Mendelian Heredity and the Chromosomal Theory of Inheritance
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| Figure 1-16 A, Gregor Johann Mendel. B, The monastery in Brno, Czech Republic, now a museum, where Mendel carried out his experiments with garden peas. |
Genetic Approach
The genetic approach consists of mating or “crossing” populations of organisms that are true-breeding for contrasting traits, and then following the hereditary transmission of those traits through subsequent generations. “Truebreeding” means that a population maintains across generations only one of the contrasting states of a particular feature when propagated in isolation from other populations.
Gregor Mendel studied the transmission of seven variable features in garden peas, crossing populations that were true-breeding for alternative traits (for example, tall versus short plants). In the first generation (called the F1 generation, for “filial”), only one of the alternative parental traits was observed; there was no indication of blending of the parental traits. In the example, the offspring (called F1 hybrids) formed by crossing the tall and short plants were tall, regardless of whether the tall trait was inherited from the male or the female parent. These F1 hybrids were allowed to self-pollinate, and both parental traits were found among their offspring (called the F2 generation), although the trait observed in the F1 hybrids (tall plants in this example) was three times more common than the other trait. Again, there was no indication of blending of the parental traits (Figure 1-17).
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| Figure 1-17 Different predictions of particulate versus blending inheritance regarding the outcome of Mendel's crosses of tall and short plants. The prediction of particulate inheritance is upheld and the prediction of blending inheritance is falsified by the results of the experiments. The reciprocal experiments (crossing short female parents with tall male parents) produced similar results. (P1 = parental generation; F1 = first filial generation; F2 = second filial generation.) |
Mendel's experiments showed that the effects of a genetic factor can be masked in a hybrid individual, but that these factors were not physically altered during the transmission process. He postulated that variable traits are specified by paired hereditary factors, which we now call “genes.” When gametes (eggs or sperm) are produced, the two genes controlling a particular feature are segregated from each other and each gamete receives only one of them. Fertilization restores the paired condition. If an organism possesses different forms of the paired genes for a feature, only one of them is expressed in its appearance, but both genes nonetheless will be transmitted unaltered in equal numbers to the gametes produced. Transmission of these genes is particulate, not blending. Mendel observed that the inheritance of one pair of traits is independent of the inheritance of other paired traits. We now know, however, that not all pairs of traits are inherited independently of each other. Numerous studies, particularly of the fruit fly, Drosophila melanogaster, have shown that the principles of inheritance discovered initially in plants apply also to animals.
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