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AMERICAN BREEDERS' ASSOCIATION*. 



But if mutation is directly from the wild or agouti condition, ABR, 

 by loss of B, leaving AR, then there is produced a red not different 

 from ordinary reds in appearance, but which will give reversion in 

 crosses with black. 



The albino (or wholly white) mutation, which is frequently found 

 in wild as well as in tame rodents, is not due, as might be supposed, 

 to simultaneous loss of the three factors A, B, and R, for albinos can 

 be shown to possess some one, some two, and some all three of these 

 factors. They have, according to Cuenot, lost a certain other factor 

 necessary for the production of pigment of any kind, an activating or 

 ferment-like factor. 



It has been observed that one mutation is often followed by another. 

 DeVries in his " Mutationstheorie " speaks repeatedly of periods of 

 mutation. We can begin to see the significance of this ; given one 

 mutation, we can produce others. Suppose, for example, that we pos- 

 sess agouti and ordinary red varieties only and desire black, we are 

 not compelled to await a mutation to produce it ; we can cross red with 

 agouti and obtain black in the second generation. This is not hypo- 

 thesis merely; its correctness has already been in part demonstrated. 

 Thus, in one experiment, there was employed an agouti of the formula 

 AB-AR (which gave only reds and agoutis in crosses with red), but 

 the agoutis so produced when mated in the same way as the parent 

 gave blacks as well as reds and agoutis, for they were of the formula 

 AB-R. From such animals homozygous blacks (B-B) are readily 

 obtained. 



To produce a red variety from agoutis and blacks alone would not 

 be so easy; it would be necessary either to await a mutation or to 

 work by the slow process of selection from continuous variations in 

 the intensity of blacks under cross-breeding with agoutis. In mice and 

 rabbits as well as in guinea-pigs, red (or yellow) varieties are well- 

 known, but in rats yellow has never been obtained separate from 

 black, though black and agouti varieties are common, both wild and 

 in captivity. 



We now know what the fixation of a heterozygous character im- 

 plies. When A and B are crossed, we obtain C. G is due either simply 

 to co-existence of A with B, or to the co-existence with them of a third 

 factor introduced with one or the other. In either case fixation will 

 consist in getting into the gamete all the factors which produce C. 

 On the first hypothesis, the zygote is A-B (Fig. 2), and the resultant 

 is equivalent to C. Fixation will consist in getting a zygote of the 

 formula AB-AB; every gamete formed will then bear the equivalent 

 of C, viz. AB. On the second hypothesis, the zygote is either AC-B or 

 A-GB, (Fig. 2) ; fixation will consist in obtaining a zygote ACB-ACB; 

 every gamete formed will then contain the three factors A, C, and B. 

 This latter case is illustrated in the production of fixed agoutis from 

 a cross of black with red, as already explained (Fig. 1). 



The first mentioned case is well illustrated in the production of 

 homozygous (i. e., fixed) brindles from crossing of black with red 



