A THIRD SEX-LINKED LETHAL 317 



female is heterozygous for pink every egg after the polar bodies 

 are extruded will contain either the pink factor or its normal 

 allelomorph. The male has two classes of sperm, viz., one with 

 and the other without the pink factor. In all cases except white 

 (which remains white in external appearance in either case) 

 the combination gives two classes in the ratio of 3:1. The 

 relation of pink to not-pink are everywhere normal except in the 

 following case. The single cross-overs between vermilion and 

 miniature are vermilion miniature (14) and vermilion miniature 

 pink (35) . Instead of the expected 3 :1 ratio the relation i.s almost 

 reversed. I can offer no explanation of the relative deficiency in 

 the vermilion miniature class or the relative excess of the vermilion 

 miniature pink. 



We may assume that the double cross-over class included in 

 white miniature is very small and negligible; if so, then the non- 

 cross-over class white miniature should equal the sum of the ver- 

 milion and vermilion pink, which is 662. In fact, it is nearly 

 double (1166) that number. This disturbance, as will now be 

 shown, is due to a lethal factor present in the stock. 



THE LOCATION OF LETHAL III 



In the experiment, made as explained above to test double 

 crossing-over, only about half as many males as females appeared 

 in some of the cultures. This kind of result we have come to 

 associate with a lethal factor, and further tests made with some of 

 the F2 females show, in fact, the presence of a new lethal. Table 

 2 gives individually the ten original mass cultures (p. 318). 



Since the cultures were made in mass they can not be used to 

 study lethal factors, for, the offspring may have arisen from both 

 lethal-bearing and non-lethal-bearing females. The high sex- 

 ratio was not noticed until the experiment was nearly finished, and 

 then it was possible to obtain only a few females. ^ In three cases 



- We have known that non-disjunction occurs in several of our stocks, notably 

 in the white. It occurred in the mother in the experiment just described, for when 

 vermilion pink females were mated to white miniaturp males there were produced: 



Verm.? Red? Verm.d" White cf 



2 92 46 2 



The two vermilion females and the two white males must have arisen through 



