PRINCIPLES OF LIVESTOCK BREEDING. 17 



before, 50 per cent colored to 50 per cent albino. These last colored 

 young are seven-eighths blood of the albino stock, yet when crossed 

 with albinos they again produce 50 per cent colored and 50 per cent 

 albino. No matter how many albino top crosses are made, the col- 

 ored animals continue to produce colored young in numbers which 

 never depart to a significant extent from 50 per cent. 



The animals of the original stock are supposed to have a certain 

 hereditary unit in their make-up which we may call factor C, follow- 

 ing the custom of representing a dominant unit by a capital letter. 

 The albino stock have a modification of this factor, c, which is no 

 longer able to play its normal part in the production of color. All 



cc 





cc 



} COLORED X ALB/NO 



i 











c 





c 



} REPRODUCTIVE CELLS 



\ 



Cc 



/ \ 





\ FIRST CROSS 

 CC J (COLORED)X ALBINO 



c 



■ \ 



c c 



N I REPRODUCTIVE 

 C J CELLS 



\ 







v. <f\ PROGENY- HALF COLORED, 

 CC J HALF ALBINO 



Fig. 2. — Diagram illustrating the mode of inheritance of a unit Mendelian factor. A pure colored strain 

 of guinea pig (CC) is crossed with albinos (cc). Their progeny are ail colored (Cc), but produce two kinds 

 of reproductive cells, one transmitting color (C) and other albinism (c), as is revealed by a cross with 

 albino stock. 



the reproductive cells of the colored stock have factor C, those of the 

 albino stock factor c. On crossing the two stocks, a fertilized egg cell 

 is produced which must possess both. The evidence indicates, how- 

 ever, that the two units remain side by side in the cell, without the 

 slightest influence on each other, and in each cell division, as the 

 annual develops, each unit divides, with the result that every cell in 

 the body is similar to the fertilized egg in containing both C and c. 

 In the appropriate parts of the body — skin, eyes, and hair — factor 

 cooperates with other factors in the production of color, and, as a 

 single unit appears to be sufficient in this case, the crossbred guinea 

 pigs (Cc) are as strongly colored as the original colored stock in which 

 factor Cis received by each pig from both eggs and sperm (CC). 

 5254°— Bull. 905 3 



