Mendel's Law 31 



and so dwarf ness did not appear in any of them. Such 

 pairs of alternative characters are now commonly called 

 allelomorphs. Thus tallness and dwarfness are allelo- 

 morphs in the pea, one dominant over the other, which 

 is therefore recessive. 



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 caimot 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 con- 

 tain 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 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 (better called segre- 

 gation), 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 fea- 

 ture of Mendel's law. The idea of dominance had been 

 somewhat vaguely proposed before Mendel's time. 

 In the old Uterature on animal breeding one meets 

 theories of prepotency, 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 



