We need a hammer to pound nails into a board. Even if we have the nails, we can't pound them without first getting hold of a hammer. This is similar to the genetic relationship known as

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Epistasis describes a genetic relationship in which one gene masks or affects the expression of another gene. Just as we cannot pound nails without a hammer, a particular allele must be present in order for another gene to be expressed. In epistasis, one gene is required for expression of another gene or genes, much like a hammer is required for pounding nails. Heterozygosity refers to the presence of different alleles at a particular gene locus, rather than a relationship between multiple genes. Codominance refers to a relationship between alleles where both are expressed equally in a phenotype, and independent assortment refers to the random distribution of alleles into gametes during meiosis. Neither of these relationships describes the need for a specific gene or allele to enable expression of another gene.

We Need a Hammer to Pound Nails into a Board