Dominant Epistasis (12:3:1)
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Dominant epistasis occurs when a dominant allele at one locus masks or suppresses the expression of alleles at a second locus, regardless of whether they are dominant or recessive. This type of epistasis often results in a phenotypic ratio of 12:3:1 in a dihybrid cross.
Key Characteristics
Interaction: A dominant allele at one locus (e.g., AAA) can mask the expression of alleles at a second locus (e.g., BBB and bbb).
Phenotypic Ratio: Typically, the ratio observed is 12:3:1, where:
12 individuals exhibit the dominant phenotype due to the dominant epistatic allele.
3 individuals show the phenotype controlled by the second locus (if the first locus is homozygous recessive).
1 individual shows the recessive phenotype of the second locus.
Example: Fruit Color in Squash
In some squash plants, fruit color is determined by two genes with dominant epistasis:
A: White color (dominant epistasis).
B: Yellow color (dominant).
a: No color (recessive).
b: Green color (recessive).
Crossing Heterozygous Plants (AaBb x AaBb):
12 White: A_ __ (A_B_ and A_bb).
3 Yellow: aaB_.
1 Green: aabb.
Genetic Mechanism
Locus 1: Alleles A (dominant) and a (recessive).
Locus 2: Alleles B (dominant) and b (recessive).
The phenotypes are determined as follows:
A_ __: Dominant phenotype (masks all alleles at the second locus).
aaB_: Phenotype of the second locus.
aabb: Recessive phenotype of the second locus.
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