Lecture Notes: Gene Interactions

Introduction

• Previous discussions included novel phenotypes, recessive epistasis, and dominant epistasis (A).
• Today focuses on dominant epistasis B and complementary gene action.

Dominant Epistasis B

• Definition: When one gene is dominant, it is epistatic to another gene.
• Conditions: Complete dominance in each gene pair.
• Example: Feather color in fowl
  • Controlled by two genes: Gene I and Gene C.
  • Gene I: Dominant allele causes color inhibition (white), recessive allele allows color expression.
  • Gene C: Dominant allele leads to color, recessive leads to non-color.
  • Cross Example: Heterozygous rooster and hen both have white feathers due to dominant I inhibiting color, despite dominant C.
• Phenotypic Ratio: 13 (white) : 3 (colored)

Complementary Gene Action

• Definition: Requires complete dominance in each gene pair; either recessive homozygous allele is epistatic.
• Example: Flower color in pea plants
  • Two genes: Gene P (purple/white) and Gene C (color/no color).
  • Both dominant alleles P and C are required for pigment production (purple).
  • Cross Example: Both parents heterozygous for P and C produce purple flowers.
• Phenotypic Ratio: 9 (purple) : 7 (white)
• Biochemical Process: Genes produce enzymes catalyzing steps in pigment production.

Duplicate Gene Action

• Definition: Either gene when dominant is epistatic; genes do the same thing (duplicate).
• Example: Seed capsule shape in shepherd's purse
  • Gene A and Gene B both control triangular shape (dominant to ovoid).
  • Phenotypic Ratio: 15 (triangular) : 1 (ovoid)

Summary of All Gene Interactions

1. Novel Phenotype

   • Example: Comb type in chicken
   • Phenotypic Ratio: 9 : 3 : 3 : 1

2. Recessive Epistasis

   • Example: Coat color in mice (agouti, black, albino)
   • Phenotypic Ratio: 9 : 3 : 4

3. Dominant Epistasis A

   • Example: Fruit color in summer squash
   • Phenotypic Ratio: 12 : 3 : 1

4. Dominant Epistasis B

   • Example: Feather color in fowl
   • Phenotypic Ratio: 13 : 3

5. Complementary Gene Action

   • Example: Flower color in pea plants
   • Phenotypic Ratio: 9 : 7

6. Duplicate Gene Action

   • Example: Shape of seed capsule in shepherd's purse
   • Phenotypic Ratio: 15 : 1

Conclusion

• Next topic will be pseudoalleles, to be discussed in another session.