234 SECRETS OF ANIMAL LIFE 



illustration of characters that exhibit Mendelian 

 inheritance, the following may be cited, the domi- 

 nant condition being named first in each case : Horn- 

 lessness and the presence of horns in cattle, normal 

 hair and long " Angora " hair in rabbits and guinea- 

 pigs, kinky hair and straight hair in man, crest and 

 no crest in poultry, extra toes in poultry and the 

 normal number four, bandless shell in wood-snail 

 and banded shell; yellow cotyledons in peas and 

 green ones, round seeds in peas and the wrinkled 

 form, absence of awn in wheat and its presence, 

 susceptibility to rust in wheat and immunity to 

 this disease, two-rowed ears of barley and six-rowed 

 ears, markedly dentate margin in nettle leaves 

 and slightly toothed margin. Why one character 

 should be dominant and another recessive is not 

 known; a positive feature may be recessive, and 

 a negative feature may be dominant. It should be 

 noted that in a large number of cases of alternative 

 or Mendelian inheritance the dominance in the 

 offspring is not complete; thus, if black Andalusian 

 fowls be crossed with white ones the progeny are 

 " blue " Andalusians. 3. The third idea in Mendel- 

 ism is the theory of segregation. Mendel supposed 

 that the hybrid or cross-bred offspring produced two 

 kinds of germ-cells in approximately equal numbers 

 one-half with the determiner or factor corre- 

 sponding to the dominant character, and the other 

 half without it, or with the factor corresponding to 

 the recessive character. In other words, each germ- 

 cell is " pure " with respect to any given unit 



