With independent assortment, the ratio of genotypes in the f2 generation of a cross between true-breeding strains (aa bb × aa bb) can be described as 1: 2: 1: 2: 4: 2: 1: 2: 1. to what genotype does the "4" in the ratio refer?

AaBb

Explanation:

Cross between true breeding strains

Parents:        AAbb x aaBB

Gametes:        Ab          aB

The F1 is homogeneous: AaBb.

To obtain the F2 generation we must do a cross between 2 F1 individuals: AaBb x AaBb

Their genotypes are the same, so they both will produce the same type of gametes: AB, Ab, aB and ab.

If you do a Punnett Square, you can check the ratio of genotypes of the F2, and that the "4" refers to AaBb.

b a mature ovary of a plant.