THE ESSENCE OF MENDELISM
It is half a century since the Austrian monk, Gregor Mendel, published in a provincial journal the results of his now famous breeding experiments with garden peas. They lay unnoticed until 1900, when three other breeders whose work had led them to similar conclusions, almost simultaneously discovered the work of Mendel and gave it to the world.
Breeding along the lines marked out by Mendel at once became the most popular method of attack, among those who were studying heredity. It became an extremely complicated subject, which can not be grasped without extended study, but its fundamentals can be briefly summarized.
Inherited differences in individuals, it will be admitted, are due to differences in their germ-plasms. It is convenient to think of these differences in germ-plasms (that is, differences in heredity) as being due to the presence in the germ-plasm of certain hypothetical units, which are usually referred to as factors. The factor, nowadays, is the ultimate unit of Mendelian research. Each of these factors is considered to be nearly or quite constant,—that is, it undergoes little, or no change from generation to generation. It is ordinarily resistant to "contamination" by other factors with which it may come in contact in the cell. The first fundamental principle of Mendelism, then, is the existence of relatively constant units, the Mendelian factors, as the basis for transmission of all the traits that go to make up an animal or plant.
Experimental breeding gives reason to believe that each factor has one or more alternatives, which may take its place in the mechanism of heredity,