Peas, unlike most flowering plants, are self-fertile. In other words, a pollen grain of a pea flower can germinate on its own stigma, allowing a sperm cell to fertilize an egg and the ovule to develop into a viable pea seed. Although Mendel did not know about mitosis and meiosis, he had noticed that if a pea plant grew to a certain height, its offspring, if grown under the same conditions, would always reach approximately the same height, generation after generation. He wondered what would happen if he crossed a tall plant with a short one. Would the offspring be intermediate in height?
To make such a cross, Mendel needed to prevent self-pollination. He did so by reaching into a flower of one plant and removing the stamens before the pollen had matured. Then, he took pollen from another plant and applied it by hand to the stigma of the first flower. He also covered the experimental flowers with small bags to prevent insects from bringing pollen from other flowers after the cross had been made.
When Mendel made such crosses, the results were astonishing. All of the offspring were tall. There were no short or intermediate plants. He found, however, that if he allowed the offspring plants to pollinate themselves, they produced offspring in a ratio of approximately three tall plants to one short plant (Fig. 13.10).
Mendel then tried crosses between peas with smooth seeds and those with wrinkled seeds. He also crossed green-seeded plants with yellow-seeded ones and ultimately used seven different pairs of contrasting characteristics.
Mendel referred to the original plants involved in making the crosses as the parental generation (P) and their offspring were the first filial (F1) generation. Filial means "of or relating to a son or daughter." The offspring of F1 plants were called the second filial (F2) generation.
There are several reasons why Mendel was successful in developing a model for inheritance when countless others before him failed. He performed carefully planned and executed crosses between pure-breeding parents. Each parent was genetically stable for the trait under study—a green-seeded parent produced only green-seeded offspring. Mendel also counted the number of offspring in each cross, something none of his predecessors had done. His use of his math background to apply statistical analyses to his data was unique, allowing him to develop and test inheritance models. Finally, he chose traits under simple genetic control. His predecessors usually studied traits such as weight, which is controlled by many genes and strongly influenced by environment. A summary of Mendel's data is shown in Table 13.1.
As Mendel began to collect data in his experiments, he realized that there must be elements in the pea plant responsible for controlling traits such as seed color and stem height. He referred to this unknown agent as a factor. He also deduced that each plant must have two such factors for each characteristic, since even though all the offspring of an F1 generation appeared identical, the F2 generation revealed some plants with one characteristic and others with the contrasting form.
These discoveries and deductions came to be known as the law of unit characters. Stated simply, this law says that factors, which always occur in pairs, control the inheritance of various characteristics. Today, we know that the paired factors are alleles of genes. Alleles are alternative forms of a gene. For example, the seed color gene has an allele for yellow color and an allele for green color. Genes are always at the same position (locus) on homologous chromosomes (see Chapter 12). We also now know that there are typically thousands of genes on every chromosome.
From his data and analyses, Mendel also deduced that one allele may conceal the expression of the other. For example, in the F1 generation following a cross between green-podded and yellow-podded parents, all the plants had green pods. However, both parents obviously had to contribute something to the cross, since some of the F2 plants had green pods while others had yellow pods. This deduction let to another principle, known as the law of dominance. This principle says that for any given pair of alleles, one may mask the expression of the other. The expressed allele is referred to as dominant, and the one that is not expressed (masked) is recessive. In the green-podded by yellow-podded cross, the green-pod allele is dominant, and the yellow-pod allele is recessive.
Mendel's crosses also made it clear that a plant's having green pods did not indicate whether or not both members of the pair of alleles controlling pod color were present, because the dominant one could mask the expression of the recessive one. To distinguish between the physical appearance of an organism and the genetic information it contains, we use the terms phenotype and genotype. Phenotype refers to the physical appearance of the organism, while genotype refers to the genetic information responsible for contributing to that phenotype. We customarily use words to describe phenotypes and letters to designate genotypes.
In Mendel's crosses, for example, seven pairs of pheno-types are shown as parents; these phenotypes include yellow seeds and green seeds, smooth seeds and wrinkled seeds, green pods and yellow pods, and so on. In designating the genotypes of each of these plants, a capital letter is used to indicate the dominant member of a pair of genes; the same letter in lowercase is used to indicate the corresponding recessive allele. In the green-podded x yellow-podded pea cross, for example, green is dominant over yellow. If a plant
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