CHAPTER XVIII 



THE BEARERS OF HEREDITARY CHARACTERS 



The conclusion reached in the preceding chapter was 

 that sex may be determined by chromosomes, but that it 

 also may be determined by physiological conditions. 

 To the geneticists this suggests the following question: 

 If the sex character may be determined by physiological 

 conditions, why may not other plant characters be 

 determined in the same way? In other words, this 

 opens up the general question of the chromosome 

 hypothesis in reference to inheritance. It has been 

 taken for granted in the preceding discussion that 

 chromosomes are the bearers of hereditary characters, 

 but is this position altogether justifiable? It will be 

 well to consider some of the arguments for and against 

 the claim that chromosomes are the bearers of what are 

 called hereditary characters. 



ARGUMENTS IN FAVOR OF THE CHROMOSOME HYPOTHESIS 



This evidence will be considered under two heads, 

 namely, that from cytology and that from breeding. 



Cytology. — (i) The program involved in the 

 ordinary mitotic division is very suggestive evidence. 

 We discover a very exact mechanism worked out, the 

 result of which is to divide the chromosomes evenly, so 

 that each daughter-nucleus receives an exactly similar 

 _ set of chromosomes, a set exactly similar to the original 

 one in the mother-nucleus. These chromosomes are 



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