CHAPTER 2 



THE MENDELIAN METHOD 



Mendel's Experiments The Segregation of Factors 



Linkage The Chromosome Basis 



Sex Determination and Sex-Linkage 



We have now seen how certain visible self-propagating structures 

 are transmitted from cell to cell and from parent to offspring. 

 We have seen the rule and order of these processes. How are 

 they reflected in the gross structures, the phenotypes, of parents and 

 offspring? The answer is given by the mendelian method. 



Mendel discovered the two requirements for success in planning 

 a critical experiment on heredity — that is, on sexual heredity. First 

 of all, crosses must be made between visibly and sharply different 

 parents. Secondly, these parents must come from true-breeding lines, 

 otherwise any differences visible in the progeny might be referable, 

 not to the known differences between these parents, but to unknown 

 differences between the ancestors of one of them. Such true-breeding 

 lines he knew from experience could be obtained with certainty only 

 in organisms with regular self-fertilization, i.e. where the male and 

 female gametes which fuse are produced, generation after generation, 

 by the same zygote, by the same or different flowers of one parent 

 plant. 



Mendel's Experiments 



The garden pea, Pisum sativum, has varieties which meet these 

 requirements and we cannot do better than describe the experiments 

 Mendel made with them. In 1857 he took 34 varieties, and after two 

 years' trial selected 22 of them for his experiments. These remained 

 constant throughout the work. They gave him seven differences of 

 character (or phenotype, as we may say) distinct enough for his 

 purpose. These differences, or "characters" as they are conveniently 

 called, showed themselves in the course of development in the order 

 given by Table i. 



One character, that of purple pigmentation, expresses itself at 



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