Mendel was curious about how traits were transferred from one generation to the next, so he set out to understand the principles of heredity in the mid-1860s. Peas were a good model system, because he could easily control their fertilization by transferring pollen with a small paintbrush. This pollen could come from the same flower (self-fertilization), or it could come from another plant’s flowers (cross-fertilization). First, Mendel observed plant forms and their offspring for two years as they self-fertilized, or “selfed,” and ensured that their outward, measurable characteristics remained constant in each generation. During this time, Mendel observed seven different characteristics in the pea plants, and each of these characteristics had two forms (Figure 3). The characteristics included height (tall or short), pod shape (inflated or constricted), seed shape (smooth or winkled), pea color (green or yellow), and so on. In the years Mendel spent letting the plants self, he verified the purity of his plants by confirming, for example, that tall plants had only tall children and grandchildren and so forth. Because the seven pea plant characteristics tracked by Mendel were consistent in generation after generation of self-fertilization, these parental lines of peas could be considered pure-breeders (or, in modern terminology, homozygous for the traits of interest). Mendel and his assistants eventually developed 22 varieties of pea plants with combinations of these consistent characteristics.

Mendel not only crossed pure-breeding parents, but he also crossed hybrid generations and crossed the hybrid progeny back to both parental lines. These crosses (which, in modern terminology, are referred to as F1, F1 reciprocal, F2, B1, and B2) are the classic crosses to generate genetically hybrid generations

When conducting his experiments, Mendel designated the two pure-breeding parental generations involved in a particular cross as P1 and P2, and he then denoted the progeny resulting from the crossing as the filial, or F1, generation. Although the plants of the F1 generation looked like one parent of the P generation, they were actually hybrids of two different parent plants. Upon observing the uniformity of the F1 generation, Mendel wondered whether the F1 generation could still possess the non dominant traits of the other parent in some hidden way.

When looking at the figure, notice that for each F1 plant, the self-fertilization resulted in more round than wrinkled seeds among the F2 progeny. These results illustrate several important aspects of scientific data:

  1. Multiple trials are necessary to see patterns in experimental data.
  2. There is a lot of variation in the measurements of one experiment.
  3. A large sample size, or “N,” is required to make any quantitative comparisons or conclusions.

In Figure 4, the result of Experiment 1 shows that the single characteristic of seed shape was expressed in two different forms in the F2 generation: either round or wrinkled. Also, when Mendel averaged the relative proportion of round and wrinkled seeds across all F2 progeny sets, he found that round was consistently three times more frequent than wrinkled. This 3:1 proportion resulting from F1 x F1 crosses suggested there was a hidden recessive form of the trait. Mendel recognized that this recessive trait was carried down to the F2 generation from the earlier P generation.