In the Reptile hobby, we classify mutant genes based on how they interact to morph the animal's appearance from its wild form. An animal may carry zero, one, or two copies of a mutant gene. If it only carries one gene (i.e., half the pair), we refer to it as the heterozygous form since it has the mutant gene paired with a the normal/wild type gene. When the pair is present, the animal is said to be homozygous for that gene.