A popular question for many years was: which is more important in determining the characteristics of an individual, heredity or environment? Scientists have learned that both play important parts, and that most qualities are the result of the interaction of heredity and environment.

The genetic makeup, or genotype, of an organism cannot be altered by its environment except through mutation. However, the way in which the organism develops is constantly affected by external influences. Its phenotype, or gross structure and function, undergoes changes throughout its life, because the phenotype is determined by both its genotype and its environment.

Traits vary widely in their susceptibility to environment. The human blood groups seem not to be affected at all. Human skin color can be altered by irradiation, chemicals, and disease. Animal bone structure may be modified by diet and by glandular disease. The size and productivity of plants are affected by the soil and climate in which they are grown.

Certain diseases, such as diabetes mellitus and some forms of gout, are hereditary, but will develop only under certain conditions. On the whole, mental and emotional traits seem to be more readily influenced by environment than are physical traits.