Recent Advances in the Study of Heredity. 319 
the absence of grey coat. The unexpected appearance of the 
purple spot in the hybrid will be seen to follow readily from the 
theory which we have enunciated. The two factors necessary for 
the production of purple spot, one of which existed in one of the 
parents and the other in the other, are brought together in the 
same zygote in and the manifestation of the purple spot is thus 
brought about. It will be seen how the phenomenon of reversion 
in this particular case is accounted for by this theory. Let us now 
proceed to see how it fits the results which obtain in F 2 , and, later 
on, consider what justification there is for the conclusion that 
these phenomena are accounted for by this theory. For the factor 
for grey coat let us write G, and for the absence of this factor g; 
for the factor for purple spot let us write P and for the absence of 
this factor p. And let us assume that these pairs of characters 
segregate in gametogenesis in the same way as the other pairs of 
characters we have already dealt with. Inasmuch as this is a case 
in which two pairs of characters are concerned, we should expect a 
9 : 3 : 3 : 1 : proportion, in F 2 , in which the characters are distributed 
as follows 
9 GP : 3 Gp : 3 gP : 1 gp. 
Let us now apply the theory, with which we started, to the F„ 
result and see what will be the characters manifested by these four 
types of zygotes. The GP will be grey coat with purple spots, the 
Gp will be a grey coat without purple spots, but the gP will be a 
white Pea; in fact, it will be of the same zygotic constitution as 
the white Pea with which the cross was made, for according to the 
hypothesis the hybrid would not have manifested purple spots if 
the white parent had not been carrying the P factor. The gp is of 
course a white-seed-coated individual. The total result is there¬ 
fore 9 individuals with grey coat and purple spot, 3 with grey coat 
only, and 4 with white seed-coats. Now although there is no 
external difference between the two kinds of white which make up 
this 4, viz., the gP and gp, there is a great difference between the 
breeding capacities of the two kinds, inasmuch as the gP will, if 
crossed with grey coat, give rise to a purple spot on a grey coat in 
Fj ; whilst the gp, when crossed with grey, should give simple 
dominance of grey. 1 
We have said that the theory which we are considering is 
1 Of course there are two types of gP, viz. ggPp and ggPP (there 
being two of the former and one of the latter in every three). 
The former when crossed with grey, will give grey and grey 
with-purple-spot in equal numbers. The latter will give all 
greys with purple spots. 
