REVERSION IN GUINEA-PIGS AND ITS EXPLANATION. 



units). Their gametes are il 5, ^, 5, 0. If an animal producing such gametes 

 is mated with a red animal lacking both units, the zygotes formed wojld be, 

 as regards these units, AB, A, B, and 0, or agouti, red, black and red respec- 

 tively, or collectively 2 reds : 1 black : 1 agouti, as obtained (table 2). Further, 

 such agoutis as came from this cross would be identical in character with the 

 i^i agoutis. This expectation is confirmed by table 2, second division. 



But if the Fi agoutis are mated inter se, then we expect to get zygotes 

 corresponding with the product of two sets of gametes, each AB +A+B+0, 

 that is A2B2* +2A2B +2AB2 +4:AB +2A +2B +A2+B2+O. In appearance 

 these zygotes would fall into three classes, agouti, black, and red, as shown in 

 the accompanying table. 



The agoutis in this (F2) generation, it will be noticed, should be of four 

 different types, as actually observed. The AB group should produce all three 

 sorts of young, agouti, black, and red, being 

 heterozygous in both characters, A and B. 

 The group designated by A2B would be pure 

 for A but heterozygous for B. All their young 

 would be potentially agoutis, but since part 

 of them would lack black pigment, such would 

 be red. Hence they would produce only the 

 two sorts of young, agouti and red ; see table 3. 

 The group designated by AB2 would transmit 

 black pigment to all their young, but only part 

 of these would receive A . Hence they would 

 produce agouti young and black ones, but 

 no reds; compare table 4. 



The group A2B2 would transmit both black and agouti to all their young; 

 hence they would produce nothing but agouti young, however mated; com- 

 pare table 5. The expected proportions of these four groups of agouti animals 

 are obviously 4:2:2:1. The numbers shown in the tables are 9 : 10 : 16 : 5. 

 The apparent deficiency of individuals in Group I and excess in Groups II and 

 III is readily accounted for. Not all the agoutis recorded as derived from an 

 agouti X agouti cross were seco?id-generation agoutis; many of them were third- 

 generation agoutis, having been obtained from two successive matings of 

 agouti with agouti. In that case one or both of their parents may have been 

 of Groups II or III, in which case the expectation for young of those groups is 

 increased, while that for Group I is correspondingly diminished. Therefore 

 both the kinds of agoutis obtained in the experiment and their numerical pro- 

 portions are in harmony with the hypothesis presented in this paper. 



The subscript 2 is used (instead of the algebraic exponent 2) to indicate double rep- 

 resentation of a factor, i. e., a homozygous condition as regards it. (See Castle, 1909.) 



