Recessive epistasis occurs when a recessive allele at one locus masks or suppresses the expression of alleles at a second locus. This results in a phenotypic ratio of 9:3:4 in a dihybrid cross.

Understanding with Example: Coat Color in Mice

In mice, coat color is determined by two genes with recessive epistasis:

Genotype  AA or Aa is responsible for making the BLACK Color, and then BLACK Color is converted into AGOUTI Color in the present of functional Product-B (BB/Bb).

If a mice has an "aa" genotype, no black color will be produced, which in turn prevents the production of agouti color, even if the other genotype is BB or Bb is present. This means that a recessive genotype (aa)  (Recessive Epistasis) at one locus can affect the production of the final color (agouti). 

Key Characteristics

1. Interaction: A recessive allele at one locus (e.g., aa) can mask the expression of alleles at a second locus (e.g., B and b).

2. Phenotypic Ratio: Typically, the ratio observed is 9:3:4, where:

   • 9 individuals exhibit the dominant phenotype.

   • 3 individuals show the phenotype controlled by the second locus.

   • 4 individuals show the recessive phenotype because the epistatic allele is homozygous recessive.

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_B_: Dominant phenotype.

• A_bb: Phenotype of the second locus.

• aaB_ and aabb: Recessive phenotype (epistatic allele aa  masks B and b).

Crossing Heterozygous Mice (AaBb x AaBb):

• 9 Agouti: AABB, AaBB, AaBb.

• 3 Black: AAbb, Aabb

• 4 Albino: aaBB, aaBb and aabb (recessive allele aa masks B and b).