and the other one is homozygous recessive (CC x cc). The resulting offspring genotype will be \(AaBbCc\). Since C is present, color will be expressed. The \(F_{1}\) phenotypic ratio is 1:0, since all progenies will share the same phenotype.

(b) Cross: \(A a B B C C \times A A B b c c\)

Perform a Punnett square for each gene pair. For Gene A: All offspring will be Aa. For Gene B: Half the offspring will be BB, and the other half will be Bb. For Gene C: All offspring will be Cc. As C is present, color is expressed. The \(F_{1}\) phenotypic ratio will be 1 (A_) : 1 (a_) for Gene A and 2 (B_) : 0 (bb) for Gene B, resulting in a 2:0 overall ratio.

(c) Cross: \(A a B b C c \times A a B b c c\)

Perform a Punnett square for each gene pair. For Gene A: The ratio is 1 (AA) : 2 (Aa) : 1 (aa). For Gene B: The ratio is 1 (BB) : 2 (Bb) : 1 (bb). For Gene C: One-fourth of the offspring will be cc (albino), and the remaining three-fourth will be C_. This results in a 9:3:4 phenotypic ratio when considering the three genes together: 9 colored with dominant A and B, 3 colored with dominant A and recessive b, and 4 albino.

(d) Cross: \(A a B B C c \times A a B B C c\)

Perform a Punnett square for each gene pair. For Gene A: The ratio is 1 (AA) : 2 (Aa) : 1 (aa). For Gene B: All offspring will be BB. For Gene C: One-fourth of the offspring will be cc (albino), and the remaining three-fourth will be C_. This results in a 3:1 phenotypic ratio: 3 colored with dominant A and B and 1 albino.

(e) Cross: \(A A B b C c \times A A B b c c\)

Perform a Punnett square for each gene pair. For Gene A: All offspring will be AA. For Gene B: The ratio is 1 (BB) : 2 (Bb) : 1 (bb). For Gene C: One-fourth of the offspring will be cc (albino), and the remaining three-fourth will be C_. This results in a 6:2 phenotypic ratio: 6 colored with dominant A and various combinations of B and 2 albino.

Key concepts

Punnett Square

The Punnett square is a simple but powerful tool used in genetics to predict the possible genotypes of offspring based on the parental alleles. This method provides a visual representation of all potential combinations and helps in understanding the probability of inheriting particular traits.

To construct a Punnett square, you list the alleles of one parent along the top, and the alleles of the other parent along the side. Each box within the square represents a possible genotype for the offspring. For example, in a dihybrid cross with two traits, you create a box for every possible combination, usually resulting in a 4x4 grid.

Using the Punnett square, you can predict the genotypic and phenotypic ratios of the offspring. This tool is especially useful when dealing with Mendelian inheritance patterns and can be applied to more complex genetic crosses by accounting for multiple alleles or genes.

Phenotypic Ratio

The phenotypic ratio refers to the relative number of offspring manifesting different phenotypes, resulting from a genetic cross. It reflects how often each phenotype appears in the offspring.

For example, in a typical Mendelian monohybrid cross involving a dominant and recessive trait, the phenotypic ratio often results in a 3:1 ratio for dominant to recessive phenotypes, assuming that the trait is governed by simple Mendelian inheritance.

Dihybrid crosses involving two genes can achieve a 9:3:3:1 phenotypic ratio, where 9/16 shows both dominant traits, 3/16 show one dominant and one recessive trait, another 3/16 exhibit the opposite combination, and 1/16 has both recessive traits. However, the presence of additional modifying factors, such as albino genes in the given exercise, can alter this expected outcome into something like the seen 9:3:4, where the final group is influenced by the modifying cc allele causing albinism.

Autosomal Genes

Autosomal genes are located on one of the 22 non-sex chromosomes, known as autosomes. These genes are not directly involved in determining the sex of an individual, unlike the genes located on sex chromosomes (X and Y).

When dealing with autosomal genes, inheritance patterns are predictable through Mendel's laws of inheritance. Each individual inherits one allele from the mother and one from the father, resulting in either homozygous (two of the same alleles) or heterozygous (one of each allele) genotypes.

In the provided genetic cross exercise, genes A, B, and C are discussed. Genes A and B are examples of autosomal genes affecting coat color in rats, influenced by their combinations. The gene C acts as a switch. If a rat inherits two recessive cc alleles, it results in albino phenotype, overriding the effects of genes A and B. This type of interaction between genes exemplifies epistasis, where one gene can suppress or modify the expression of another.