


AS CAUSE OF THE APPARENT CONSTANCY OF THE MUTANT SPREAD”. 337 
But the theory must, however, be rejected from the reason that 
all the normal-winged individuals occurring in the Spread cultures . 
then had to be mutants, which is not believable; and the theory 
must be discredited especially from the reason that — as afore- 
mentioned — a New Spread individual has been shown ina pure 
Old Spread culture, which must necessarily have been brought 
about by crossing-over. The fact that Spread must be supposed 
not to have a lethal effect in a single dose, even when no part- 
ner-chromosome is present, renders the theory unreasonable. Be- 
sides, by using other factors in the IIIrd chromosome the problem 
might easily be examined. 
What I consider as the essential result of this investigation is, 
that through it a case has been shown that a given type (in this 
case Spread) which on a superficial view seemed to be homozy- 
gotic, on a closer examination appeared to be double-heterozygotic. 
A still more intense linkage between the factors Spread and Vital 
would cause that despite thorough experiments one might be quite 
deceived by the facts. The present case, therefore, urge upon us 
the necessity of being prudent when it is a question of deciding 
whether a material before us is homozygotic, for the consequence 
must be that the morphological constancy of the type in practice 
in itself is an insufficient guarantee of the type being homozygo- 
tic; at the same time it must be clear that a suddenly appearing 
new type need not being a mutant, but may be the result of a 
rare crossing-over between two closely linked factors. 
I thank Professor O. Monr, D. Sc. for the amiability with 
which he provided to me with some Drosophila cultures in 1921; I 
also thank Mr. J. CLAUSEN, M. A. who has assisted me in carrying 
through the present investigations. 
RESUME. 
1. By examination of the mutant „Spread” (wings) of Drosophila 
melanogaster it appeared that Spread, located in the IlIrd 
chromosome at 61, is not — asstated in the literature -— reces- 
sive but dominant (to normal wings). 
Genetica IV. 22 
