MENDEL'S LAWS OF HEREDITY 239 



for tallness or for dwarfness, but not both. In other words, allelo- 

 morphs cannot be represented in the same gamete. If the gamete 

 having the character for tallness unites with one having the character 

 for dwarfness, the resulting zygote will contain both, but will produce 

 a tall individual because tallness is dominant over dwarfness. When 

 this tall hybrid produces gametes, however, one-half of them will 

 contain the character for dwarfness. Thus the alternative characters 

 are "segregated" in gamete formation and no gamete will have both 

 characters. 



These three theses, independent unit characters, dominance, and 

 purity of gametes (better called segregation), make up the theoretical 

 explanation of Mendel's law. Independent unit characters was of 

 course a necessary conception. It was original with Mendel, and has 

 also been original with other investigators, but this conception does not 

 represent the essential feature of Mendel's law. The idea of domi- 

 nance had been somewhat vaguely proposed before Mendel's time. 

 In the old literature on animal breeding one meets theories of pre- 

 potency, which were proposed again and again before the discovery 

 of Mendel's work in 1900. In any event Mendel was the first to 

 formulate definitely the theory of dominance among unit characters. 

 It should be realized 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 theoreti- 

 cal 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 : 1 ratio will illustrate the situation. 



For convenience we will assume that the nuclei of Mendel's peas 

 have four chromosomes each (Fig. 55). In the case of a tall plant two 

 of the four chromosomes carry the character for tallness, that is, some- 

 thing that determines the production of the tall character in the 

 somatoplasm, which is practically the body builder. This unknown 



