Extensions to Mendelian Inheritance: Gene Interaction and Epistasis

Extensions to Mendelian Inheritance

Gene Interaction (Two Loci)

Gene interaction occurs when alleles at multiple loci collectively determine a single phenotype. This concept extends basic Mendelian inheritance by considering how genes can influence each other's expression, leading to novel phenotypes and modified inheritance ratios.

• Definition: Gene interaction refers to the phenomenon where the effect of alleles at one locus depends on the alleles present at another locus.

• Dihybrid Crosses: Crosses involving two loci (e.g., AaBb x AaBb) yield 16 possible genotypic combinations.

• Interpretation: The phenotypic outcome depends on how these genotypes interact, which may deviate from the classic Mendelian ratios.

Novel Phenotypes from Gene Interaction

Some gene interactions produce phenotypes not seen in either parent, known as novel phenotypes. This is often observed in traits such as pepper color, where two loci interact to produce new color combinations in the F2 generation.

• Example: In peppers, crossing red and cream varieties can produce red, peach, orange, and cream offspring in a 9:3:3:1 ratio.

• Genotypic Explanation: Each phenotype corresponds to a specific combination of alleles at two loci.

Dihybrid Crosses and the 9:3:3:1 Ratio

The classic dihybrid cross (AaBb x AaBb) assumes independent assortment and complete dominance at each locus, resulting in a 9:3:3:1 phenotypic ratio.

• Assumptions:

  1. Both loci assort independently.

  2. Each locus exhibits complete dominance.

  3. Each of the four phenotypes is distinct and unambiguous.

• Punnett Square: Used to visualize the 16 possible genotypes and their corresponding phenotypes.

Equation:

$9:3:3:1$

Deviations from the 9:3:3:1 Ratio

Gene interactions can cause deviations from the expected 9:3:3:1 ratio. These deviations occur when the assumptions of independent assortment, complete dominance, or distinct phenotypes are violated.

• Linked Genes: Genes on the same chromosome may not assort independently.

• Epistasis: One gene may mask or modify the effect of another, altering the expected ratio.

• Indistinguishable Classes: Some genotypes may produce the same phenotype, reducing the number of observable classes.

Epistasis: One Gene Masks the Effect of Another

Types of Epistasis

Epistasis is a specific type of gene interaction where one gene masks or suppresses the expression of another gene at a different locus.

• Recessive Epistasis: The epistatic effect is only seen when the masking gene is homozygous recessive (e.g., Labrador retriever coat color, Bombay phenotype).

• Dominant Epistasis: A single copy of the dominant allele at the epistatic locus is sufficient to mask the other gene (e.g., fruit color in summer squash).

Epistatic vs. Hypostatic Alleles

• Epistatic Alleles: The alleles that mask the effect of another gene.

• Hypostatic Alleles: The alleles whose effect is being masked.

Analyzing Epistasis Problems

Epistasis problems often involve polygenic traits. Dihybrid crosses and Punnett squares or branch diagrams are used to predict phenotypic ratios.

• Polygenic Traits: Traits controlled by multiple genes.

• Tools: Punnett squares, branch diagrams.

Recessive Epistasis: Labrador Retriever Coat Colors

Genetic Basis

Fur color in Labrador Retrievers is determined by two genes: one controlling pigment color (B locus) and one controlling pigment deposition (A locus).

• B locus: B (black), b (brown)

• A locus: A (allows pigment deposition), a (prevents pigment deposition)

Phenotypes:

• At least one dominant A allele: pigment deposited (black or brown, depending on B locus)

• aa genotype: no pigment deposited, resulting in golden fur regardless of B locus

Phenotypic Ratios

Recessive epistasis modifies the dihybrid cross ratio from 9:3:3:1 to 9:3:4.

Equation:

$9:3:4$

Genotype	Phenotype	Ratio
A_B_	Black	9/16
A_bb	Brown	3/16
aaB_	Golden	3/16
aabb	Golden	1/16

Example: Bombay Phenotype

The Bombay phenotype in humans is a classic example of recessive epistasis affecting blood type expression. Individuals homozygous for the recessive h allele cannot produce the H antigen, resulting in blood type O regardless of their ABO genotype.

• Epistatic locus: H/h

• Hypostatic locus: ABO

• Phenotype: hh genotype masks ABO blood type expression

Dominant Epistasis: Fruit Color in Summer Squash

Genetic Basis

Fruit color in summer squash is determined by two genes. The presence of a dominant allele at the first locus (A) masks the expression of the second locus (B).

• A locus: A (white), a (allows other colors)

• B locus: B (yellow), b (green)

Phenotypes:

• A_B_ or A_bb: white

• aaB_: yellow

• aabb: green

Phenotypic Ratios

Dominant epistasis modifies the dihybrid cross ratio from 9:3:3:1 to 12:3:1.

Equation:

$12:3:1$

Genotype	Phenotype	Ratio
A_B_	White	9/16
A_bb	White	3/16
aaB_	Yellow	3/16
aabb	Green	1/16

Biochemical Pathways and Epistasis

Epistasis often reflects underlying biochemical pathways, where the product of one gene is required for the function of another. For example, pigment production in squash involves a two-step pathway, and a dominant allele can block the pathway at an early stage, preventing downstream products from forming.

Summary Table: Modified Dihybrid Ratios Due to Gene Interaction

Ratio	A_B_	A_bb	aaB_	aabb	Type of Interaction	Example
9:3:3:1	9	3	3	1	None	Seed shape/color in peas
9:3:4	9	3	4	0	Recessive epistasis	Coat color in Labrador retrievers
12:3:1	12	3	1	0	Dominant epistasis	Color in squash
9:7	9	7	0	0	Duplicate recessive epistasis	Albinism in snails
15:1	15	1	0	0	Duplicate dominant epistasis	Additional info: Example not specified

Practice Problems and Applications

• Predicting Phenotypic Ratios: Use Punnett squares and knowledge of gene interactions to predict offspring ratios in crosses involving epistasis.

• Identifying Epistatic and Hypostatic Loci: Determine which gene is masking (epistatic) and which is being masked (hypostatic) in genetic problems.

• Real-World Examples: Labrador retriever coat color, Bombay phenotype in humans, fruit color in squash, and coat color in large cats.

Key Terms

• Gene Interaction: The effect of one gene depends on the presence of alleles at another gene.

• Epistasis: One gene masks the effect of another gene at a different locus.

• Epistatic Allele: The allele that does the masking.

• Hypostatic Allele: The allele whose effect is masked.

• Polygenic Trait: A trait controlled by multiple genes.

Additional info: This study guide expands on the provided notes by including definitions, examples, and tables for clarity and completeness, suitable for Genetics college students preparing for exams.