THE PRODUCTION AND FIXATION OF NEW BREEDS. 



37 



in its inheritance of both black and red; it is in fact an independent 

 Mendclian character, which can become visible only in the presence of 

 both black and red, because it is a mosaic of those two pigments. 



The chart (Fig. 1) is intended to show how the independent fac- 

 tors behave in heredity. The black parent contributes the factor B; 

 the red parent, R and A, so that the zygote, or new individual, con- 

 tains the three factors necessary to the production of agouti. When 

 the new individual forms gametes (sex-cells), these will be of four 

 different kinds, for A is independent of B and of B and may pass out 

 with either one in the reduction division which separates B from R. 



Black Red Black 

 Red 



4 2 2 1 



Fig. 1. — Diagram showing how the three independent characters, B (black), R 

 (red), and A (agouti), are transmitted in the production and fixation of an agouti 

 race. A circle represents a reproductive cell (gamete) ; gametes united in pairs 

 form zygotes (new individuals). The tirst horizontal row shows the kinds of gam- 

 etes which unite to produce the reversion (second row). In the third row, are 

 shown the four kinds of gametes formed by the reversionary individual ; and. in the 

 fourth row. the four kinds of agouti individuals produced by different combination's 

 of those gametes. Below are indicated their expected numerical proportions and 

 the kinds of young which they will respectively produce in crosses with red indi- 

 viduals (R). 



That division accordingly may occur either so as to form gametes B 

 and RA respectively, or what is equally probable so as to produce 

 gametes BA and R. Observation confirms this interpretation, for it 

 is found that the reversionary agoutis do not breed true, but produce 

 young of the three sorts — agouti, black, and red, as expected. We 

 expect black individuals from unions of B with B, or of B with R; we 

 expect red individuals from unions of R with R or with RA, and from 

 unions of RA with RA; we expect agoutis to be produced by any 

 gametic union which brings together the three factors B, h. and 

 A. There are 4 chances in 16 for the occurrence of such a union, when 

 the reversionary agoutis are bred together. In fact, however, agoutis 

 are produced much oftener. Approximately 9 out of 16 of the young 



