a) supplementary genes - 9 : 3 : 4
b) Complementary genes - 9 : 7
c) Dominant epistasis - 12 : 3 :1

To identify the plant genotype whether homozygous or heterozygous Mala can perform test cross, where the individual is crossed with homozygous recessive for the trait. If the plant is heterozygous then the resultant progenies would be in the ratio 50:50 (i.e., 1: 1)

The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance. The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substance. Example: Gossypium hirsutum and Gossypium sturtianum, their F ₁ hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, additive banding pattern was noticed. Their hybrid shows the presence of both the types of proteins similar to their parents.

Four gametes - RY, Ry, rY, ry

He chooses pea plant because,
(i) It is an annual plant & bears contrasting characters controlled by a single gene separately.
(ii) Self-fertilization occurred in garden pea plants.
(iii) Mendel used both self-fertilization and cross-fertilization.   
(iv) The flowers are large hence emasculation & pollination are very easy for hybridization.