CHAPTER VI 

 LINKAGE 



The fundamental mechanism of inheritance which 

 was proposed by Mendel, and which was later supported 

 by cytological studies, has been confirmed time and 

 again by breeding experiments. Its scope is consid- 

 erably enlarged by the factor hypothesis, but its basic 

 concepts are not altered. It will now be necessary to 

 consider some well-established facts of inheritance which 

 can be interpreted only by analyzing still further the 

 hereditary mechanism. 



It has been assumed that the chromosomes are the 

 bearers of the hereditary units or genes. (The term 

 ''gene" is used where it is not intended to imply whether 

 the hereditary unit acts as a factor or determiner.) This 

 has been warranted by the fact that the distribution of 

 the chromosomes in inheritance fits exactly into the Men- 

 delian scheme. In the cases that have been considered 

 in the last few chapters, the genes have always been 

 located on separate chromosomes, with the result that 

 they have been passed on in inheritance quite independ- 

 ently of each other. The intensive study of inheritance 

 that has been made during the last decade, however, has 

 revealed cases where the total number of genes known for 

 the organism exceeds the number of chromosome pairs. 

 The obvious conclusion is that more than one gene may 

 be carried on a single chromosome. If this is true, it 

 should result in decided modifications of the breeding 



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