In a cross of a homozygous dominant parent and a homozygous recessive parent, what is the ratio of offspring that will exhibit the recessive phenotype

Assuming complete dominance, *none* of the offspring will exhibit the recessive phenotype. If A is dominant to a, then the cross between a homozygous dominant parent (AA) and a homozygous recessive parent (aa) would produce offspring all sharing the same genotype (Aa, or all heterozygous).

Because A is assumed to be completely dominant over a, only those individuals with the aa genotype would exhibit the recessive phenotype.

Subtracting is the opposite of adding