Mendel and Hardy-Weinberg

Dihybrid Nine-to-Three-to-Three-to-One

Independent assortment in a two-gene cross gives four phenotype classes.

highlighted = computed this step

Independent assortment makes four gametes

AaBb makes 4 gamete types. Crossing AaBb with AaBb fills 16 body cells in a four-by-four Punnett square.

4×4=164\times4=16
AaBb by AaBb Punnett squareIndependent assortment produces the four gametes shown on each side.·ABAbaBabABAABBAABbAaBBAaBbAbAABbAAbbAaBbAabbaBAaBBAaBbaaBBaaBbabAaBbAabbaaBbaabb

The dihybrid phenotype ratio has four classes

Aggregating the offspring by phenotype gives 9:3:3:1 over the classes A_B_, A_bb, aaB_, and aabb, where an underscore means either allele.

9:3:3:19:3:3:1
Dihybrid phenotype ratioPhenotype probabilities are aggregated from the two-gene cross.PhenotypeProbabilityRatioA_B_9/169A_bb3/163aaB_3/163aabb1/161