For two years, the scientist grew generation after generation of pea plants to develop true-breeding lines for each of these 14 characteristics. A true-breeding line is one that always produces one particular trait, such as yellow seeds. If green seeds showed up in this line later during the experiment, Mendel could be sure that it would be the result of his intervention in the pollinization process.
A common misconception about heredity during Mendel’s time was that characteristics blended, producing, for example, medium-high stems from crossing a true-breeding line for tall stems with a true-breeding line for short stems. Some scientifically inclined breeders had noticed characteristics disappearing in one generation and reappearing in another, but they did not take their observations any farther.
Mendel followed these traits through enough generations to find that all the inherited traits he was studying are distinct and remain intact. They do not blend together during sexual reproduction; instead, they appear, disappear and reappear from generation to generation.
In addition to determining that the traits were discrete, Mendel discovered that their appearance followed certain patterns. Some were more common than others. For example, when Mendel bred a true-breeding tall plant with a true-breeding short plant, all the offspring were tall. Therefore, he determined that the trait for tallness is dominant and the trait for shortness was recessive, at least concerning garden peas.
Remarkably, all seven sets of characteristic traits he studied behaved in the same way in the first generation of offspring. Yellow seeds, smooth seeds, green pods, inflated pods, purple flowers, axial flowers, and tall stems were all dominant traits. Meanwhile, their corresponding alternative characteristics were all recessive.
Mendel meticulously counted all the plants’ characteristics in the second generation of offspring and noted a three-to-one ratio of dominant to recessive traits. Since Mendel was also a mathematician; he recognized a pattern in this ratio.
Mendel concluded that every characteristic he was studying must be controlled by two “elements” that are present in every pea plant. As part of sexual reproduction, these elements separate and only one is passed down to the offspring.
Thus, if a plant from a true-breeding line for a dominant trait (with the gene pair AA) was crossed with one from a true-breeding line for a recessive trait (with aa genes), all the offspring would manifest the dominant trait because the only possible combination of genes would be Aa. In the second generation of offspring, however, crossing a plant with Aa genes with another plant with Aa genes would reveal four possible gene combinations in their offspring: AA, Aa, aA and aa gene pairs.
In this case, the dominant trait would manifest itself in three out of four plants. This would explain the three-to-one ratio of dominant to recessive traits for all seven pairs of characteristics that Mendel observed in his plants’ second generation of offspring. In other words, it would explain why the recessive trait disappeared in the first generation of crosses (AA genes with aa genes) and yet manifested itself in one-fourth of the second-generation offspring.
Today, the idea of two separate hereditary factors that split during sexual reproduction is known as the principle of segregation. What Mendel called elements are now known as genes.
(The conclusion of this article appears of the next page.)