FURTHER OBSERVATIONS ON THE "MUTANT" SERIES. 



Castle and Phillips described, under the name "mutants," 2 rats 

 of the plus-selection series of very high grade. They proved to be 

 heterozygotes between the average condition of the plus-selected race 

 at that time, about +3.75, and a new condition, not previously known 

 in our hooded races, but resembling that seen in "Irish" rats, which are 

 black all over except for a white spot on the belly and would be classed 

 on our grading scale as about + 5|. In later generations we secured 

 animals homozygous for the darker condition just described (that of 

 Irish rats). The homozygous "mutant" race proved to be very stable 

 in color-pattern, varying only from 5| to 5f , with a majority of ani- 

 mals graded 5|. Attempts to alter the modal condition of the race 

 by selection have thus far proved futile because of our inability to 

 increase the race sufficiently to afford a basis for selection. Its inbred- 

 ness and its feebleness are perhaps causally related. 



The suggestion was made that the change from our plus-selected 

 race, which had occurred in the mutant stock, might be due to some 

 supplementary modifying factor, not to a change in the hooded factor 

 itself. If so, a cross with a race lacking the hooded factor or its "modi- 

 fiers" might serve to demonstrate their distinctness by separating one 

 from the other. A wild race seemed best suited for a test of this 

 hypothesis, since it would be free from suspicion on the possible ground 

 of harboring either the hooded pattern or its supposed modifier, which 

 had converted the hooded pattern into the mutant. It was to be 

 expected, if the hypothesis were correct, that the mutant character 

 was hooded plus modifier; that then a cross with wild should produce 

 in F 2 hooded young (lacking the modifier) as well as mutants and selfs. 

 But if the mutant race had arisen through a change in the hooded factor 

 itself, then the cross should produce only mutants and selfs, without 

 hooded young in F 2 . Crosses have now been made on a sufficient scale 

 to show beyond question the correctness of the latter alternative, which 

 is entirely in harmony also with the results described in the preceding 

 parts of this paper. 



Six homozygous "mutant" females of grade +5| were mated with 

 wild males of the same race described in Part I. They produced 46 

 young, all gray like wild rats and of grades as follows: 



Grade 5^ 5| 5| 6 



No 1 15 7 23 



Exactly half of the 46 Y l rats bore no white spot, ^. e., were of grade 

 +6. Seven more bore only a few white hairs (grade 5|) . The remain- 

 der were very similar to the mutant parent in grade. 



Several matings were made of the FI rats, brother with sister, which 

 produced 212 F 2 young. About a quarter of these were black (non- 

 173 



