As Box 2.1 shows, Mendel was an unusual person to be doing such scientific research. As an Augustinian friar, he tended gardens at the monastery at Brno and studied, over a period of about a decade, the genetics of garden peas. These were a good choice because peas can be raised rapidly and exhibit clearly recognizable traits that can breed true for many generations. Also, they are capable of either self fertilization or cross-fertilization (Figure 1 and Table 1). The unusual aspect of Mendel’s work, for this field at the time, was his careful quantitation of the outcome of every breeding experiment.
Fig1. Mendel’s experiments with garden peas. (A) Anatomy of the pea flower. Peas are self-fertilizing but can also be cross-fertilized. Clear-cut alternative, or antagonistic, forms of particular traits exist: seed color and shape, flower color, pod color and shape, stem length, and flower position. Crossing of plants that have different forms of these traits allowed the formulation of Mendel’s laws of inheritance. (B) Mendel isolated and perpetuated true-breeding, or pure-breeding, pea lines, in which a trait that he was studying had remained constant from generation to generation for eight generations. He cross-fertilized antagonistic forms to produce hybrids; for each experiment he also did reciprocal crosses. The first filial or F1 generation was hybrid; it was then allowed to self-fertilize to produce second, third, and further filial generations: F2, F3, etc. Mendel then followed the inheritance patterns of the traits over several generations and quantified the data, leading to the most profound scientific understanding of inheritance laws.
Table1. Examples of pairs of contrasting traits in peas studied by Mendel.
In his experiments, Mendel would first choose a pair of stocks exhibiting contrasting traits in a particular character, yellow versus green seeds, for example, each of which he knew to breed true in self-fertilized pea lines. These are referred to as the parental phenotypes, the P generation. When these were cross-fertilized, it was always observed that only one of the two alternate traits was expressed in the progeny, called the F1 or first filial generation. In our example, the trait expressed in F1 (the F1 phenotype) is yellow (Figure2; see Table 1). This showed Mendel that one trait, yellow seeds, was dominant over the other, green seeds (recessive). Why this occurred was shown by the next experiments, in which members of the F1 generation were self fertilized. Now the other trait, the recessive trait, reappeared but in only one-fourth of the F2 progeny. This meant that there must be units of heredity, now called genes, that could be distributed according to fixed rules, and which dictated the dominant and recessive states. Most eukaryotes are diploid, carrying two copies of each gene, in their somatic cells (that is, cells of the body) (Box 2.2). In the case of seed color, the copies can be of two different forms (alleles), each controlling a different trait (yellow or green) of the same character (seed color); the dominant allele is denoted Y and the recessive allele y. We can now understand the whole process by recalling the fact that, at fertilization, each parent donates one gamete, either sperm (from pollen in plants) or ovum, to the resulting fertilized egg or zygote; during self-fertilization, the same plant acts as both male and female parents. In this example, each of the two parents contains only one allele (genotype YY for the true-breeding yellow seed phenotype, and genotype yy for the true-breeding green seed phenotype); because the two copies of the gene in the zygote of the parent are the same allele, each parent is a homozygote, YY or yy. Because the gametes are haploid, this means that each parent donates one or the other of the two alleles, chosen at random. All the gametes from the YY parent contain Y and all the gametes from the yy parent contain y. The F1 generation, obtained from cross-fertilization of the two different homozygotes, each carrying two copies of just one of the alternate alleles, must be a heterozygote, with the genotype Yy (is said to be heterozygous), and the phenotype shows that only the dominant trait (yellow seeds) is expressed. In the F2 generation, however, random combination of gametes (half of the female gametes will be Y and half will be y, with the same for the male gametes) will lead to equal frequencies of the genotypes YY, Yy, yY, and yy. Because Y is dominant, this yields the 3:1 ratio of yellow (YY, Yy, yY)/ green (yy) phenotypes observed. Mendel’s experimental results, which are based on thousands of crosses, are summarized for several traits in Table 1.
Fig2. Mendel’s results showing the distribution of a pair of different traits. Seed color after four rounds of fertilization is illustrated. Capital Y stands for yellow color, whereas lowercase y stands for green color. For each trait, the plant carries two copies of a unit of inheritance, or two copies of a gene in contemporary understanding. The trait that appears in all F1 hybrids is controlled by the dominant allele; the alternative trait that remains hidden in F1 but reappears in F2 is controlled by the recessive allele. In this specific example, yellow color is dominant and green is recessive. The schematic also illustrates the difference between homozygous and heterozygous individuals. Homozygous individuals of each generation breed true, as shown by red arrows, whereas heterozygous do not, as shown by black arrows.
From these experiments, Mendel formulated the two laws, which are really hypoth eses, that constitute the basis of classical genetics. Mendel’s first law, the law of seg regation, states that the two alleles for each trait separate or segregate during gamete formation and then unite at random, one from each parent, at fertilization. The first law can be expressed in a number of ways. Here we choose to break it into several statements, in modern nomenclature.
• Variation in phenotype is explained by the existence of alternate versions of genes. These versions are called alleles.
• The alleles of each gene segregate, independently, one to each gamete.
• Every individual inherits two copies of each gene, one from the gamete from each parent.
• If the alleles differ, one will be dominant and one recessive. If the individual is heterozygous for an allele, only the dominant allele and trait will be expressed in the first generation.
But what happens if one hybridizes peas that differ in two traits? Does the segregation
of alleles for one trait affect the other? Mendel also carried out such experiments and
derived what is called Mendel’s second law, the law of independent assortment. This
law states that, during gamete formation, the segregation of the alleles of one gene
is independent of the segregation of the alleles of a different gene. In other words,
traits segregate independently; there is no linkage between genes for different traits
(Figure 3). This happens to be not always true, as we shall see.
Fig3. Mendel’s results showing the segregation of two independent traits. Capital Y stands for yellow seed color, and lowercase y stands for green seed color; capital R stands for round seed shape, and lowercase r stands for wrinkled seed shape.
