338 WINGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS _ 
2. Spread acts lethally in double dose. 
3. The wild type possesses a dominant indispensable factor 
Vital (Vit) that is closely linked to spread. Its allelomorph, 
vit, also has a lethal effect in double dose. 
. Spr Vit tre 
4. The formula of the Spread mutant is ———-; thus it is 
spr vit 
double heterozygotic. It is only the strong linkage between 
Spr and Vzt that causes that the Spread type seems to be 
constant. 
5. The distance between Spread and Vital is scarcely 1.0. 
6. The Spread mutant is an instance of a double heterozygote, 
which on account of the interaction of two lethal factors (Spr 
and vif) reminds of and has been construed as a constant 
homozygotic type. 
Copenhagen, June 7, 1922. 

