432 - I'.&redity and Evolution 



SPLASHED WHITE 



Fig. 26-7. Cross between a splashed white and a black Andalusian fowl. The 

 hybrid is popularly called a "blue" Andalusian fowl. (Jane Wyatt. From 

 MacDougall and Hegner, Biology. McGraw-Hill Book Co.) 



Provided the F, hybrids are clearly dis- 

 tinguishable from both of the pure line par- 

 ents, similar results are always obtained in 

 any one-factor cross, in which the mated in- 

 dividuals possess just a single hereditary dif- 

 ference. 



The results of this and all other similar 



which fosters the production of black pig- 

 ment (Fig. 26-7). Similarly, inbred white fowls 

 always produce only white progeny, so that 

 both of the feather color genes of this stock 

 must be of the same kind (b) which is not a 

 pigment producer (Fig. 26-7). Or to phrase 

 these ideas more technically, the black An- 



breeding experiments can be explained by a dalusian is said to be homozygous (BB) with 

 very simple hypothesis, which has been sub- reference to the "black gene"; whereas the 

 stantiated by many lines of evidence. The white fowl is homozygous (bb) as to the 

 feather color differences are due to a single "white gene." Furthermore, this genie condi- 

 pair of genes that are localized in a particu- tion obtains in all the diploid cells of the 

 lar pair of homologous chromosomes. Black organism, since all the cells have been de- 

 fowls, when inbred, never give rise to any rived by mitosis from the same fertilized egg. 

 white offspring, and consequently it is cer- Granting these assumptions, the results of 

 tain that the black stock has both of the the breeding experiments follow automati- 

 feather color genes of the same kind (B), cally from the known events of gametogene- 



