Epistasis: Gene Interaction Ratios

Cross Type

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Cross AaBb × AaBb

Observed ratio (16 offspring)

9 : 3 : 3 : 1

Distinct phenotype classes

P(largest class)

56.3%

How it works

Two heterozygous parents, AaBb × AaBb, each produce four gamete types — AB, Ab, aB, ab — with equal probability ¼ (Mendel's Law of Independent Assortment, valid when the two genes sit on different chromosomes). Combining every gamete pair gives the 4×4 = 16-cell Punnett square shown in the grid.

P(A_) = 3/4   P(aa) = 1/4      (Law of Segregation, one locus)
P(A_B_) = P(A_)·P(B_) = 3/4 × 3/4 = 9/16
baseline dihybrid ratio:  9 : 3 : 3 : 1
  A_B_ : A_bb : aaB_ : aabb

Epistasis is when the genotype at one locus masks or modifies the phenotype normally produced by another locus. It doesn't change the 16 underlying genotype counts — it changes how many of those 4 baseline categories look identical, collapsing 9:3:3:1 into a smaller, characteristic ratio:

Recessive epistasis   9:3:4   aa masks locus B entirely      (e.g. Labrador coat colour, e/e)
Dominant epistasis    12:3:1  A_ masks locus B entirely       (e.g. summer squash colour)
Duplicate recessive   9:7     needs A_ AND B_ together        (complementary gene action)
Duplicate dominant    15:1    either A_ or B_ alone suffices  (redundant pathways)
Dom. & recessive     13:3   A_ or aabb → phenotype 1
Polymeric (additive)  9:6:1   each dominant allele set adds a level of the trait

Pick a cross type on the left: the grid recolours by phenotype class and the live readouts recompute the ratio, class count and top-class probability straight from the 16 genotype cells — nothing is hard-coded per option.

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