42 Outline of Genetics 



known as the reduction division, for each of the resulting 

 nuclei has the reduced number of chromosomes, just 

 half of the characteristic number in the body cells. It is 

 important to remember that this reduction is not indis- 

 criminate, but always involves a separation of the two 

 components of each chromosome pair. It is the reduc- 

 tion division that gives rise to the gametes. Gametes, 

 therefore, are characterized by the reduced or haploid 

 number of chromosomes, in contrast with the body cells 

 which have the diploid number. Gametes have just one 

 representative of each chromosome pair that appears in 

 the body cells. When two gametes unite at fertihzation 

 there is, of course, a return to the diploid number in the 

 resulting zygote. 



This is exactly the mechanism required by Mendel's 

 scheme, on the assumption that the chromosomes are 

 the bearers of hereditary characters. So much data has 

 accumulated to justify this assumption that it will be 

 treated as an established fact in the subsequent descrip- 

 tions. 



The chromosome mechanism may be applied to the 

 case in hand as follows. For convenience, we will assume 

 that the nuclei of the body cells in Mendel's peas have 

 each four chromosomes (two pairs). This is represented 

 in fig. 3. In the case of a tall plant, two (one pair) 

 of the four chromosomes carry the character for tallness, 

 that is, something that determines the production of 

 the taU character in the somatoplasm. This unknown 

 something is called by various names in the literature 

 of genetics; for the present we shall refer to it as a deter- 

 miner. In our illustration, therefore, two of the four 

 chromosomes carry the determiner for tallness. 



