The term “hybrid” refers to the offspring of crosses between organisms differing in one or more characters. In Mendel’s first experiments, he crossed parental (P) varieties with contrasting traits for a single character, producing monohybrids (from the Greek monos, “single”) in the F₁ generation. He subsequently planted the F₁ seeds and allowed the resulting plants to self-pollinate to produce the F₂ generation. This technique is referred to as a monohybrid cross.

Mendel performed the same experiment for seven pea characters. You can follow his method in Investigating Life: Mendel's Monohybrid Cross, using seed shape as an example. When he crossed a variety that made round seeds with one that made wrinkled seeds, all of the F₁ seeds were round—it was as if the wrinkled seed trait had disappeared completely. However, when F₁ plants were allowed to self-pollinate to produce F₂ seeds, about one-fourth of the seeds were wrinkled. These two kinds of crosses were key to distinguishing the blending and particulate theories:

These observations led to a rejection of the blending theory of inheritance and provided support for the particulate theory. We now know that hereditary determinants are not actually “particulate,” but they are physically distinct entities: sequences of DNA carried on chromosomes, which we now call genes.

All seven crosses between varieties with contrasting traits gave the same kind of data (Table 12.1). In the F₁ generation only one of the two traits was seen, but the other trait reappeared in about one-fourth of the offspring in the F₂ generation. Mendel called the trait that appeared in the F₁ and was more abundant in the F₂ the dominant trait, and the other trait recessive. In the F₂ generation, the ratio of exhibiting dominant to plants with recessive traits was about 3:1. (To calculate the ratios shown in Table 12.1, divide the number of F₂ plants with the dominant trait by the number with the recessive trait.)

You can see in Table 12.1 that for each character, Mendel counted hundreds or even thousands of F₂ seeds or plants to see how many carried each trait. As we will discuss in more detail below, the probability of a plant inheriting a particular trait is independent of the probability of another plant inheriting the same trait. If Mendel had looked at only a few F₂ progeny from the “round × wrinkled” cross, he might, by chance, have found only round seeds. Or he might have found a higher proportion of wrinkled seeds than he did. In order to discover recurring patterns and to develop his laws of inheritance, Mendel used very large numbers of plants.

Mendel went on to expand on the particulate theory. He proposed that hereditary determinants—which we will refer to as genes, although Mendel did not use that term—occur in pairs and segregate (separate) from one another during the formation of gametes. He concluded that each pea plant has two copies of the gene for each character (such as seed shape), one inherited from each parent. We now use the term diploid (2n) to describe the state of having two copies of each gene; haploids (n) have just a single copy. You saw these terms in the last chapter when you learned about the cell cycle and meiosis.

Mendel concluded that while each gamete contains one copy of each gene, the resulting zygote contains two copies, because it is produced by the fusion of two gametes. Furthermore, he surmised that different traits arise from different forms of a gene (now called alleles) for a particular character. For example, Mendel studied two alleles for seed shape: one that resulted in round seeds and the other resulting in wrinkled seeds.

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In a heterozygote, one of the two alleles may be dominant (such as round, R) and the other recessive (wrinkled, r). By convention, dominant alleles are designated with uppercase letters and recessive alleles with lowercase letters. Note that the terms dominant and recessive refer only to which phenotype is expressed when the two allele are present together. They do not refer to which alleles are stronger, better, or more common.

The physical appearance of an organism is its phenotype. Mendel proposed that the phenotype is the result of the genotype, or genetic constitution, of the organism showing the phenotype. Round seeds and wrinkled seeds are two phenotypes resulting from three possible genotypes: the wrinkled seed phenotype is produced by the genotype rr, whereas the round seed phenotype is produced by either of the genotypes RR or Rr (because the R allele determines a dominant trait and the r allele determines a recessive trait).