22 
BULLETIN 905, U.'S. DEPARTMENT OF AGRICULTURE. 
This would seem to be the same kind of cross as that discussed 
previously, in which black guinea pigs transmitting both red and 
albinism (CcEe) were crossed with albinos of red stock (ccee). We 
should expect half the reproductive cells of the crossbreds to contain 
factor P, and half of these should also contain factor R. Thus, one- 
quarter of the young should receive both normal factors from the 
crossbred gray parent and be gray themselves. This, however, was 
not the result. When grays derived from the cross between the 
original two strains of yellows (PPrr and p-pRR) were used, only 
174 out of 1,714 young were gray, about 10 per cent. When the 
gray parent came from the cross between wild grays (PPRR) and 
PAfi£NTS 
WILD GRAY PPPR 
DOUBLE RECESSIVE PINK- 
EYED YELLOW pprr 
PEPRODUCTIVE CELLS 
FIRST CROSS - GRAYPfRr 
REPRODUCTIVE CELLS 
Cj ) \ *Q PER CENTPR -4QPER CENTpr 
/OPER CENTPr-IO PER CENT pR 
Fig. 5. — Diagram illustrating linkage. The cross between mid gray rats and double recessive pink-eyed 
yellows results in gray young with the same formula as in the cross between the two yellow strains. 
They breed differently, however, since in this case the two normal factors enter the cross together, instead 
of apart. About 40 per cent of their reproductive cells are found to transmit both normal factors in this 
case instead of 10 per cent. 
the double recessive pink-eyed yellows (pprr), 1,255 out of 3,032 
young were gray, or more than 40 per cent. In the second case there 
is as much excess over the expected 25 per cent as there was defect 
in the first case. The explanation is that the two sets of factors P, p 
and R, r are not wholly independent of each other in heredity. An 
individual produced, as in the second case, by the union of repro- 
ductive cells PR and pr tends to produce reproductive cells of these 
kinds in excess over the kinds Pr and pR. The situation is reversed 
in an individual produced, as in the first case, by the union of repro- 
ductive cells Pr and pR. This tendency of certain sets of factors to 
