Fig. 3-1 Changes in Traits after Cross-breeding (Peas)
When purebred round and wrinkled peas are combined, F1 produces only round peas. When F1 hybrids are combined, the ratio of round to wrinkled peas is 3:1.
In 1866, Gregor Mendel crossed peas and carefully observed the allelotype (contrasting traits, such as whether seed shape is “round” or “wrinkled”). He found that hybrids occurred according to an ordered principle (Fig. 3-1). First, he obtained pea plants that always produced round peas during self-fertilization and those that always produced wrinkled peas and combined them as “parents” (P). He found that the hybrid first generation F1 was always entirely round. When these round pea plants self-fertilized, the hybrid second generation F2 produced 5474 round peas and 1850 wrinkled peas.
These results are shown in Fig. 3-1. When the factors determining the pea shapes are set as A for round peas and a for wrinkled peas, then P is considered a purebred AA or aa (With regard to alleles, a homozygote is an individual with two identical alleles as in AA or aa and a heterozygote is an individual with different alleles as in Aa; a lineage consisting of homozygotes is called purebred). Organisms that undergo sexual reproduction always receive one allele from each parent. F1 results in Aa because it receives two dissimilar alleles: one is A and the other is a. Mendel defined traits as “dominant” when they appeared in F1 (heterozygotes) (Mendel’s law of dominance). In this case, roundness is dominant, while wrinkles are recessive. When gametes from F1 are combined, the AA:Aa:aa ratio becomes 1:2:1 and the round:wrinkled ratio becomes 3:1 in F2. Practical examples of this finding are available. However, the ratio is not always 3:1 because intermediate types are occasionally observed. When red- and white-colored flowers of Mirabilis jalapa are crossed, pink-colored flowers are formed and this intermediate type corresponds to Aa. Such cases are called incomplete dominance. These hereditary factors hypothesized by Mendel were later named “genes.”