Q.10 When one gene masks the effect of another gene in a phenotype, then it is known as:
(1) Pleiotropy
(2) Epistasis
(3) Homeostasis
(4) Incomplete dominance
Gene Masking Explained: Epistasis
Epistasis occurs when one gene masks or modifies the effect of another gene at a different locus, creating novel phenotypic ratios. In the MCQ: “When one gene masks the effect of another gene in a phenotype, then it is known as,” the correct answer is (2) Epistasis.
Classic example: Coat color in Labrador retrievers—B gene (black/brown) masked by E gene (pigment deposition). ee genotype blocks color regardless of B gene (yellow labs).
Correct Answer: Option (2) – Epistasis
Types & Examples:
-
Dominant Epistasis (12:3:1): Fruit color in squash
-
Recessive Epistasis (9:3:4): Mouse coat color
-
Duplicate Recessive (9:7): Flower color
Labrador Example:
E_ B_ = Black ee B_ = Yellow (E masks B)
E_ bb = Chocolate ee bb = Yellow
Key: Epistatic gene (modifier) controls hypostatic gene (masked).
Why Not the Other Options? Complete Breakdown
-
Option (1) Pleiotropy: One gene affects multiple traits (e.g., sickle cell anemia: RBC shape + pain crises). Not gene-gene interaction.
-
Option (3) Homeostasis: Physiological balance maintenance (e.g., body temperature regulation). Not genetic masking.
-
Option (4) Incomplete Dominance: Two alleles of same gene blend (e.g., pink flowers from Rr). Same locus, no masking between different genes.
Comparison Table: Genetic Interactions
| Concept | Definition | # Genes Involved | Classic Example | Ratio |
|---|---|---|---|---|
| (2) Epistasis | One gene masks another | 2+ different loci | Labrador coat (9:3:4) | Modified |
| (1) Pleiotropy | One gene → multiple traits | 1 gene | Sickle cell disease | Normal |
| (3) Homeostasis | Physiological balance | Multiple systems | Temperature regulation | N/A |
| (4) Incomplete dominance | Alleles blend | 1 locus, 2 alleles | Snapdragon pink | 1:2:1 |
Biology Applications & Exam Strategy
Epistasis Examples:
-
Agouti mice: 9 black: 3 brown: 4 albino (recessive epistasis)
-
Chicken comb: 4 phenotypes from 2 genes
-
Human skin color: Polygenic + epistatic modifiers
Exam Tip: Epistasis = “Epi (upon) + stasis (standing)” = one gene stands upon/over another. Modified Mendelian ratios (not 9:3:3:1) signal epistasis.
Distinguish epistasis (different genes) vs. dominance/codominance (same gene).
