158 



GENETICS 



young four varieties are obtained in the next genera- 

 tion, viz., black, brown, black-eyed yellow, and brown- 



10 



bE \Be 



Black 



14 



'be \ I Be 



'. Yellow 



IbE 



BE \\bE 



Black 



BellbE 



Black 



11 



bE \\bE 



Chocolate 



15 



be I bE 



Chocolate 



|6e 



Be \\be 



Black-eyed Yellow 



12 



bE\\b* 



Chocolate 



16 



be\ \be 



3rown-eyed Yellow 



FIG. 34. Diagram to illustrate the origin of a brown-eyed yellow 

 guinea-pig from two heterozygous black parents, based upon 

 Castle's experiments. The gene for yellow (Y) is present in 

 every gamete and is consequently duplex in every zygote but 

 is hidden whenever the gene B is present. B, black pigment 

 hiding brown or chocolate; b, chocolate (absence of B) ; E, 

 extension of B over the entire body hiding Y; e, restriction 

 of B to eyes alone thus exposing Y over the entire body. 



eyed yellow, the latter a variety unknown before 

 Castle's experiment in breeding was made. 



For the sake of clearness the formation of the 

 brown-eyed yellow is shown above in Figure 34. 



