110 Mr. G. P. Mudge. On the Hereditary [May 8, 
The actual offspring (experiment 66a) consisted of :— 
1 Cr 4+ Cri+2 Cr 2+1 Cr 541 Cr 7. 
Thus, two types of black individuals have appeared, z.c., Cr 5 and Cr 7. 
The predicted “ Irish” form, together with the grey Cr 3 type, is absent. But 
if we bear in mind that, at the lowest limit, eight individuals at least must be 
produced in order to represent all types expected, and that actually only a 
total of six individuals were produced, it is obvious that two types at least. 
must be absent. Unfortunately, though I kept these two rats mated for 
a year and a-half after this first litter, they never gave rise to a second one, 
and are now dead. 
If the hypothesis above made be correct, then the Cr 7 member of the: 
offspring should be homozygote. It was tested by mating with three albinoes. _ 
(experiments 43, 44, 45) in succession, the last of which carried the grey 
determiner. With the first two albinoes, it had a single litter in each case, of 
eight individuals, making a total of 16, all of the “Irish” 6 type. No albinoes 
appeared. The “ Irish” 0 individuals of this offspring were crossed inter se 
(experiments 51, 52, 53), and gave a progeny wholly confirmative of the 
homozygote nature of the Cr 7. When crossed (experiment 45) with the 
third albino (carrying the grey determiner), it gave a wholly pigmented offspring, 
five in number (infra, p. 114). 
Thus, in a total offspring of 21, from three albinoes mated with it, not a 
single albino appeared. There can, then, be no doubt as to the homozygote 
nature of the Cr 7. 
Testing the assumption further by following some other of the offspring of 
the F 1 generation of this “Irish” 6 x Cr 2, to the F 3 generation, we may 
consider the behaviour of two of the F2 Cr2, which I mated with albinoes 
(Pedigree Chart I, experiments 73 and 74). 
If the assumption be correct that the Cr 2 parent (experiment 66a) 
carried B and P recessive, then the Cr 2 members of its offspring should be 
of several zygotic compositions, 7.¢., CecGgBSP, CGegBSsP, and CcGgBSsP. 
If the F 2 Cr 2 of CGgBSsP composition be mated with an albino of 
reputed cgBsP composition, two particular features are expected in the F 3 
generation. There should be no albinoes, and black forms should appear. 
The expected offspring is :— 
Cr 2 4A Cri 3-4 Crepe emer ono: 
In experiment 73, an F 2 Cr 2 was mated with an albino derived from 
albino grandparents obtained from an ordinary dealer, and presumably, there- 
fore, of cgBsP composition. The actual result is :— 
2 Cr 2+-4 Cr 3+5 Cr 5. 
