no GENETICS 



great numbers of different organisms, from the pea plant 

 and Drosophila to man. Tlie fact that inheritance gives 

 these ratios is commonly spoken of as Mendel's law, and 

 the type of inheritance which it exemplifies is spoken of as 

 Mendelian inheritance. The type that gives in the F2 gen- 

 eration 3D to iR is the commonest method of inheritance, 

 much commoner than the sex-linked method, in which the 

 characteristics depend on differences in the X-chromosomes. 

 This is obviously because autosomes are more numerous 

 than X-chromosomes. 



We find then that inheritance according to Mendel's law 

 takes place exactly as it would if the characteristics con- 

 cerned depended on the two members of a pair of auto- 

 somes. As will be shown later, it can be proved experi- 

 mentally that such characteristics do indeed depend upon 

 autosomes. 



Thus the rules of Mendelism are the rules of distribution 

 of the two members of a pair of autosomes, one having a 

 recessive effect, the other a dominant effect. Mendelian 

 heredity is heredity of characteristics that depend on differ- 

 ences in autosomes. 



Mendel did not know of the relation of the character- 

 istics to the chromosomes. He discovered however certain 

 other important facts about this type of heredity, facts 

 which have become clearly explicable since it has been 

 learned that this type depends on differences in autosomes. 

 These facts must be examined. 



For this purpose it will be convenient first to define cer- 

 tain terms which are universally employed in dealing with 

 inheritance. 



By the union of two germ cells or gametes, as A and a, 

 there is produced a zygote. If the two germ cells are alike 

 with respect to the chromosomes or characteristics that they 

 carry, the zygote so produced is spoken of as a hoviozygote; 

 thus AA and aa are homozygotes. If the two germ cells that 



