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, thereby changing the visible character of the individual plant or animal in which it occurs. To put the matter a little differently, one germ-cell differs from another in having alternatives present in place of some of the factors of the latter. A given germ-cell can never have more than one of the possible alternatives of each factor. These alternatives of a factor are called its allelomorphs.
Now a mature germ-cell has a single system of these factors: but when two germ-cells unite, there result from that union two kinds of cells—namely, immature germ-cells and body-cells; and both these kinds of cells contain a double system of factors, because of course they have received a single entire system from each parent. This is the second of the fundamental principles of Mendelism: that the factors are single in the mature germ-cell, but in duplicate in the body-cell (and also in the immature germ-cell).
In every cell with a double system of factors, there are necessarily present two representatives from each set of allelomorphs, but these may or may not be alike—or in technical language the individual may be homozygous, or heterozygous, as regards the given set of alternative factors. Looking at it from another angle, there is a single visible character in the plant or animal, but it is produced by a double factor, in the germ-plasm.
When the immature germ-cell, with its double system of factors, matures, it throws out half the factors, retaining only a single system: and the allelomorphic factors which then segregate into different cells are, as has been said above, ordinarily uninfluenced by their stay together.
But the allelomorphic factors are not the only ones which are segregated into different germ-cells, at the maturation of the cell; for the factors which are not alternative are likewise distributed, more or less independently of each other, so that it is largely a matter of chance whether factors which enter a cross in the same germ-cell, segregate into the same germ-cell or different ones, in the next generation. This is the next fundamental principle of Mendelism, usually comprehended under the term "segregation," although, as has been pointed out, it is really a double process, the segregation of alternative factors being a different thing from the segregation of non-alternative factors.
From this fact of segregation, it follows that as many kinds of germ-cells can be formed by an individual, as there are possible combinations of factors, on taking one alternative from each pair of allelomorphs present. In practice, this means that the possible number of different germ-cells is almost infinitely great, as would perhaps be suspected by anyone who has tried to find two living things that are just alike.
