This section is from the book "Biology In Human Affairs", by Walter Van Dyke Bingham. Also available from Amazon: Biology In Human Affairs.
Descriptions of the properties of various chemical compounds are found in Egyptian papyri; the science of chemistry is no older than the American commonwealth. Exact knowledge of the composition of substances and of the transformations which they undergo was quite impossible before the atomic hypothesis was established by the experimental demonstration of the Law of Definite and Multiple Proportions. With the conception of the molecule as the unit of matter identifiable in mass and of the atom as the elementary unit, precise information concerning the reaction of substances under known conditions was attainable, though no one has ever seen either molecule or atom. A similar statement may be made about heredity. Doubtless some of our paleolithic ancestors noticed whom the new baby resembled, and formulated theories to explain the situation. At all events, the ancient Egyptians and Babylonians must have known something about inheritance, for they left graphic records showing highly improved breeds of domestic animals and of cultivated plants. Yet the sum total of previous experience up to the middle of the nineteenth century had yielded no more penetrating solution of the mystery than the adage "Like produces like," a proverb which, like many another, is not true. There is something radically wrong with such a statement as an expression of natural law, in view of the knowledge that two snow-white rabbits may produce litter after litter of coal-black progeny. The saying probably became current simply to generalize on the undeniable fact that the cub of a fox is another fox, while the foal of a mare is a horse; but as a genetic principle it is lame and impotent. It would not have gained popular approval had people appreciated the logical consequences of the common observation that a child never wholly resembles one of its two parents; for the obvious deduction from this fact is that the physical basis of heredity must be a set of discrete units gathered together in the germ cells, and that the laws of heredity must be the principles which govern the distribution of these units during the maturation of the germ cells and the fertilization of the egg. When such an idea did take form in the brain of a man who could not rest until he had tested its validity, the science of genetics was born. Gregor Mendel was the man.
Mendel was a physicist by training. As such he was keenly aware of the value of dealing with the lowest possible number of variables, of controlling all experimentation carefully, and of searching for statistical relationships among the observations recorded. He chose to study inheritance in the garden pea rather than light or sound; but the choice did not make him forget the rigorous discipline of earlier days. He selected pairs of varieties to be crossed which differed but by a single striking character. He made sure that he was dealing with uniform material by self-pollinating plants of each type and studying the resulting progeny. The variety which bred true for its distinctive trait was used; the variety which did not breed true was discarded. He then made seven crosses, each involving only one pair of contrasting characters; and followed each lone character through several generations produced by self-fertilization. Previous hybridization experiments had come to naught because the varieties used as parents had differed by hundreds of characters, and the results obtained had been too complex for analysis. Mendel avoided this mistake. Only when he was convinced that he knew exactly what would happen under the simplest conditions, did he try to steer his way through the complications introduced when two or three character distinctions were under observation.
The first important fact brought to light by Mendel's experiments was that the two members of a given pair of contrasting characters may express themselves differently in the hybrids. In the pea, if a pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant, the flowers of the hybrids are always red. Mendel called the character that was expressed in the hybrid the dominant trait. The other member he called recessive\ since it had been suppressed only temporarily and appeared again in later generations. Dominance has not proved to be a universal phenomenon, however; in many crosses the hybrids are intermediate between the parents in their appearance.
It was the behavior of the second hybrid generation rather than the original cross that gave Mendel the clue to the first real law of inheritance, now known as the Law of Segregation. In this generation dominants and recessives appeared in ratios which always approached the limiting proportion three dominant to one recessive. These recessives always bred true. The dominants, on the other hand, could be divided into two types; one-third bred true, while two-thirds behaved as did the first hybrid generation.
It is plain that such results could be obtained if half of the reproductive cells of each sex contained unit factors, or genes, for the dominant character, and half contained genes for the recessive character, provided the union of these cells was a matter of chance. And such was Mendel's interpretation. It involved several radically new conceptions. The body cells of an organism were assumed to possess two sets of the units of inheritance, one from the father and one from the mother; and in pure-breeding types each set was assumed to be alike. The reproductive cells, naturally, were supposed to contain only one set of genes, which functioned as if they were wholly uninfluenced by association with the companions forming the other set, during the succession of cell divisions previous to the maturation of the germ cells. This being the case, when a germ cell containing a dominant gene unites with one containing a recessive gene, the body cells of the hybrid will each contain a gene D and a gene R; but when the germ cells of the hybrid are formed, the dominant gene D will pass into one, while the recessive gene R will pass into another.
 
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