English: Example of genetic complementation. Two strains of flies are white eyed because of two different autosomal recessive mutations which interrupt a single pigment-producing metabolic pathway at two different points. Flies from Strain 1 have complementary mutations to flies from Strain 2 because when they are crossed the offspring are able to complete the full metabolic pathway and thus have red eyes. A fly from Strain 1 is not complementary to another fly from Strain 1 because they both are homozygous for mutations affecting the same point in the pathway, so their offspring will be white-eyed.