Students should be able to describe simply how the genome and its interaction with the environment influence the development of the phenotype of an organism. • Differences in the characteristics of individuals in a population is called variation and may be due to differences in: the genes they have inherited (genetic causes); the conditions in which they have developed (environmental causes); a combination of genes and the environment. • Students should be able to state that there is usually extensive genetic variation within a population of a species, and recall that all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.