d) Duplicate genes with cumulative effect (9:6:1):

Certain phenotypic traits depend on the dominant alleles of two gene loci. When dominant is present it will show its phenotype. The ratio will be 9: 6: 1.Example:Complete dominance at both gene pairs, interaction between both dominance to give new phenotypes.Fruit shape in summer squashGene pair ‘A’ sphere shape dominant over long.Gene pair ‘B’ sphere shape dominant overlong.

Interaction at ‘AB’ when present together, form disc-shaped fruit (Fig .39.1)
Disc shaped fruits 9/16
Sphere shaped fruits 6/16
Long shaped fruit 1/16