No. 578] MULTIPLE ALLELOMORPHS 



91 



tan was associated with each one of these Mendelizing 

 characters, as well as others in which it was not associated 

 with them. Finally Haecker (1912) has shown that black- 

 and-tan, like the ordinary form of agouti, is independent 

 of albinism, since when black-and-tans are crossed with 

 Himalayan albinos, not only these two varieties are ob- 

 tained in the F 2 generation, but also blacks. The propor- 

 tions in which these three varieties were obtained by 

 Haecker approximate the modified dihybrid ratio, 9 black- 

 and-tan: 3 black: 4 Himalayan. One of the two Men- 

 delian pairs concerned is color vs. albinism ; the other and 

 independent one, black-and-tan vs. black. 



It is known that if a gray rabbit is used, instead of a 

 black-and-tan one, in a cross with Himalayan albinos, the 

 same 9:3:4 ratio is obtained in F 2 , of grays, blacks, and 

 albinos, respectively. The observed results differ, in the 

 two cases, only in the substitution of gray for black-and- 

 tan, which is further evidence that it is only another form 

 of the same genetic factor. 



Notwithstanding all this consistent and converging evi- 

 dence, it is possible that the modified form of agouti seen 

 in black-and-tan is not due to a changed agouti factor 

 itself, but to the modifying action of a factor associated 

 with it which partially inhibits its action. Here we must 

 consider two subordinate possibilities: (a) that the sup- 

 posed modifier is wholly independent of the agouti factor, 

 and (b) that it is coupled with the agouti factor. The 

 first possibility is readily disproved ; the second one is not 

 so easily disposed of. 



(a) If black-and-tan were due to the action of an inde- 

 pendent modifying factor associated with agouti, a cross 

 of black-and-tan with ordinary black should permit the 

 separation of agouti from its supposed modifier in a con- 

 siderable part of the F t gametes and F 2 zygotes, so that 

 we should expect F 2 to contain gray animals as well as 

 blacks and black-and-tans. But experiments started sev- 

 eral years ago at the Bussey Institution show that when 

 black is crossed with black-and-tan no gray offspring are 

 obtained either in F x or in F 2 , but only black-and-tans in 



