Situations in which one allele for a gene is not completely dominant over another allele for that gene are

This is called incomplete dominance. When an something is heterozygous for an incomplete dominant gene, the resulting phenotype will be somewhere between the two. For example if a plant has a tall allele and a short one, but neither shows complete dominance over the other, the plant height will be somewhere in the middle.

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem