128 
GENETICS: W. E. CASTLE 
These are the alternative views that we had in mind in our experiments with 
the hooded character of rats. It had been estabhshed that the hooded char- 
acter varied but that it gave unifactorial inheritance ratios. The question to 
be determined was whether the single gene plainly in evidence was or was not 
variable. To test the point it was necessary to make the 'residual heredity' 
as nearly constant_as possible, whether this consisted of modifying genes or 
not. For the purpose of determining whether the gene proper for the hooded 
character had or had not varied in the course of our selection experiments, we 
proposed to utiHze two very diverse races of hooded rats produced by many 
generations of selection in opposite directions. They were (1) a plus selected 
race in which the pigmented areas had been increased as much as possible by 
selection, and (2) ' a minus selected race in which the pigmented areas had been 
reduced as much as possible by selection, both having been derived at the out- 
-2 -1 0 +1 +2 +3 +4 
FIG. 1. A SET OF "GRADES" USED IN CLASSIFYING THE OBSERVED VARIATIONS OF THE 
HOODED CHARACTER OF RATS 
Pictures at the extreme right and extreme left of the series show the modal conditions 
of the plus selected and of the minus selected races respectively. 
set from a common stock. In the course of the selection they had become 
very different in appearance, the plus race being practically black all over as 
seen from above, the minus race white all over except for a black hood on 
head and shoulders. In classifying the young of each generation of rats an 
arbitrary set of grades was found useful. (See fig. 1.) 
In order to compare the genetic value of the gene proper for the hooded char- 
acter in one of these races with its value in the other, it was necessary first to 
eliminate all modifying genes or else to make them similar in the two races. 
To do this the plan was adopted of making repeated crosses of each race with 
a third race, entirely free from the hooded character, thus combining with the 
residual heredity of the third race the hooded character from each of the se- 
lected races. A race of wild rats was chosen as the third race and tables 1 
and 2 show how crosses with this race affected the grade of the hooded char- 
acter as recovered in hooded individuals in the F2 generation. 
