MENDEL'S SECOND LAW 67 



those of the wild fly. The miniature fly carries the domi- 

 nant {DD) wild-type allelomorph of the dumpy gene, and 

 the dumpy carries the dominant (MM) wild-type allelo- 

 morph of the miniature gene. Since the hybrid contains 

 the two wild-type genes (DM) it "reverts" to the long- 

 winged fly. The proof that two pairs of factors are in- 

 volved is found by inbreeding an F-^ male and female, 

 which give 9 long, 3 miniature, 3 dumpy, and 1 miniature 

 dumpy (Fig. 30, c) fly. 



There are certain modifications of the two-pair ratio 

 that arise sometimes when different factors produce a like 

 effect on the same organ. Such cases have sometimes been 

 treated as special cases, and rather peculiar interpreta- 

 tions given to them on the basis that the situation is 

 unusual. In reality they are only interesting cases of 

 Mendelian behavior, the results obscured to some degree 

 by superficial character relations. The absence of 

 color, albinism, is, perhaps, the most familiar example 

 of this sort. There are certain recessive factors 

 that when homozygous interfere in some unknown way 

 with the development of color. Albinos of the ordinary 

 house mouse ai^e white because they are homozygous for 

 the albino factor, although they may be pure for all other 

 factors that are essential for color. If a certain kind of 

 albino mouse is crossed to a pure black mouse the off- 

 spring will be gray because black (bb), being recessive to 

 its wild- type allelomorph (BB), brought in by the albino, 

 disappears; and white (ww) being recessive to its wild 

 type allelomorph for color (WW), brought in by the 

 black, also disappears, so that the color of the resulting 

 animal, gray, is due to the hybrid having recovered all 

 the factors that give this color. The two factor-pairs 

 involved are black (b) and its normal allelomorph {B = 

 gray), and white {w) and its normal allelomorph 

 {W=Go\or). The F2 results, put into the recombination 

 square, are as follows : 



