THE AMERICAN NATURALIST [Vol. XLI 



The blue Andalusian fowl is a race which in breeding produces 25% 

 of black offspring, 50% of blue, and 25% of white splashed ^\ith 

 black. It is evident from these proportions that the blue race desired 

 by fanciers is essentially mongrel, and can never be made to breed 

 true. The black race and the splashed whites remain true when each 

 is mated with its own kind, but when crossed they produce the blue 

 Andalusian. 



The fixed proportion of pure and mongrel forms in the offspring 

 of hybrids may be readily explained. The factors A and a, derived 

 by the hybrid from its parents respectively, are transmitted through 

 its germ cells in equal abundance. The factors become segregated, 

 so that one half of the germ cells contains only A, and the other half 

 a. In the process of fertilization an A will unite with a as often as 

 Avith A ; and an a will join A as often as a. Thus there will be 2 

 Aa for each aa and AA. 



When two different inheritable factors occur in each parent the 

 number of combinations in the offspring is much greater. Mendel 

 found, with peas, that the height of the plant (tall or dw^arf) and the 

 color of the seeds (green or yellow) were transmitted independently 

 of one another. A and B may represent respectively the factors for 

 tallness and greenness which are dominant; a and b the factors for 

 shortness and yellowness which are recessive. If a tall green-seeded 

 plant AB, is crossed with a dwarf yellow ab, tall green-seeded hybrids 

 containing the factors AaBb result. Every germ cell of such a hybrid 

 contains one factor for height and one for color; tlu^y are e(|ually 

 distributed in the four possible combinations AB, Ah, ciB, and ab. 

 When such a group of germ cells fertihzes a .similar (jjioup, the follow- 

 ing combinations are to be expected: 



AABB AAbb aaBB aabb 



2AA Bb 2Aa bb 2aa Bb 



2AaBB 



4AaBb 



Thus among sixteen individuals nine contain both dominant factors 

 and in the case of the peas are tall green-seeded plants. Three con- 

 tain only the dominant A, and are tall yellow-seeded forms; three 

 contain only the dominant B and are green-seedcnl dwarfs. ( )nc 

 contains neither dominant and is a yellow-seeded dwarf. Tlii. ratio, 

 9:3:3:1 Mendel verified by experiment. 



The sweet pea known as the 'Painted Lady' has a bright pink color 

 due to its sap, and this is dominant over the absence of such sap color 



