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| Modern Genetic Analysis Chapter 6. Gene Interaction A Diagnostic Test for Alleles Figure 6-1. The molecular basis of genetic complementation. Three phenotypically identical white mutants, $, £, and ¥, are intercrossed to form heterozygotes whose phenotypes reveal whether or not the mutations complement each other. (Only two of the three possible crosses are shown here.) If two mutations are in different genes (such as £ and ¥), then complementation results in the completion of the biochemical pathway (the end product is a blue pigment in this example). If mutations are in the same gene (such as $ and £), no complementation occurs because the biochemical pathway is blocked at the step controlled by that gene, and the intermediates in the pathway are colorless (white). (What would you predict to be the result of crossing $ and ¥?)

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