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AMERICAN BREEDERS' ASSOCIATION. 



have been found to be agoutis. The unexpected excess of agoutis in 

 our experiments was fully explained when these second generation 

 agoutis were tested individually. It was then found that they are of 

 three sorts as regards breeding capacity. The first sort produces the 

 three kinds of young, agouti, red, and black ; the second sort produces 

 only agouti young and red young; and the third sort produces only 

 agouti young and black young. A fourth sort, known to exist but not 

 yet obtained in these experiments, will produce only agouti young, i. e., 

 will represent the fully fixed agouti type. 



In the chart (Fig. 1) I have indicated certain gametic unions which 

 would lead to the production of these four classes of agoutis. The 

 probable frequencies of their occurrence on the basis of chance are 

 4:2:2:1. Only a few animals of this generation have yet been fully 

 tested, but it is clear that the class indicated at the left in the chart 

 is as expected the largest, and the class indicated at the right (not 

 yet obtained) will be the smallest. 



Fig. 2. — Diagram Illustrating the nature of heterozygous characters and the mode 

 of their fixation. For explanations, see text. 



These experiments make it clear that B as an independent gametic 

 factor is not necessary to the production of the agouti character, but 

 that any gametic union which includes both B and A will produce an 

 agouti individual. Yet a microscopic examination of the agouti hair 

 shows that red pigment is present in a distinct band near the hair- 

 tip. As a matter of fact all black individuals, even when they breed 

 true, probably form some red pigment along with the black, but its 

 presence is overlooked when the more opaque black is distributed 

 throughout the whole length of the hair. When, however, black is ex- 

 cluded from the hair-tip, the red then becomes visible as the agouti 

 marking, elsewhere the hair appears black. Eed, then, we may as- 

 sume, is always present with black in sufficient quantities to produce 

 the agouti marking if the factor A is present (absence of black from 

 the hair-tip). This explains why blacks never give the reversion in 

 any sort of cross, but it is always brought about through the agency 

 of the red parent. If a black individual contained the factor A, it 

 would not be a black individual, but an agouti one. 



