16.10 Epistasis

1. What is Epistasis?

• Definition: Epistasis refers to the interaction between two or more different genes where the alleles of one gene influence or modify the expression of alleles of another gene, ultimately affecting the same phenotypic trait.

Key Concept:

• Gene Interaction: The expression of one gene depends on the presence or absence of alleles in another gene.
• Phenotypic Modification: This interaction can enhance, suppress, or alter the expected phenotypic outcome based on each gene individually.

Visual Representation: Imagine genes as players in a game where one player’s action can influence another’s performance, leading to outcomes different from what each player could achieve alone.

2. Real-World Examples of Epistasis

• To solidify your understanding, let’s explore two detailed examples of epistasis in action:

Example 1: Epistasis in Chicken Feather Color

Genes Involved:

1. Gene G/g (Pigment Production)
   • G (Dominant): Produces pigment, resulting in colored feathers.
   • g (Recessive): No pigment production, resulting in white feathers.
2. Gene F/f (Feather Inhibition)
   • F (Dominant): Inhibits pigment production, leading to white feathers regardless of the G gene.
   • f (Recessive): Does not inhibit pigment production, allowing colored feathers if the G allele is present.

Genotype to Phenotype Mapping:

Explanation:

• Dominant F Allele (F): Masks the effect of the G gene, resulting in white feathers irrespective of G’s alleles.
• Recessive ff Genotype: Allows the G gene to express pigment production, leading to colored feathers if at least one G allele is present (ffGG or ffGg).
• gg Genotype: Prevents pigment production, resulting in white feathers regardless of the F gene.

Example Crosses:

1. Cross Between White Leghorn (FFGG) and White Wyandotte (ffgg) Chickens
   • Parental Genotypes and Gametes:
     • White Leghorn (FFGG): Gametes = FG
     • White Wyandotte (ffgg): Gametes = fg
   • Punnett Square:

1. F1 Generation:
   • Genotype: All offspring are FfGg.
   • Phenotype: White feathers (due to dominant F allele).
2. Cross Between Two F1 Chickens (FfGg × FfGg)
   • Parental Gametes: FG, Fg, fG, fg
   • Punnett Square:

1. F2 Generation Phenotypic Ratio:
   • White Feathers: 9 (FFGG, FFGg, FfGG, FfGg, FFgg, Ffgg)
   • Colored Feathers: 3 (ffGG, ffGg)
   • White Feathers due to gg Genotype: 4 (ffgg)
2. Resulting Ratio: 9 White : 3 Colored : 4 White
   (Note: The additional white phenotype from gg genotype exemplifies epistasis, where gg masks the effect of the F gene.)

Example 2: Epistasis in Flower Color in Salvia Plants

Genes Involved:

1. Gene B/b (Flower Color)
   • B (Dominant): Purple flowers.
   • b (Recessive): Pink flowers.
2. Gene A/a (Color Activation)
   • A (Dominant): Necessary for color expression.
   • a (Recessive): No color expression; flowers are white.

Genotype to Phenotype Mapping:

Explanation:

• Dominant A Allele (A): Required for any color to be expressed. Without at least one A allele (aa), flowers remain white regardless of the B gene.
• B Allele in Presence of A:
  • B: Produces purple flowers.
  • b: Produces pink flowers.
• Recessive a Allele (a): Masks the effect of the B gene, leading to white flowers when homozygous (aa).

Example Crosses:

1. Cross Between Homozygous Pink-Flowered (AAbb) and White-Flowered (aaBB) Salvia Plants
   • Parental Genotypes and Gametes:
     • Pink-Flowered (AAbb): Gametes = Ab
     • White-Flowered (aaBB): Gametes = aB
   • Punnett Square:

1. F1 Generation:
   • Genotype: All offspring are AaBb.
   • Phenotype: Purple flowers (presence of both A and B alleles).
2. Cross Between F1 Generation (AaBb × AaBb)
   • Parental Gametes: AB, Ab, aB, ab
   • Punnett Square:

1. F2 Generation Phenotypic Ratio:
   • Purple Flowers: 9 (AABB, AABb, AaBB, AaBb)
   • Pink Flowers: 3 (AAbb, Aabb)
   • White Flowers: 4 (aaBB, aaBb, aabb)
2. Resulting Ratio: 9 Purple : 3 Pink : 4 White
   (The white phenotype arises from the aa genotype, which masks the color produced by the B gene, demonstrating epistasis.)

3. Types of Epistasis

• Epistasis can manifest in various forms, each characterized by different interactions between genes:

1. Recessive Epistasis:
   • Description: Recessive alleles of one gene mask the expression of alleles of another gene.
   • Example: In chicken feather color, the gg genotype results in white feathers regardless of the F gene’s alleles.
2. Dominant Epistasis:
   • Description: Dominant alleles of one gene mask the expression of alleles of another gene.
   • Example: The dominant F allele in chicken feather color inhibits pigment production, leading to white feathers regardless of the G gene.
3. Duplicate Recessive Epistasis:
   • Description: Both genes must be homozygous recessive to produce a recessive phenotype.
   • Example: In Salvia flower color, white flowers appear only when both A and B genes are homozygous recessive (aabb).
4. Duplicate Dominant Epistasis:
   • Description: Presence of at least one dominant allele in either gene produces the same dominant phenotype.
   • Example: Not illustrated in the provided examples but important for comprehensive understanding.

Summary:

4. Visualizing Epistasis: Punnett Squares and Phenotypic Ratios

• Understanding epistasis often involves analyzing Punnett squares to predict phenotypic ratios. Follow these steps to effectively map out and interpret genetic interactions:

Step-by-Step Approach:

1. Identify Genes and Alleles:
   • Determine which genes are interacting and their respective alleles.
   • Example: In chicken feather color, genes F/f and G/g are involved.
2. Determine Dominance Relationships:
   • Understand which alleles are dominant or recessive and how they influence each other.
   • Example: F (dominant) inhibits pigment production; g (recessive) allows pigment production.
3. Set Up Punnett Square:
   • List possible gametes from each parent.
   • Combine them systematically to predict offspring genotypes.
4. Map Genotypes to Phenotypes:
   • Use genotype-to-phenotype mappings to determine the physical traits of the offspring.
   • Identify which gene is epistatic (masking) and which is hypostatic (masked).
5. Calculate Phenotypic Ratios:
   • Count the number of each phenotype to establish the ratio.
   • Highlight how epistatic interactions alter expected Mendelian ratios.

Example Application:

Using the chicken feather color cross between FfGg × FfGg:

• Punnett Square Outcome:
  • 9 White (due to F allele or gg genotype)
  • 3 Colored (ffGG, ffGg)
  • 4 White (ffgg)
• Phenotypic Ratio: 9 White : 3 Colored : 4 White
• Interpretation: The additional white phenotypes from gg genotype demonstrate epistasis, where gg masks the effect of the F gene.

5. Key Terms and Definitions

• Familiarize yourself with these essential terms to enhance your understanding of epistasis:

• Epistasis: Interaction between different genes affecting the same trait.
• Allele: Different forms of a gene (e.g., G vs. g).
• Genotype: The genetic makeup of an organism (e.g., FfGg).
• Phenotype: The observable physical traits (e.g., white or colored feathers).
• Dominant Allele: An allele that masks the effect of a recessive allele.
• Recessive Allele: An allele whose effects are masked by a dominant allele.
• Punnett Square: A diagram used to predict the genotype and phenotype combinations of offspring.
• Epistatic Gene: The gene that masks or modifies the expression of another gene.
• Hypostatic Gene: The gene whose expression is masked or modified by another gene.