In Chapter 3, we learned that each phenotype is the result of a genotype that develops within a particular environment; each genotype may produce several different phenotypes, depending on the environmental conditions in which development takes place. For example, a fruit fly that is homozygous for the vestigial mutation (vg vg) develops reduced wings when raised at a temperature below 29°C, but the same genotype develops much longer wings when raised at 31°C.

For most of the characteristics we have discussed so far, the effect of the environment on the phenotype has been slight. Mendel’s peas with genotype yy, for example, developed green seeds regardless of the environment in which they were raised. Similarly, persons with genotype I^AI^A have the A antigen on their red blood cells regardless of their diet, socioeconomic status, or family environment. For other phenotypes, however, environmental effects play a more important role.