No. 578] DBOSOPHILA AMPELOPHILA 



119 



ampelophila, the number of eggs failing to reach matu- 

 rity is between 25 per cent, and 75 per cent, of the total 

 output; and concluded that this peculiarity probably be- 

 haves as a Mendelian recessive factor. More recently, 

 Morgan ( '14) describes recessive lethal factors in Droso- 

 phila, which he defines, "as any factor that brings about 

 the death of the individual in which it occurs, provided 

 that its effect is not counteracted by the action of its 

 normal allelomorph." 



In the light of this evidence, the following conclusions 

 suggest themselves : 



1. The original pink-eye mutant was heterozygous for 

 some non-sex-linked factor which, in the homozygous state, 

 acts like Morgan's lethal. This factor was, in the course 

 of time, to a large extent eliminated, as is to be expected 

 if the individuals homozygous for it are more likely to 

 die. The chance of such homozygous forms appearing 

 again, has thereby been much reduced. This is borne 

 out by, and also explains, the improvement in the pink 

 race. ' 



2. A similar recessive, though not necessarily the same 

 factor, might also be present in some individuals of the 

 wild, red-eyed stock. Hyde's work mentioned above 

 gives weight to this assumption— which is not at all an 

 unreasonable assumption in a species as unstable as this, 

 judging by the vast number of mutations reported. For 

 this reason, the red sometimes fall behind the expected 

 ratio. 



3. The mode of action of these lethals shows that they 

 are linked to the "pink" factor or to its normal "red" 

 allelomorph. This will be clear from the following 

 analysis : 



Of the flies recorded in Fig. 2, one parent was RP (with 

 gametes R and P) ; the other was PP (with gametes P 

 and P) . The zygotes resulting from these gametes almost 

 invariably give fewer PP's than PP's. In other words, 

 the homozygous forms run behind the heterozygous forms. 

 The relation between these two classes may also be sup- 

 posed to hold in the F 2 cross (Fig. 1). Here, however, 

 the reds (RR and RP) run relatively less often ahead of 



