170 HEREDITARY CHARACTERS 



these beans be withdrawn from the bag haphazard in pairs, 

 it will be found that about 25 pairs are composed of two 

 white beans, 25 composed of two black beans, and 50 com- 

 posed of one black and one white. The greater the number 

 of beans used in the experiment, the more nearly will it come 

 to these proportions, or the greater number of times these 200 

 beans are mixed together and drawn in pairs the more nearly 

 will the average of the draws come to 25, 50, and 25 per cent. 



This theory of Mendel's is known as the segregation of 

 the gametes, and it is assumed that, in so far as the char- 

 acters behaving in this manner are concerned, offspring with 

 the perfectly pure characters of the two parent organisms 

 may be bred out from the hybrids. The accompanying 

 illustration shows diagrammatically how this is supposed to 

 be brought about (Fig. 20). The large circles containing 

 two small ones represent individuals ; the small circles 

 represent the gametes. The white circles represent the 

 dominant character, the black the recessive. The large 

 circles are shaded to show whether the dominant or re- 

 cessive character appears in the individual. It will be 

 seen that in the impure dominants the dominant char- 

 acter only appears, although the recessive gametes are also 

 contained in the individual. 



Experiments have since been carried out with a number 

 of other domesticated plants and animals, and dominance 

 and recessiveness have been determined with regard to 

 certain pairs of characters. Thus in sweet peas and stocks, 

 coloured flowers are dominant, white recessive. In mice 

 a coloured coat is dominant, an albino coat recessive. In 

 rabbits a coloured coat is dominant, an albino coat re- 

 cessive ; and angora fur is dominant, short fur recessive. 

 In poultry a rose comb is dominant, a serrated single comb 

 recessive, and so on with a number of other pairs of char- 

 acters. There are very frequently several pairs of characters 

 involved in the crossing of two pure-bred individuals. In 

 these cases each different pair of characters seems to act 

 independently of the rest. For instance, if two characters, 



