Qualitative traits have alternative phenotypes that fit into discrete categories. The spotted vs. melanistic forms of leopards, the round vs. wrinkled skin in peas, and the ABO blood groups in humans are all examples of qualitative traits. Many qualitative traits are controlled by variation at a single locus, but multiple loci can be involved when there are epistatic interactions. Epistasis describes the phenomenon where the effect of a gene is dependent on the effect of one or more modifier genes. For example, coat color variation in many mammals (including yellow, chocolate, and black Labrador retrievers) is dependent on both the ability of an individual to produce a particular color pigment (controlled by the B locus in Figure 8.1) and the ability of an individual to deposit pigments in relevant tissues (controlled by the A locus). Variation at the B locus that determines brown vs. black phenotype has no effect on coat color unless an individual also inherits the ability to deposit pigment; otherwise the coat color is just white. Epistatic interactions can profoundly impact evolutionary dynamics at single loci and cause deviations from the predictions of the simple inheritance models that we have used so far. Example of epistasis in coat color genetics: If no pigments can be deposited, the other coat color genes have no effect on color expression, no matter if they would be dominant or recessive, and no matter if the individual is homozygous. Figure 8.1: Example of epistasis in coat color genetics: If no pigments can be deposited, the other coat color genes have no effect on color expression, no matter if they would be dominant or recessive, and no matter if the individual is homozygous.