The manner in which inheritance units from the two parents unite in fertilization and later segregate in the formation of gametes, so that the latter are pure with respect to any character, is a familiar part of Mendelian inheritance (Fig. 58). What are these units in terms of
Fig. 58. Diagram Showing Union of Factors in Fertilization and their Segregation in the Formation of Germ Cells. With 4 pairs of factors (Aa, Bb, Cc, Dd), 16 types of gametes are possible, as shown in the two series of small circles at the right. (From Wilson.)
cell structures and where are they located in the cell? We have in the chromosomes, as Wilson especially has emphasized, an apparatus which fulfils all the requirements of carriers of Mendelian factors (Fig. 59). Both factors and chromosomes come in equal numbers from both parents; both material and paternal factors and chromosomes pair in the zygote and separate in the gamete, as shown in diagrams 58 and 59; and so far as known the chromosomes are the only portion of the germ cells which fulfil these conditions. Furthermore, there is much additional evidence that the chromosomes are especially concerned in heredity, as was pointed out in the last lecture, and it is not reasonable to suppose that this remarkable coincidence between the distribution of Mendelian factors and of chromosomes is without significance.
Of course Mendelian factors are not all the factors of development, but merely the differential factors which cause, for example, one guinea pig to be white and its brother to be black. Very many factors are involved in the production of white or black color, but there is at least one differential factor for every unit character, and this alone is the Mendelian factor. Of course there is no such thing as a "sex-producing chromosome," sex being the result of the interaction of the X-chromosome upon other chromosomes, and of all of these upon the cytoplasm. The X-chromosome is only one factor in the determination of sex, but
