142 THE SCIENCE OF BIOLOGY 



characters like the black and white and that combination 

 occurs by chance. 



The application of the cell-theory in the foregoing explana- 

 tion of the 1:2:1 ratio is obvious. The germs to which 

 Mendel referred in general terms are germ-cells. Fertiliza- 

 tion is a fusion of two cells, each containing one-half the 

 normal number of chromosomes. The modern chromosome- 

 theory of heredity, which is a theory of intra-cellular organ- 

 ization, furnishes an even more precise explanation of the 

 phenomena. The most important discoveries since the 

 work of Mendel have been those relating to the chromosomes 

 in heredity. 



In illustration of the relationship between chromosome 

 behavior and Mendelian heredity, a pair of characters 

 may be arbitrarily represented by A and a. The use of the 

 large letter expresses the fact that A is dominant while a is 

 recessive, The use of two letters of the same sort, rather 

 than two different letters, shows that A and a are a con- 

 trasting pair of Mendelian unit-characters. The pure adults 

 would, therefore, be designated respectively as AA and aa; 

 the hybrids as Aa. Suppose the number of chromosomes in 

 the body cells is four pairs, which means four single chromo- 

 somes to a germ-cell, i. e., one member of each pair (cf. 

 Fig. 16). The case may be represented as in Fig. 20, again 

 using squares for adults and circles for germ-cells. The 

 addition of globular bodies indicates the chromosomes as 

 they occur in every cell of the adult body and in the germ- 

 cells. 



Assume that the A determiner is carried in one of the 

 pairs of chromosomes of the dominant individual and the a 

 determiner in the corresponding chromosomes of the reces- 

 sive. This may be indicated by writing the letters upon the 

 chromosomes in question. When reduction in the number of 

 chromosomes occurs, during the final ripening of ova and 

 spermatozoa, the result is as shown in the Pi germ-cells of 

 Fig. 20. Each germ-cell possesses but one of the chromo- 



