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. What height is expected in the F₁ progeny of a cross between A₁A₁B₁B₁C₁C₁ and A₂A₂B₂B₂C₂C₂.