PHENOMENA OF INHERITANCE 



429 



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 Seg- 

 regation 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 par- 

 ents; 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 addi- 

 tional evidence that the chromosomes are especially concerned in he- 

 redity, 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 in- 

 volved 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 Men- 

 delian factor. Of course there is no such thing as a " sex-producing 

 chromosome," sex being the result of the interaction of the X -chromo- 

 some upon other chromosomes, and of all of these upon the cytoplasm. 

 The X-chromosome is onlv one factor in the determination of sex, but 



