A look at the work of Gregor Mendel provides an appreciation of his contribution to learning along with a basic introduction to the study of plant improvement. Gregor Mendel was an Augustinian monk who lived from 1822 to 1884. Throughout his adult life, Mendel enjoyed plants and entertained a curiosity about how their hereditary features were transmitted from generation to generation. Prior to his studies, scholars accepted the idea that inheritance in plants was not predictable, and that the characteristics of the parents just blended together in the offspring. According to that mistaken concept, the fusion of gametes from a six-foot-tall parent and a two-foot-tall parent might produce a four-foot-tall offspring, as though inheritance resembled mathematical averaging. Since such results were not the case, Mendel looked for other explanations.
He worked with the common garden pea and selected two strains that were notably different in height; one grew to a mature height of six or seven feet and the other to a mature height of one or two feet. He declined to study other differences between the parents, isolating for his observations only the height variable. He crossbred selected parents, carrying out the pollinations under carefully controlled conditions and keeping precise records of the offspring that resulted.
When Mendel allowed two tall parents to self-pollinate, the offspring were all tall. These first generation offspring are termed F1. When F1 generation plants were then allowed to self-pollinate, the second generation (F2) offspring were divided, with three-fourths of the plants tall and one-fourth of the plants short. In neither generation, F1 nor F2, were there any plants of intermediate or average height.
Explaining Mendel’s observations with modern terminology, it is now recognized that factors such as height are controlled by genes and that these genes occur in pairs, as with short and tall, on homologous chromosomes. When the sexual gametes are formed, the paired genes separate. To illustrate, the genes for tall and short are usually represented as T and t respectively. The homologous chromosomes from the original tall plants are therefore represented as TT. Those from the original short plants are represented as tt. The chromosomes and genes separate during meiosis and the resulting haploid gametes each contain a single gene for height. They recombine after pollination to create a diploidal Tt offspring (Figure 5-12).
paren, s: TT (tall) ¥ tt (short)
(tall F1 offspring)
figure 5-12. First generation of cross between TT and tt parents (Delmar/Cengage Learning)
figure 5-13. Second generation of cross between TT and tt parents (Delmar/Cengage Learning)
Since all the F1 generation mature as tall pea plants despite containing a gene for shortness, the T gene is regarded as dominant and the t gene as recessive. The phenotype (external appearance) of Mendel’s plants was the same (tall) whether the genotype (genetic composition) was TT or Tt.
The self-fertilization of the F1 generation plants produces a 3:1 ratio of tall to short plants, three tall to one short (Figure 5-13).
A simpler method of diagramming the crosses is shown in Table 5-2. The genotype of one parent is written across the top and the genotype of the other parent is written down the side. By multiplying the letter symbols down and across, the possible types of offspring and their predicted ratio result.
The example of Mendel’s work just described refers to parents that differ in only one characteristic. A cross between such parents is termed a monohybrid cross. When two or more independently inherited characters distinguish the parents, a cross is termed dihybrid. Gregor Mendel selected a parent pea plant that always produced yellow, round seeds when self-pollinated. Another parent always produced green, wrinkled seeds when self-pollinated. When the two plants were cross-pollinated, the F1 offspring all produced yellow, round seeds. He deduced that the yellow and round traits were dominant over the green and wrinkled traits.
Using Y and y to represent yellow and green and R and r to represent round and wrinkled, the two original parents can be represented as YYRR and yyrr. The F1 plants would be YyRr, heterozygous for the two characteristics. Since the characteristics exist on two different chromosomes, the two pairs of genes separate independently of each other. The result is that F1 parents produce four types of haploid gametes during meiosis: YR, Yr, yR, and yr. When F1 plants are self-pollinated, the checkerboard method of predicting how the gametes will combine corresponds closely with what Mendel actually observed: nine yellow and round, three yellow and wrinkled, three green and round, and one green and wrinkled (refer to Table 5-3).
As self-pollination continues, the proportion of homozygous offspring increases. That is why self-pollinating plants usually breed true. In animals, especially humans, self-pollination or inbreeding may be detrimental because a homozygous offspring manifesting recessive harmful genes is likely to result. With plants, the inbreeding technique can be used to produce homozygous offspring that exhibit desirable recessive genes.
The great danger inherent to a simplified discussion of a complex subject is that readers will assume the limited information is all they need to know. Therefore, it bears repeating that what is sought here is not substantive insight into the science of genetics or the technology of plant breeding, but rather a basic understanding of the kind of knowledge needed for plant improvement.
The work of Gregor Mendel illustrates many of the principles on which the science of genetics is based, but the science continues to grow and knowledge to accumulate. A few more concepts are worth mentioning.
• Dominance does not exist between all gene pairs. F1 may appear intermediate in a characteristic compared to the parents. For example, a red flowering parent and a white flowering parent may produce an F1 offspring having pink flowers.
• Some plants are self-sterile, or unable to self-pollinate, and produce seed. Crossfertilization gains importance in such species and results in increasing the number of variations in the offspring.
• Certain genes are termed lethal. Their presence on the chromosome of a plant will kill the plant. For example, plants need chlorophyll to
TABLE 5-3. Ratio of Phenotypes 9:3:3:1
photosynthesize. If a lethal recessive gene for no chlorophyll appears as a homozygous individual, the plant will eventually die.
• When inbred plant species are crossed, the F1 hybrid generation may have qualities superior to those of either parent. The phenomenon is known as hybrid vigor. It can be predicted and created by selective breeding, but its cause is not clearly understood.