"a" is a dominant mutant allele of the A gene, and "b" is the recessive mutant allele of the B gene. To see the mutant phenotype, one requires either an "a" mutant phenotype or a "b" mutant phenotype (both mutations are not necessary).
What would the F2 segregation ratio be (wild type to mutant) of the cross of an aabb mutant with AABB wild type?
(Note that Arabidopsis nomenclature is used. Mutant alleles are lower-case whether they are dominant or recessive, wild type alleles are upper-case.)
The first step in this type of problem is to make a Punnet square. The aabb X AABB cross would result in 100% AaBb ...