3G 



AMERICAN" BREEDERS' ASSOCIATION. 



results from a cross. This seems to be true in mammals as regards 

 general size, dimensions of the skull and long bones, of the tail, and 

 of the ears. A variation produced as a blend needs no fixation. It 

 breeds true from the start, as I pointed out at our meeting two years 

 ago in the case of ear-length of rabbits. In the case of blending char- 

 acters no less than in that of alternative ones, the principle of unit 

 characters holds good. Hair color, hair length, and coat pattern, are 

 in rabbits independent unit characters, which can be combined in any 

 desired way in conformity with the laws of alternative heredity. Ear 

 length is another unit, distinct from all three hair characters and 

 capable of combination in any desired way with them, but harder to 

 manipulate simultaneously because of its blending nature. I pointed 

 out at our meeting two years ago a method of fixing simultaneously 

 characters which blend and these which do not. 



Perhaps the most important extension which has been made of 

 Mendel's law consists in the demonstration (chiefly by Tschermak, 

 Cuenot, and Bateson) that certain characters are produced only when 

 two or more separately heritable factors are present together. Such a 

 character does not conform with the simple Mendelian laws of inheri- 

 tance, but its factors do. Herein lies the key to the explanation of so- 

 called heterozygous characters and to the practical process of their fixa- 

 tion. This same principle serves to explain also atavism or reversion, 

 and the process by which reversionary characters may be fixed. 



When purebred black guinea-pigs are mated with red ones, only 

 black offspring are as a rule obtained. The hairs of the offspring do 

 indeed contain some red pigment, but the black pigment is so much 

 darker, that it largely obscures the red. In other words black behaves 

 as an ordinary Mendelian dominant. In the next generation black and 

 red segregate in ordinary Mendelian fashion, and the young produced 

 are in the usual proportions — three black to one red. All black races 

 behave alike in crosses with the same red individual, but among red 

 animals individual differences exist. Some, instead of behaving like 

 Mendelian recessives, produce in crosses with a black race a third ap- 

 parently new condition, but in reality a very old one, the agouti type 

 of coat found in all wild guinea-pigs, as well as in wild rats, mice, 

 squirrels and other rodents. In this type of coat, red pigment alone 

 is found in a conspicuous band near the tip of each hair, while the 

 rest of the hair bears black pigment. The result is a brownish or 

 grayish ticked or grizzled coat, inconspicuous and hence protective in 

 many natural situations. Some red individuals produce the reversion 

 in half of their young by black mates, some in all, and others, as we 

 have seen, in none, this last condition being the commonest of the 

 three. It is evident that the reversion is due to the introduction of a 

 third factor, additional to simple red and simple black. It is evident 

 further that this new third factor, which we will call A (agouti), has 

 been introduced through the red parent, and that as regards this fac- 

 tor, A, some individuals are homozygous (A-A) in character, others 

 are heterozygous (transmit it in half their gametes only), while others 

 lack it altogether. Further observations show that it is independent 



