352 EXPERIMENTAL STUDY OF INHERITANCE 



In books that deal with Mendelism in particular (see Biblio- 

 graphy) the reader will find an account of further complications, 

 e.g. when the parents differ in three pairs of contrasted unit- 

 characters. These more complicated cases are of great interest 

 to the breeder or cultivator who wishes to know how to combine 

 various excellences in a type that will breed true. 



Blue Andalusian Fowls. When black and white fowls are 

 crossed there sometimes results a blue or Andalusian fowl " with 

 a minute patchwork of black and white." When these are 

 inbred they produce 25% black, 50% blue, and 25% white with 

 black splashes. This splitting-up is characteristically Mendelian, 

 but what gives rise to the " blue " feature is obscure. 



The ingenious Mendelian interpretation in the case of the An- 

 dalusian fowl is that the black and the splashed white are the 

 pure breeds, and that the blue Andalusian is a peculiar mongrel. 

 We must refer to Mr. Punnett's essay on Mendelism for the 

 interesting theoretical working out of the case, which is exceed- 

 ingly instructive, since it shows that Mendelian interpretation 

 is feasible even when the hybrid (the Andalusian) is quite distinct 

 from either parent (black or splashed white). 



Yellow Mice. Somewhat similar is the much-discussed case 

 of yellow mice. The yellow is dominant over all other colours, 

 but it is itself quite unfixable. No pure or homozygous yellows 

 can be obtained. When two yellows are mated, two-thirds of 

 the offspring are yellow and one-third some other colour. It 

 has been suggested that the fertilisations which give pure yellow 

 do occur, but that they come to nothing for some unknown 

 reason. Another case, worked out by Baur, is that of a so-called 

 " golden " snapdragon, which is also unfixable. It produces 

 when self-pollinated two-thirds golden offspring and one-third 

 green. And here there is some evidence of the existence of a 

 few feeble entirely yellow seedlings which are not viable. 



Compound Allelomorphs. A differentiating unit character 

 capable of replacing another or of being replaced by another 



