Gregor Mendel lived in an Austrian monastery and tended the monastery garden. In 1865, through his observations of the garden pea plants that grew there, Mendel developed three basic principles that—although ignored at the time by his scientific colleagues—would later become the foundation for the new science of genetics.

Every pea plant contains both male and female reproductive parts and will normally reproduce through self-pollination. Mendel noticed that the self-pollinating pea plants in his garden were true breeding: they all produced offspring with characteristics identical to their own. Mendel looked at seven different characteristics, or traits, that showed up in all of the plants. Each of these traits had two contrasting natures, only one of which would show up in a given true-breeding plant. For example, plant height could be either short or tall: short, true-breeding plants would only produce short offspring, and tall plants would only produce tall offspring. At some point, Mendel wondered what would happen if he manually mated these true-breeding plants with each other—would a tall plant mated with a short plant produce a tall, medium, or short offspring? Focusing on only one trait at a time, Mendel cross-pollinated plants with each of the seven contrasting traits and examined their offspring. He called the original true-breeding parents the P (for parental) generation and called their first set of offspring the F₁ (for “first filial,” from the Latin word filius, meaning son). The F₁ offspring that result from two parents with different characteristics are also called hybrids.

When Mendel crossed a purebred tall plant with a purebred short plant, all of the offspring in the first generation (the F₁ generation) were tall. The same thing happened with the other pairs of contrasting traits he studied: hybrid offspring in the first generation always showed just one of the two forms.

Mendel used the word dominant to describe the form that dominated the phenotype, or physical appearance, in the F₁ generation. The other form he called recessive, because the characteristic receded into the background in the F₁ generation. Mendel was the first to realize that hereditary information for two different forms of a trait can coexist in a single individual, with one form masking the expression of the other form. This principle, referred to as the law of dominance, provided the basis for Mendel’s subsequent work.

Mendel discovered that mating a tall pea and a short pea would produce an F₁ generation of only tall pea plants. But, he wondered, were these offspring tall pea plants really identical to their tall parents, or might they still contain some element of their short parents? To answer this question, Mendel let all seven types of hybrid F₁ generation plant self-pollinate, producing what he called the F₂ (second filial) generation.

Lo and behold, in each F₂ generation some of the recessive forms of the traits—which had visibly disappeared in the F₁ generation—reappeared! Approximately one fourth of the F₂ plants exhibited the recessive characteristic, and three fourths continued to exhibit the dominant form of the trait, like their F₁ parents. This 3:1 ratio of dominant to recessive remained consistent in all of the F₂ offspring.

Mendel came up with a simple but revolutionary explanation for the results he saw in the F₂ generation. He concluded that within an individual, hereditary information came in paired units, with one unit derived from each parent. Each simple physical trait, such as stem height, was determined by the combined action of a single pair of units. Each unit could come in either a dominant form, which he denoted with a capital letter “A,” or a recessive form, which he denoted with a lowercase “a.” Two units with two possible forms gave four possible combinations: AA, Aa, aA, and aa; since Aa and aA were equivalent, there were really only three functional combinations. Because “A” is dominant over “a,” both AA and Aa produced plants with the same physical characteristics. Only “aa” produced a plant that showed the recessive characteristic.

Mendel realized that the results he saw in the F₂ generation could only be explained if, during the formation of reproductive cells, paired units are separated at random so that each gamete contains only one of the two units. This postulate is now known as the law of segregation.