Heredity, Environment, and Mutation
Everyday language uses the word "heredity" both for biological inheritance and for legal inheritance of property. The statement that a person has inherited from his parents a dark, or a light, skin color does not mean the same thing as the statement that this person has inherited from his relatives a house or a farm. Inherited houses and farms are buildings or pieces of land which change their owners; the skin color is not transferred in this manner. The narrow bridge which connects parents and offspring is formed by the sex cells, and human sex cells have no color, and for that matter no skin. Inheritance of skin color refers to a developmental pattern which leads to the formation of a certain amount of pigment in the skin.
Just how much skin pigment is formed depends, however, not only on the presence of certain genes but also on the environment in which the carriers of these genes live. Exposure of the skin to sunlight, or to ultraviolet rays of certain wavelengths, darkens the skin color. The trait (skin color) that develops is thus determined by interaction of the heredity and the environment.
The environment concerned, however, is not only that prevailing at a given moment, but also the whole sequence of environments which the organism met during its lifetime. The color of my skin today is determined not only by the sunshine or its absence now but also by the amount of time spent outdoors and indoors during the preceding months. The personality of every human being is determined by his heredity, upbringing, education, relationships to other persons, disease, nutrition, etc. Every one of us is a product of his life experience, his biography. Nobody can escape his past. Living organisms are time- binding machines; they are products of their histories.