GENETICS is the biological science that deals with inheritance and variation among living organisms. In the reproduction of plants and animals it is obvious, even to the casual observer, that parents and offspring tend to resemble each other in many respects. For example, when a particular species of plant reproduces, it gives rise to other plants with the same general characteristics; the same holds true for the various species of animals. Even in human beings certain traits can be traced through families. This transmission of traits from one generation to another, and consequently the resemblance between closely related organisms, is called heredity.
Although there may be many hereditary resemblances between individuals, no two individuals are ever exactly alike. All organisms, particularly higher animals, show some differences or variations. These may be due to the definite manner in which traits are distributed to the offspring, to environmental factors, such as temperature, light, moisture, food supply, etc., or to many other causes. Genetics, then, is that branch of the biological sciences that is concerned with the mechanisms of heredity and variation and with the relationship between the two phenomena.