Duplicate Dominant Epistasis (15:1)
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Duplicate dominant epistasis is a form of gene interaction where at least one dominant allele from either of two loci is sufficient to produce a specific phenotype. This means that the presence of a dominant allele at either locus can mask the effect of the recessive alleles. As a result, the recessive phenotype is only observed when both loci are homozygous recessive.
Key Characteristics
Key Characteristics
1. Two Gene Loci:
Involves two different gene loci, each with a pair of alleles (dominant and recessive).
2. Dominant Epistasis:
A single dominant allele at either locus is enough to express the dominant phenotype.
3. Phenotypic Ratio:
The typical phenotypic ratio in the offspring of a dihybrid cross exhibiting duplicate dominant epistasis is 15:1. This means that 15 of the 16 possible genotypic combinations result in the dominant phenotype, and only 1 combination results in the recessive phenotype.
4. Complementary Dominance:
The dominant alleles at the two loci are functionally redundant for producing the dominant phenotype. Therefore, the presence of a dominant allele at either locus suffices to exhibit the trait.
Example: Fruit Shape in Cucurbits
Example: Fruit Shape in Cucurbits
In some cucurbit species (like squash), fruit shape is controlled by two genes showing duplicate dominant epistasis. Both genes contribute to producing a dominant phenotype, where the presence of a dominant allele from either gene results in a disc-shaped fruit. The recessive phenotype (spherical fruit) is observed only when both loci are homozygous recessive.
Genes Involved:
A: Dominant allele for disc shape.
B: Dominant allele for disc shape.
a: Recessive allele for spherical shape.
b: Recessive allele for spherical shape.
Crossing Two Heterozygous Plants (AaBb x AaBb):
The phenotypic ratio would be:
15 Disc: A_B_, A_bb, aaB_
1 Spherical: aabb
Phenotypic Ratio
Phenotypic Ratio
The phenotypic ratio of 15:1 is explained by considering:
15 combinations have at least one dominant allele at one or both loci (A_B_, A_bb, aaB_).
1 combination (aabb) has both loci homozygous recessive, leading to the recessive phenotype
Dominant Phenotype: Combinations where at least one dominant allele is present at one or both loci (A_B_, A_bb, aaB_).
Recessive Phenotype: The combination where both loci are homozygous recessive (aabb).
Genetic Mechanism
Genetic Mechanism
Locus 1: Alleles A (dominant) and a (recessive).
Locus 2: Alleles B (dominant) and b (recessive).
The phenotypic expression is determined as follows:
The dominant phenotype appears if at least one dominant allele is present at either locus (A_B_, A_bb, or aaB_). (Note: Here "_" blank space represents any type of allele i.e., dominant or recessive).
The recessive phenotype appears only when both loci are homozygous recessive (aabb).
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