OLD TYPES AND NEW 



167 



these hybrid young four varieties are obtained in 

 the next generation, viz., black, brown, black-eyed 

 yellow, and brown-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 below in Figure 50. 



Be 



BeMBE 



Black 



bE||EE 



Black 



13 



be||BE 



Black 



BE Be 



Black 



Be Be 



10 



Black 



14 



b e | |B e 



Black-ened Yellow 



Be bE 



Black 



11 



bE||bE 



Chocolate 



15 



be||bE 



Chocolate 



|be 



BE||be 



Black 



Be||be 



Black ened Yellow 



bE||be 



Chocolate 



16 



be| [be 



3rown-exjed Yellow 



Fia. 50. Diagram to illustrate the origin of a brown-eyed yellow guinea- 

 pig from two heterozygous black parents based upon Castle's experi- 

 ments. The factor for yellow (Y) is present in every gamete and is 

 consequently duplex in every zygote but is hidden whenever the fac- 

 tor 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. 



