Nos. 620-621] LXHERITAXCE IX PEROMYSCTS 



443 



merely to a change in the proportion of element^ pre- 

 viously present, the new type lias arisen abruptly and has 

 diverged so widely that its range of variation does not 

 overlap that of the normal race. Among the many hun- 

 dreds of individuals which I have dealt with, I have never 

 found any mice which would serve in a true sense to 

 bridge the gap between these two types. Nor have any 

 other yellows appeared in my cultures, except among the 

 descendants of the single pair in question. 21 * 



As stated in an earlier paper (1917«), I trapped several 

 years ago a mouse which I feel fairly certain was a juve- 

 nile yellow gambeli. It is possible that this character, in 

 a heterozygous condition, may be of not uncommon occur- 

 rence among the mice of this vicinity. Thus, the muta- 

 tion through which my stock came into existence may 

 have taken place among the wild ancestors, many gen- 

 erations earlier. On the other hand, the same genetic 

 instability which led to such a factorial loss or modifica- 

 tion in one case may be responsible for its occurrence on 

 many independent occasions. I have no data by which 

 to decide between these two alternatives. 



As regards the genetic behavior of these yellow mice, I 

 have fairly satisfactory evidence that they are simple 

 Mendelian recessives. As was stated above, 7 yellows 

 and 14 normal animals constituted the fraternities in 

 which they first appeared. The departure from Men- 

 delian expectation may well have been accidental here, 

 though a differential mortality may possibly have been 

 responsible. The first yellows, bred to their (presumably 

 heterozygous) parents, have given 5 dark and 5 yellow 

 offspring. Bred to homozygous dark animals, they have 

 thus far produced only a single brood, consisting of three 

 dark individuals. Yellows bred to yellows have produced 

 young of the aberrant type only (thus far 10). These fre- 

 quently do not attain the full yellow color until they as- 



20 Mr. H. H. Collins has, however, found a number of sports of this 

 general appearance among the offspring of a single pair of normally 

 colored individuals which were trapped at La Jolla. Mr. Collins' mice 



