Figure 7.13: The results of a three-point testcross can be used to map linked genes. In this three-point testcross of Drosophila melanogaster, the recessive mutations scarlet eyes (st), ebony body color (e), and spineless bristles (ss) are at three linked loci. The order of the loci has been arranged arbitrarily. Each phenotypic class includes both male and female flies; the sex of the pictured flies is random.