PRINCIPLES OF LIVESTOCK BREEDING. 
21 
strains with each other, all the young looked practically like ordi- 
nary gray rats with black eyes. This result was not, however, 
wholly unexpected. It is rather common to find that two varia- 
tions which look alike are due to different factors, so that, on crossing, 
each supplies the normal factor lacking in the other, and the young 
appear to be normal. If we represent the wild gray rats by formula 
PPRR, the red-eyed yellows by PPrr, and the pink-eyed yellows by 
ppRR, we indicate that each kind of yellow is recessive and breeds 
true by itself, but on intercrossing produces a variety PpRr, which 
contains both of the dominant normal factors, and thus appears like 
a wild gray. 
Parewts 
P/NK-EYED YELLOW (ppRR) 
RED-EYED YELLOW (PPrr) 
REPRODUCTtVE CELLS 
P/RST CROSS - GRAY(PpRr) 
REPRODUCT/VE CELLS 
40 PER CENT pR 40 PER CENT Pr 
/OPER CENTpr /O PER CENT PR 
Fig. 4.— Diagram illustrating linkage. The pink-eyed and the red-eyed strains of yellow rats depend on 
different recessive factors. As each supplies the normal factor lacking in the other, crossing results in 
normal black-eyed grays. These grays produce reproductive cells in which the factors tend to be asso- 
ciated in the same combination as that in which they entered the cross, due, it is believed, to their trans- 
mission in the same chromosome. Only about 10 per cent of the reproductive cells are found to trans- 
mit both normal factors (PR). 
On raising a second generation, a few pink-eyed yellows were 
found, which proved to transmit the red-eyed as well as the pink- 
eyed variation in all of their reproductive cells. Their formula would 
be pprr. On crossing this strain with ordinary wild rats (PPRR), we 
obtain gray young, which should have the same formula (PpRr) as 
the first cross between the two strains of yellows. The two kinds 
of first-cross grays were backcrossed with the above-mentioned double 
recessive pink-eyed yellow strain (pprr) by Prof. Castle in order to 
test the formulae. 
