MODERN THEORlIiS OF HEREDITY. 



A.B.c? A.b.<f a.B.,? a.b. J 



383 



Adding up the 16 zygotes, we get A.B., A.b., a.B., a.b., 2A.a.B., 

 2a.B.b., 2A.a.b., 4A. a.B.b. 



The last vertical line alone shows the result of crossing the 

 hybrid with the recessive. 



It must be emphasized that dominance is by no means an 

 essential part of Mendelism, though it frequently occurs. Our 

 next example is one in which there is no dominance. This is 

 the case of " blue '" Andalusian fowls, which had puzzled many 

 generations of breeders, until it was shown to be a perfectly 

 simple case of Mendelian inheritance. It appeared common 

 sense to the breeders that if one continued long enough to breed 

 from these " blue " fowls, mated with their like, one would in 

 time get a pure breed of " blue " Andalusians. But in practice 

 about half the offspring are invariably found to be entirely 

 different, some black, and some white with black splashes. In 

 every generation ■ the breeders diligently discarded these 

 ■' rogues," being most careful not to allow them to mate and so 

 still further spoil the .strain, and they bred only from the blues, 

 but with always the same result. It has now been shown that 

 when the white and black "rogues" are crossed only "blues" 

 are hatched, these being simply the hybrids between the blacks 

 and whites, neither being dominant. ( )n mating the hybrids the 

 normal proportion of one pure white, two hybrid blues, and one 

 pure black, results. 



As an example from the plant kingdom we may take the 

 case of two species of primula (P. sinensis and P. stcllata) , 

 which, when hybridized, give a distinct hybrid form which has 

 been named P. pyrainidalis. This on self-pollination gives the 

 usual proportion of one P. sinensis: two P. pyrantidalis: one 

 P. stellata, in F.2. 



These are perfectly simple examples of IMendelian inheri- 

 tance, but some cases are not so .straightforward as these. In 

 pome plants it has been found that an affinity seems to exist 

 between allelomorphs belonging to different pairs, such that, 

 in.^tead of every possible combination of characters being found 

 in the usual proportions in F.2, certain combinations occur more 

 frequently than in those cases we have been considering. This 

 is spoken of as " gametic coupling," and it has been shown, in 

 the case of some such allelomorphic pairs that number.^ in F.2, 



