Duplicate Recessive Epistasis (9:7)
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Duplicate recessive epistasis also known as complementary gene interaction is a genetic interaction where two recessive alleles from different loci must be present together to produce a specific phenotype. If either locus has a dominant allele, the typical phenotype is observed. This form of epistasis involves two genes that must both have recessive alleles to suppress or alter the expression of a phenotype.
Key Characteristics
Key Characteristics
1. Two Gene Loci:
Involves two separate gene loci, each with two alleles (dominant and recessive).
2. Recessive Epistasis:
The recessive allele at each locus can mask the expression of the dominant allele at the other locus.
3. Phenotypic Ratio:
The typical phenotypic ratio for duplicate recessive epistasis in the offspring of a dihybrid cross is 9:7 (9 showing the dominant phenotype and 7 showing the recessive phenotype).
4. Complementary Interaction:
Both loci must have at least one dominant allele for the dominant phenotype to be expressed. If both loci are homozygous recessive, the recessive phenotype is observed.
Example: Flower Color in Sweet Peas
Example: Flower Color in Sweet Peas
In sweet peas, flower color is determined by two genes that show duplicate recessive epistasis.
The dominant alleles of each gene are required for the production of the purple color, while the recessive alleles lead to a white color.
Gene A and Gene B:
A: Purple allele (dominant)
a: White allele (recessive)
B: Purple allele (dominant)
b: White allele (recessive)
Crossing Two Heterozygous Plants (AaBb x AaBb): (Figures)
This results in a phenotypic ratio of 9 purple (dominant phenotype) to 7 white (recessive phenotype).
Phenotypic Ratio
Phenotypic Ratio
The 9:7 ratio in duplicate recessive epistasis can be explained by considering the genotypic combinations:
9 combinations (A_B_) have at least one dominant allele at each locus, thus expressing the dominant phenotype.
7 combinations (aaB_, A_bb, and aabb) have at least one locus with both recessive alleles, leading to the recessive phenotype.
Genetic Mechanism
Locus 1: This locus has two alleles, A (dominant) and a (recessive): When the genotype is AA or Aa, Enzyme A is produced, which forms a colorless/white intermediate.
Locus 2: This locus has two alleles, B (dominant) and b (recessive): When the genotype is BB or Bb, Enzyme B is produced, which converts the intermediate into a purple-colored pigment, resulting in a purple flower.
Phenotype outcome:
The dominant phenotype (purple color) is expressed if at least one dominant allele is present at both loci (A_ and B_).
The recessive phenotype (white color) appears only when both loci have homozygous recessive alleles (aabb).
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