Three pairs of genes with two alleles each (A₁ and A₂,B₁ and B₂, and C₁ and C₂) control the height of a plant. The alleles of these genes have an additive relationship: Each copy of alleles A₁,B₁, and C₁ contributes 6 cm to plant height, and each copy of alleles A₂,B₂, and C₂ contributes 3 cm.Identify the number of different phenotypes (expected plant heights) that are possible with these three genes.