Understanding the modification of dihybrid ratios due to gene interactions (epistasis).
Recognizing that a phenotype can be controlled by multiple interacting genes.
Utilizing model organisms and the complementation test to determine the number of genes controlling a characteristic.
Key Concepts
Gene Interaction: Most genes do not operate in isolation. Phenotypes are often controlled by multiple interacting genes, leading to epistasis which modifies Mendelian ratios.
Examples of Gene Interaction (Epistasis)
1. Complementary Gene Action
Example: Purple flower color (anthocyanin) in peas.
Two loci: P/p and C/c. Dominant alleles needed at both loci for color.
Genotype PP and Pp: Purple; pp: White
Genotype CC and Cc: Color; cc: White
9:7 phenotypic ratio indicative of complementary gene action.
2. Recessive Epistasis
Example: Coat color in mice
Loci: A (agouti), B (pigment production)
9:3:4 phenotypic ratio shows recessive epistasis where bb masks A locus.
3. Dominant Epistasis
Example: Color in summer squash
Loci: A (colorless), B (white dominant)
12:3:1 phenotypic ratio indicates dominant epistasis, where B is epistatic to A.
4. Duplicate Gene Action
Example: Seed shape in wheat
Loci: T, V
15:1 phenotypic ratio indicates duplicate gene action.
Genetic Testing and Model Organisms
Complementation Test: Used to identify if mutations are allelic or in different genes.
Crossing mutants can establish complementation groups.
If offspring are wild-type, mutations are in different genes.
Summary of Genetic Ratios
9:7 Ratio: Indicates complementary gene action
9:3:4 Ratio: Indicates recessive epistasis
12:3:1 Ratio: Indicates dominant epistasis
15:1 Ratio: Indicates duplicate gene action
Conclusion
Most traits are controlled by a number of interacting genes leading to departures from Mendelian ratios.
Gene identification involves mutation of model organisms and uses tests like the complementation test to understand gene interactions.