312 F. B. SUMNER AND H. H. COLLINS 



difference proved to be, hereditary. Since the hybrid offspring 

 of these two strains were all 'yellows,' and displayed, on the 

 whole, an intermediate tint, we probably have to do either with 

 a case of 'multiple allelomorphs' or a case in which one or more 

 'modifying factors' condition the difference. 



5. In none of these cases is the evidence sufficient to show 

 whether the actual mutation, or modification of a genetic factor, 

 occurred in our own cultures, or whether the mutant factor had 

 been present for many generations in a simplex condition. 



6. The number of 'mutants' originally produced was consid- 

 erably higher than would be expected, on the assumption that 

 the parents were both heterozygous. This excess was found in 

 five out of six of the independent outcroppings of yellow. 

 It was also found in the pallid strain, but not in the very small 

 number of original albinos. Combining all the offspring of these 

 original heterozygous parents, we have 87 individuals, of which 

 57 were of the wild-type and 30 were mutants (recessives) , giv- 

 ing a ratio of 1.9:1, instead of 3.:1. The departure from the 

 normal is not, however, of very probable significance.-" Fur- 

 thermore, there is another possible interpretation of this excess 

 of recessives. We are necessarily dealing only with the offspring 

 of parents known to have produced some recessives. It may 

 well be that there have been other pairs of heterozygous parents 

 in the stock, which have not been recognized as such owing to 

 their having produced only normal offspring. Inclusion of these 

 last would increase the ratio of dominants to recessives. 



7. All of these mutations, like the vast majority of those 

 described by previous writers, plainly involve the loss of some- 

 thing normally present. In the case of the albinos, all pigment 

 has been lost, both from the hair, the skin, and the retina. The 

 pallid mice have lost most of their dark pigment, and probably 

 some of their yellow, and here also the loss has been general, 

 affecting all pigmented parts of the body. In the yellows there 

 has been an almost complete suppression of the all-black (un- 

 handed) hairs and a shortening of the basal, deeply pigmented 

 zones of the others. In the 'agouti' hairs of these mice on the 



2*' The probability is only about nine out of ten. 



