Individuality of the Chromosomes 255 



already described for body cells. The final division, how- 

 ever, results in producing cells that have only one-half the 

 usual number of chromosomes (see Fig. 6j) . Later, when 

 fertilization takes place, the chromosomes of the sperm and 

 those of the cg^ cell combine and start the new individual 

 with a full complement of chromosomes. The process of 

 fertilization thus brings into the individual's make-up ger- 

 minal material — or more specifically chromosome material 

 — from each parent. When the germ cells are being formed, 

 it is the pairs of chromosomes that break up. Each sperm cell 

 and each egg cell may therefore get a different complement 

 of chromosomes. As a result the individuals of the following 

 generation will have entirely new combinations of chromo- 

 somes in the cells. The uniting of chromosomes from the 

 two germ cells into pairs, and the subsequent random separa- 

 tion of the members of each pair in the production of new 

 germ cells, furnish a mechanism in perfect accord with Men- 

 delian inheritance. These facts serve not only to explain 

 segregation in the proportions observed, but also to prophesy 

 new combinations of characters in the making of new hybrids 

 (see page 256 and Fig. 68). 



Individuality of the Chromosomes 



The chromosomes had been for a long time recognized 

 as the carriers of heredity. They are, however, very few in 

 number — few, that is, compared with the number of char- 

 acteristics that make up an individual, even a small individual 

 like a fruit fly. It is therefore necessary to consider what con- 

 nection there is between the numerous inherited traits and the 

 few chromosomes. The intensive work on these problems 

 with the fruit fly Drosophila has shown first of all that, cor- 

 responding with the four pairs of chromosomes, there are 

 four sets of inherited pairs of characters. In the study of 

 hundreds of character pairs and of mutational forms, it be- 

 came apparent that certain characters appear together more 

 often than others, and by grouping these characters it was 



