A hammer is needed to pound nails into a board.Even if one has the nails,they cannot be used without first getting ahold of a hammer.This is similar to the genetic relationship known as

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The genetic relationship known as epistasis is when one gene masks or affects the expression of another gene. In this scenario, the gene for the hammer is needed in order for the nails (the other gene) to be expressed or used. Therefore, this is similar to epistatic interactions. Heterozygosity, codominance, and independent assortment do not accurately reflect the relationship between the need for a hammer and nails.

A Hammer Is Needed to Pound Nails into a Board