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 ceitain 
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 A LB/ NO 
C } REPRODUCTIVE CELLS 
■ '^ X P/RST CROSS 
CC J (COLORED) X ALBINO 
\ REPRODUCTIVE 
C J CELLS 
Cc 
1 PROGENY- HALF COLORED, 
CC ] 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 all 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 0, 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 
animal 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 (7 is received by each pig from both eggs and sperm (CC). 
45957°— 23— Bull. 905 3 
