REVERSION OR ATAVISM 115 



duction of a new factor, additional to simple red or simple 

 black. It is evident further that this new factor, which we 

 will call A (agouti), has been introduced through the red 

 parent, and that as regards this factor, A, some red indi- 

 viduals are homozygous (AA) in character, others are hetero- 

 zygous (Aa) , while others lack it altogether (aa) . The agouti 

 character becomes visible only in the presence of both black 

 and red, because it is a mosaic of those two pigments. If the 

 Fi agouti individuals are bred together they produce in the 

 next generation (F2) three sorts of young, viz., agouti, 

 black, and red, which are numerically as 9:3:4. This evi- 

 dently is a modification of the dihybrid Mendelian ratio 

 9:3:3:1, resulting from the fact that the last two classes are 

 superficially alike. They are red animals with and without 

 the agouti factor respectively; but this agouti factor is in- 

 visible in the absence of black, so that both sorts of reds look 

 alike. Together they number four in sixteen of the F2 off- 

 spring. Figure 54 is intended to show by the checkerboard 

 method how this modified dihybrid ratio is obtained. 



Black and red varieties differ from each other by a varia- 

 tion in what has been called the extension factor (E), the 

 reference being to the fact that black (or brown) pigment, 

 found in the eyes of both varieties, extends throughout the 

 coat in the black variety but is restricted to the eye in the 

 red variety. The allelomorphic conditions of this factor are 

 designated E (in black) and e (in red) respectively. The 

 agouti factor (A) may exist in red animals without producing 

 visible effects because there is no black pigment in the fur 

 of such animals to bring out the ticking, but its existence in 

 animals which would otherwise be black changes the coat to 

 agouti. Hence the constitution of the parental gametes is: 

 Black parent, Ea; red parent eA. Fi is EeAa, a double 

 heterozygote. Its gametes are EA -f Ea -|- eA + ea, which 

 with dominance complete will produce F2 zygotes, 9 EA + 

 3 Ea -|- 3 eA + 1 ea. (See Figure 54.) But EA contains the 

 two factors which together produce agouti; Ea contains the 

 factors for black; eA contains the factor for agouti but with- 



