Presume that the number of leaves in clovers is controlled by the combined actions of two genes,A and B,with no contributions from the environment.A/A results in four leaves,A/a in three leaves,and a/a in two leaves.Moreover,the genotypes B/B and B/b result in one additional leaf (b/b has no effect on leaf number). a)Based on the above information:
A)A has an additive mode of action and B has a dominant mode of action.
B)A has a dominant mode of action and B has an additive mode of action.
C)both A and B have a dominant mode of action.
D)both A and B have an additive mode of action.
E)the respective modes of action of A and B cannot be determined.

Question

Quizplus · Accepted Answer

The observation that A/A results in four leaves, A/a in three leaves, and a/a in two leaves indicates that presence of A allele leads to an increase in the number of leaves in a dose-dependent manner. This is indicative of a dominant mode of action where the presence of one copy of A allele is sufficient to generate the observed phenotype. 
The observation that genotypes B/B and B/b result in one additional leaf than their respective counterpart (i.e., b/b) indicates that presence of B allele leads to an increase in the number of leaves without any dose dependence. This is indicative of an additive mode of action where the presence of one or two copies of B allele leads to an equal increase in the phenotype. 
Therefore, the best choice is B: A has a dominant mode of action and B has an additive mode of action.

Presume That the Number of Leaves in Clovers Is Controlled