Mendel started with 34 varieties of peas and spent 2 years selecting those varieties that he would use in his experiments. He verified that each variety was pure-breeding (homozygous for each of the traits that he chose to study) by growing the plants for two generations and confirming that all offspring were the same as their parents. He then carried out a number of crosses between the different varieties. Although peas are normally self-fertilizing (each plant mates with itself), Mendel conducted crosses between different plants by opening the buds before the anthers (male sex organs) were fully developed, removing the anthers, and then dusting the stigma (female sex organ) with pollen from a different plant’s anthers (Figure 3.3).

Mendel began by studying monohybrid crosses—crosses between parents that differed in a single characteristic. In one experiment, Mendel crossed a pea plant that was pure-breeding (homozygous) for round seeds with one that was pure-breeding for wrinkled seeds (see Figure 3.3). This first generation of a cross is called the P (parental) generation.

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After crossing the two varieties in the P generation, Mendel observed the offspring that resulted from the cross. For seed shape, the phenotype develops as soon as the seed matures because the seed traits are determined by the newly formed embryo within the seed. For characteristics associated with the plant itself, such as stem length, the phenotype doesn’t develop until the plant grows from the seed; for these characteristics, Mendel had to wait until the following spring, plant the seeds, and then observe the phenotypes of the plants that germinated.

The offspring of the parents in the P generation are the F₁ (filial 1) generation. When Mendel examined the F₁ generation of this cross, he found that they expressed only one of the phenotypes present in the parental generation: all the F₁ seeds were round. Mendel carried out 60 such crosses and always obtained this result. He also conducted reciprocal crosses: in one cross, pollen (the male gamete) was taken from a plant with round seeds and, in its reciprocal cross, pollen was taken from a plant with wrinkled seeds. Reciprocal crosses gave the same result: all the F₁ seeds were round.

Mendel wasn’t content with examining only the seeds arising from these monohybrid crosses, however. The following spring, he planted the F₁ seeds, cultivated the plants that germinated from them, and allowed those plants to self-fertilize, producing a second generation—the F₂ (filial 2) generation. Both of the traits from the P generation emerged in the F₂ generation: Mendel counted 5474 round seeds and 1850 wrinkled seeds in the F₂ (see Figure 3.3). He noticed that the numbers of the round and wrinkled seeds constituted approximately a 3 to 1 ratio: that is, about ¾ of the F₂ seeds were round and ¼ were wrinkled. Mendel conducted monohybrid crosses for all seven of the characteristics that he studied in pea plants, and in all of the crosses he obtained the same result: all the F₁ resembled only one of the two parents, but both parental traits emerged in the F₂ in an approximate ratio of 3:1.