MENDELIAN INHERITANCE QI 



these blue Andalusian fowls be mated, their offspring (the 

 " F2 " generation) are not all blue ; some are black and 

 others are white. On the average, in this generation, there 

 will be equal numbers of pure black and pure white, and 

 twice as many of the mixed or blue form. 



Far more commonly, however, it is found that there is 

 no blending of the contrasted parental characters, but 

 that one prevails over the other. Thus when a black 

 guinea-pig is mated with a white one, the offspring in the 



Fi ccf) 



Fn 



/^ 



Aa 



A A A a ^^ 

 . > r ^— 



AA AA Aa aA aa AA Aa aA 



0.0. aa 



Fig. 43. — Mendelian inheritance illustrated in wood snail 



[Helix nemoralis). 



P., The parents, bandless [A), dominant, and banded {a), recessive. 

 F.I., First filial generation, all bandless (.-ta). 



F.ii., Second filial generation, 25 per cent, pure bandless {A A), yielding bandless 

 offspring in the next generation (F.iii.). 



25 per cent, pure banded {aa), yielding banded offspring in the next 

 generation (F.ni.). 



50 per cent, bandless {Aa), with the banded character recessive as in F.i. 

 These, if inbred, yield in the F.iii. generation the same ratio : 



lAA + zAa + iaa. 



F^ generation are not intermediate in colour, but perfectly 

 black ; blackness is said to be dominant over whiteness 

 or albinism, which is a recessive character. In the Fg 

 generation the majority are again pure black and the 

 minority pure white, the ratio between blacks and whites 

 being three to one. Now if these white guinea-pigs of the 

 Fg generation be inbred, their offspring will always be pure 

 white ; in the same way either the black or white Anda- 

 lusian fowls may be inbred without any other shade of 



