INHERITANCE IN ORGANISMS 



535 



generation there are individuals like both grandparents as well as inter- 

 mediates. This occurs in the case of plants called four-o'clocks; plants 

 with red flowers crossed with those with white flowers produce hybrids 

 with pink flowers, and when these pink hybrids are crossed the result is a 

 ratio of one red dominant, two pink hybrids, and one white recessive. 

 Another case of blending inheritance is that of the blue Andalusian fowl 

 (Fig. 326), which is a hybrid between a black individual and an indi- 

 vidual which is white splashed with black. When these blue Andalusians 



P Gen 



F, Gen, 



The 



c • 



Fig. 326. — Blending inheritance, as illustrated by the blue Andalusian fowl. 



P generation is splashed white and black, the F\ generation all blue, and the Fi generation, 

 one splashed white, two blue, and one black. 



are bred, there is in the F^ generation a ratio of one splashed white, one 

 black, and two blue individuals. 



Another variation is that due to the cumulative effect of duplicate 

 genes. This is observed in mulattoes, whose skin color may vary from 

 very dark to very light. It has been found that there are lethal genes 

 which when present in pairs cause the death of the organism. In some 

 cases a character is believed to result from the interaction of two genes, 

 neither one of w^hich when alone shows itself. These are called com- 

 pleniental genes. 



It has recently been demonstrated that radiations, particularly 

 X-rays, are capable of bringing about gene mutations (Sec. 587). 



