Gregor Mendel

Mendel's experiments were designed to investigate the most widely accepted model of inheritance, blending, which held that the traits of an offspring would be a blend of the parental traits. For example, the theory of blending predicts that a tall and short parent would give rise to a medium-height offspring. Mendel's results showed that for many simple traits, at least, this model was wrong. Instead, the offspring displayed traits in exactly the same form as they appeared in one or the other of the parents.

Mendel chose to study a small group of traits that occur in either of two forms, such as round versus wrinkled pea shape. He began by developing "pure-breeding" lines of each form. In a pure-breeding line, crossing two members gives only offspring that are identical to the parents for that trait. Mendel then crossed pure-breeding parents who had different forms of a trait. For example, he crossed a pea plant that produced only round peas with one that produced only wrinkled peas. All the offspring from this cross-developed only round peas; no wrinkled peas were found. When these off-spring were crossed among themselves, however, both round and wrinkled were observed, in a numerical ratio of three round-pea plants for every one wrinkled-pea plant.

Mendel explained these results by proposing that each visible trait is governed by the presence of two "factors," which may be the same or different in any individual. One of these factors is "dominant," while the other is "recessive." In the above example, the round-producing factor is dominant, and the wrinkled-producing factor is recessive. If two recessive factors are present, the organism will display the recessive trait. If the organism has two dominant factors, or one dominant and one recessive, the dominant trait will be displayed.