No. 594] 



COLOR MUTATIONS IN MICK 



339 



It will be noticed that the approximation to the expect- 

 ancy on the basis of three pairs of Mendelian factors 

 is extremely close. The occurrence of the two "white- 

 bellied" agouti young is the matter of especial interest. 

 These two young, one a male the other a female, are ap- 

 parently identical with the gris a ventre blanc of Cuenot, 

 judging from his description. 



The question may quite naturally arise as to the possi- 

 bility that the white-bellied agouti animals in F 2 had arisen 

 by an accidental cross with some white-bellied agouti 

 male in an adjoining cage. This possibility, however, is 

 obviated by the fact that the F 2 generation white-bellied 

 agoutis appeared in different cages at the Harvard Med- 

 ical School where there had been no other ivhite-bellied 

 agoutis for more than a year before the appearance of the 

 mutant animals. The two F 2 white-bellied agoutis must 

 then be considered as true mutants epistatic to the type 

 from which they originated. 



Further evidence is provided by the F 3 generation, a 

 tabulation of which is given below. 



It will be observed that there is one "white-bellied" 

 agouti among the 624 animals comprising the F 3 genera- 

 tion. This one mutant occurred as a descendant of a 

 family of F 2 animals shown in Table II, mating J. This 

 mating in F 2 gave 9 black agouti and 3 brown agouti 

 young, none of which were "white-bellied" and the mu- 

 tation therefore originated in a gamete formed by one of 

 the F 2 generation animals while the other two white-bel- 

 lied agoutis originated as mutations in the gametes of Fj 

 animals. 



This recurrence of the mutation is a matter of consider- 

 able interest and indicates a germinal change, the occur- 

 rence of which is largely irregular, having some under- 

 lying cause or causes which become operative in animals 

 of this particular hybrid race. 



That hybridization between the same wild agouti and 

 dilute brown races does not always produce this or any 

 other recognizable mutation from the eight expected color 



