392 THE BIOLOGY OF BIRDS 



To return : if a crested fowl is crossed with one without 

 a crest, the progeny are all crested. This is expressed in the 

 formula 



D X R 

 yields 

 D(R) 



The recessive character is indicated in brackets beside the 

 dominant character, because the next generation shows that 

 it is not absent from the inheritance. It is represented by a 

 factor in the fertihsed egg-cell, though it may not find any 

 expression in the body. Should it find a shght expression 

 in the body, as it often does, then we speak of imperfect or 

 incomplete dominance. 



Now if the hybrid offspring D(R) be paired together or 

 with others of precisely similar history, their offspring (the 

 second filial generation) will be 25 per cent, pure dominant, 

 50 per cent, like the hybrids, and 25 per cent, pure recessive. 

 In the case mentioned, there will be 25 per cent, pure- 

 crested, 50 per cent, crested, but not pure-crested, and 25 per 

 cent, pure non-crested. In external appearance there may 

 be no difference between the pure-crested and the impure- 

 crested, but there is a difference in their germ-cells, for the 

 pure-crested, when paired with others of similar history, 

 will yield only pure-crested, whereas the impure-crested, 

 when paired with others like themselves, will yield the same 

 characteristic Mendelian proportion 



25 per cent, pure-crested + 50 per cent, impure-crested + 25 per cent, 

 pure-uncrested. 



To complete the schematic expression : — 



D X R (Parents) 



Yield 



D(R) Hybrid or First 



which inteibred Filial Generation 



will yield 



\ 



I [ /Second 

 25 per cent. D -f 50 per cent D(R) -j- 25 per cent. R ; Filial 

 which interbred which interbred which interbred Genera- 

 will yield will yield will yield Ition 

 only pure D | only pure R 



I I 



25 per cent. D + 50 per cent. D(R) + 25 per cent. R 



