What would you expect the F1 generation flies to look like if two scarletmutants were crossed with each other?

The easiest examination to differentiate between the two possibilities is the complementation test. The test is simple to perform: two mutants cross and F1 is analyzed. If F1 expresses the wild-type phenotype, we conclude that each mutation is in one of two possible genes necessary for the wild-type phenotype. When it is shown that it is genetically shown that two (or more) genes control a phenotype, the genes are said to form a complementation group. Otherwise, if F1 does not express the wild type phenotype, but rather a mutant phenotype, we conclude that both mutations occur in the same gene.

These two results can be explained considering the importance of genes for phenotypic function. If two separate genes are involved, each mutant will have an injury to one gene while maintaining a wild-type copy of the second gene. When F1 occurs, it will express the mutant allele of gene A and the wild-type allele of gene B (each contributed by one of the mutant parents). F1 will also express the wild type allele for gene A and the mutant allele for gene B (contributed by the other mutant parent). Because F1 is expressing the two necessary wild-type alleles, the wild-type phenotype is observed.