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 ppRR) 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 



PA/&A/TS 



WILD GRAY PPRR 

 DOUBLE PECESS/VE P/NK- 

 EYED YELLOW pprr 



REPRODUCT/VE CELLS 



F/RST CROSS - GRAYPpRr 



REPRODUCT/VE CELLS 

 4Q PER CENT PR -4QPER C&VTpr 



/O PER CENT Pr ~/0 PER CENT pR 



Fig. 5. — Diagram illustrating linkage. The cross between wild 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 



