The Laws of Heredity: Mendel
In 1866 Gregor Mendel, Abbot of Brünn, published the results of certain experiments he had made in cross-breeding varieties of the common garden pea and the conclusions he had drawn from them. Though the value of his work was not recognized until 1900, the first half of this present century has seen great advances in the knowledge of heredity based on the original discoveries he made, and the breeding of new varieties of plants and of animals is now a scientific process.
In one of his experiments, Mendel cross-pollinated a variety of pea having round seeds when ripe with one having wrinkled seeds when ripe. This he effected by removing the unripe stamens from unopened flowers of one variety and then, when the stigmas were ready for pollination, he placed pollen on them from the other variety. When the ripe pods so produced were examined, all the peas in them were found to be round. This was the case whichever variety was used as the 'female' parent, i.e. the one used to receive pollen from the other. These peas Mendel called the first filial generation (designated Fl in diagram). Mendel sowed these seeds the following year, and this time he allowed the flowers produced to be self-pollinated, as they normally are in the garden pea, provided insects are excluded. The ripe pods this time contained both round and wrinkled peas. He called these peas the second filial generation (F2), and on counting the two kinds found that there were 5,474 round and 1,850 wrinkled peas. The ratio of round peas to wrinkled was thus nearly 3:1. Mendel then proceeded to sow these peas in the next year, and again allowed the plants to be self pollinated. The wrinkled peas produced plants which bore only wrinkled seeds, but only one-third of the round peas produced plants bearing round peas only; the other two-thirds had pods containing both round and wrinkled peas in the former ratio of 3 round: 1 wrinkled. The result of this experiment may be summarized thus:
Mendel explained these results in the following terms.
Factors - the pollen grains and ovules bear character-producing factors. These are now called genes, and are known to be situated in the nuclei of the pollen and of the ovule. When fertilization occurs one of the 'male' nuclei of the pollen tube fuses with the 'female' nucleus of the ovum in the ovule. From the zygote thus produced the new plant arises by cell division. The factors (genes) carried by the male nucleus are combined with those carried by the female nucleus, so that the new plant inherits half its factors from the male parent and half from its female parent. These factors (genes) determine the characters formed in the new plant during its development. The germ cells (male and female nuclei) of the 'round' pea variety carry a factor for the production of the round character, while those of the 'wrinkled' variety carry a factor for producing the wrinkled character. Since a pea cannot be both round and wrinkled, but only one or the other, Mendel called the two factors a pair of opposed factors.
Dominance - Mendel supposed that in any one pair of opposed factors, one factor was dominant and the other recessive. In the hybrid (Fl) round pea both factors for 'roundness' and 'wrinkledness' are present, but the effect of the 'round' factor only is seen since this factor is dominant, while the 'wrinkled' factor is recessive. In this hybrid the 'round' factor had been inherited from the 'round' parent and the 'wrinkled' factor from the 'wrinkled' parent, so that both factors were present in the same plant. A plant which contains only one of two opposed factors is said to be homozygous, while the hybrid which contains both opposed factors is said to be heterozygous.
Segregation - When the 'germ cells' of the hybrids are formed, Mendel supposed that the opposed factors are separated (segregated), so that two kinds of pollen grains and of ovules are formed. These carry either one or other of the two factors but not both. Thus the hybrid 'round' pea plant produces some pollen grains carrying the 'round' factor and some carrying the 'wrinkled' factor and similarly the ovules. He supposed, moreover, that the two kinds of pollen grains and ovules were produced in approximately equal numbers, and that, on self-pollination, random (chance) combination of these two types of germ 'cells' would produce four possible combinations, also in approximately equal numbers, just as when tossing two coins (A and B) four results may occur: A1 and B1; A1 and B2, A2 and B1; A2 and B2. The result of this random pollination can be seen in figure 2.
The ripened pods of the Fl plants show the effect of segregation and recombination of the factors. Of the four possible combinations (1), (2) and (3) produce round seeds, since the dominant factor for 'roundness' is present in each. Combination (4) alone produces wrinkled peas, since only the recessive factor for 'wrinkledness' is present. In F3 the homozygous combinations RR and ww on self-pollination can only produce round and wrinkled peas respectively, while the heterozygous combinations Rw and wR will show segregation, as in the case of the original Fl hybrid. This is what Mendel actually found by experiment.
He also confirmed that in the segregation of the 'round' factor from the 'wrinkled' factor, equal numbers of the two types of pollen grains were formed, one carrying the 'round' factor and the other the 'wrinkled' factor, by pollinating the 'wrinkled' pea variety with pollen from the Fl hybrid plants. Since Fl pollen (R) plus egg cell (r) produces a round pea (Rr) and Fl pollen (r) plus egg cell (r) produces a wrinkled pea (rr), then if the two types of pollen grains (R) and (r) were produced in equal numbers, equal numbers of round and wrinkled peas should result. Again, the experimental result was that expected, equal numbers of round and wrinkled peas being found in the pods.
Following the revival of Mendel's work in 1900, many breeding experiments have been made, and have shown that Mendel's principles are of general application both among plants and animals. Mendel's work has provided the basis for a new branch of science called genetics or the study of inheritance. Already outstanding achievements in the production of new varieties of plants with valuable properties such as disease resistance have been recorded.
(From Chapter XXVI of Biology by H. J. Cooke, K. F. P. Burkitt and W. B. Barker.)