40 Outline of Genetics 



3. Purity of gametes. — A gamete can contain only 

 one of two alternative characters. For example, it may 

 contain the character for tallness or for dwarfness, but 

 not both. In other words, allelomorphs cannot be repre- 

 sented in the same gamete. If the gamete having the 

 character for tallness unites with one having the char- 

 acter for dwarfness, the resulting zygote will have both, 

 but will produce a tall individual because tallness is 

 dominant to dwarfness. When this tall hybrid produces 

 gametes, however, one-half of them will contain the 

 character for tallness and one-half of them 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 (due to 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 concep- 

 tion does not represent the essential feature of Mendel's 

 law. The idea of dominance had been somewhat vaguely 

 proposed before Mendel's time. In the old literature 

 on animal breeding one meets theories of ''prepotency," 

 which were proposed again and again before the dis- 

 covery of Mendel's work in 1900. In any event, Men- 

 del 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 Men- 

 del's theory. Many cases are known in which domi- 

 nance fails, but in other regards the Mendehan inherit- 

 ance is strictly followed. 



