The Mendelian interpretation of heredity having been shown to be satisfactory for all characteristics in widely separated groups of animals and plants, a fundamental underlying mechanism, common to these groups, was sought which would provide the gene distribution required by the experimental data. It was found in the behavior of the chromosomes during sexual reproduction.

The essential feature of sexual reproduction is the formation of a new organism by the union of two cells, the egg and the sperm. Now the egg is usually very much larger than the sperm, yet the tiny sperm is just as effective in transmitting genes to the offspring as the huge egg. The physical basis of inheritance, therefore, must lie in some cell component present in equal quantity in both germ cells. That component is the chromatin. The chromatin undergoes numerous changes during the life history of a cell, but at some point in the process it always fragments into bodies known as chromosomes, having a definite number, shape, and size for each species. There are twenty in corn, forty-two in certain wheats, forty-four in rabbits, forty-eight in man.

The chromosomes are the chief performers in ordinary cell division (mitosis). They initiate the process; and they alone, among the constituents of the cell, go through an exact dichotomy. Probably they also have a good deal to do with directing growth and differentiation; though what their precise duties are in this connection, is still questionable. But their really star role is associated with reproduction. During the preparation of the germ cells for fertilization (meiosis), their behavior is precisely what is required by the data from all crossing experiments for carriers of the genes.

As the cells of every organism are characterized by a definite number of these bodies, and as every new individual is ordinarily produced by the union of two cells, it follows that some provision must be made for reducing chromosome numbers prior to fertilization. What happens is virtually this: when either type of germ cell matures, the chromosomes line up in pairs, and one member of each pair passes to a daughter cell. Thus, if there are two pairs of chromosomes which we may designate by the letters A and a and B and b, the resulting daughter cells may be AB and ab, or they may be Ab and aB.

It is not difficult to see that this arrangement provides the physical basis of heredity which the pedigree culture investigations found to be necessary. It must be assumed that each chromosome carries numerous genes which act as determiners of the various traits which will characterize the mature individual. Each organism possesses two complete sets of these determiners because it possesses two complete sets of chromosomes. We know that each chromosome set is complete because certain plants and animals, with proper treatment, can be induced to develop when but one set is present. One set comes from the father and the other set from the mother, as can be demonstrated by crossing two species where the chromosomes of each set differ in size. At maturity, this creature will produce either eggs or sperms, and its germ cells will possess but one complete set of chromosomes. Any given egg or sperm will have one representative of the first pair of chromosomes, and it will be a matter of mere chance whether it came from the father or from the mother; similarly, it will have one representative of the second pair of chromosomes, which also may have originated with either the father or the mother; and so on throughout the whole series. There is, then, an orderly means by which the genes from the two parents pass to the germ cells which are to function in producing the new generation; and, generally speaking, this process is one by which any given germ cell receives one, and only one, gene-packed chromosome from pairs of such chromosomes which have come from the maternal and from the paternal side of the house.

Obviously with such a mechanism for passing genes from one generation to another, the number of chromosome pairs possessed by the species in question must represent the number of independent assortments of gene packets which can be made, since the genes carried by a single chromosome would tend to be inherited together. Experimentation has shown that such a principle actually does exist. It is known as the Limitation of the Linkage Groups. If n chromosomes characterize a species, then only 2" different germ cells can be formed through the choice of chromosomes at the reduction division.

The chromosomes do indeed pass as entities to the daughter cells at the reduction division, which is the essential feature of germ cell maturation; yet this is not the only way that genes can be distributed. If it were, we should have to consider the whole chromosome as the unit of heredity. Actually, any given chromosome contains a packet of genes which tend to be inherited together; but individual members from this packet may be exchanged for homologous genes from the mate of this chromosome at the period when like pairs of chromosomes come together, just before the reduction division occurs. This phenomenon is known as crossing-over; and by studying its peculiarities intensively, an extraordinarily precise conception of the architecture of the germ plasm has been obtained.

Individual chromosomes can be tagged by the possession of particular genes, and their behavior followed in hybridization experiments. Such investigations have shown that the genes are placed in the chromosomes in a linear order, like beads on a string. The Linear Order of the Genes is, in fact, one of the basic tenets of genetics. Moreover, when crossing-over occurs, the genes ordinarily recombine in the same linear order. If, for example, a pair of homologous chromosomes contain genes ABCDEFGHI and abcdefghi, then a cross-over may occur which will result in chromosomes having the genes lined up as ABCdefghi and abcDEFGHI; but the order of the genes is usually retained. If any change in order does occur, it is such that the whole piece of chromosome which has been exchanged with the piece belonging originally to its homologue is inverted. In other words, the chromosome may sometimes have the genes lined up as ABCihgfed. When such a thing happens, it is known as a chromosome mutation, since it usually produces a change in the traits exhibited by the organism.