MENDELISM 131 



we have two new true-breeding, pure types Ab, yellow 

 and angular, and aB, green and round. In other words, 

 we are able, on the Mendelian principle, to create in two 

 successive generations new types breeding true indefinitely. 

 The diagram from J. P. Lotsy on p. 130 will make the 

 descent of all the types clear at a glance. 



With three allelomorphs, Aa, Bb, Cc, we get a hybrid 

 of the constitution ABCabc. The F 2 offspring will give 

 eight visible types, in the following proportion : 



F 2 = 2/ ABC+9flBC+9 A6C+9 ABc+3 A6c+3 aBc+3abC+i abc, 



and so on with more allelomorphs. 



IV.COMPLICA TIONS. 



Not all cases of Mendelian inheritance work out so 

 simply as those given in the previous account. There are 

 phenomena which at first sight appear rather complicated, 

 and need some further elaborations in order to be made 

 amenable to Mendel's law. 



In the first place, the dominance of one of the characters 

 may be incomplete. Thus, when white Leghorn poultry 

 is crossed with brown, the hybrid is not completely white, 

 as would be expected according to Mendel's law, but it 

 has invariably a few dark splashes. In other respects the 

 hybrid behaves in the regular manner, and, on inter- 

 breeding, splits up into one-quarter white birds, one-quarter 

 brown ones, and two-quarters white birds with " ticks " 

 of colour. 



A still more interesting case is that of the Andalusian 

 fowl. Here the hybrid differs completely from either the 

 dominant or the recessive parent. It is blue, while the 

 dominant parent is black, the recessive being white with 

 black splashes. On inbreeding, this hybrid yields, in the 

 ordinary Mendelian manner, one-quarter black birds, one- 

 quarter " splashed whites," and two-quarters blues. If 

 we cross the blacks with the whites, we get all the off- 

 spring blue, while the blue always show on inbreeding 



