32 Plant Genetics 



the theory of dominance among unit characters. It 

 should be reahzed also that dominance is not an 

 essential feature of Mendel's theory. Many cases 

 are known in which dominance fails, but in other 

 regards the Mendelian inheritance is strictly followed. 



The essential feature of Mendel's theory is his 

 conception of the purity of gametes, brought about by 

 the segregation of alternative characters. The striking 

 fact is that this conception, purely theoretical with 

 Mendel, has since been confirmed by cytology. In the 

 mechanism of cell division each chromosome is divided 

 into two equal parts and each daughter-cell receives 

 one of these parts. It is a reasonable inference that 

 chromosomes are bearers of hereditary characters. In 

 the production of gametes the number of chromosomes 

 characteristic of the organism is reduced one-half. As 

 a consequence each gamete carries only one-half the 

 characters of the individual that produced it. An 

 application of these statements to an explanation of 

 Mendel's 3 : i ratio will illustrate the situation. 



For convenience we will assume that the nuclei of 

 Mendel's peas have four chromosomes each (fig. 3). 

 In the case of a tall plant two of the four chromosomes 

 carry the character for tallness, that is, something that 

 determines the production of the tall character in the 

 somatoplasm, which is practically the body builder. 

 This unknown something is called by various names 

 in the literature of genetics, the commonest one 

 being determiner. In our illustration, therefore, two 

 of the four chromosomes carry the determiner for tall- 

 ness (p. 33). At this point two questions may be 

 asked. 



