GENETICS: H. J. MULLER 
621 
from its influence on crossing over, no other effect of this factor has 
been discoverable. The locus of C is within 10 units of that of sooty 
body color, being to the left of the locus of lim, and to the right of the 
locus of kidney eyes. C probably existed in the chromosome of 
beaded stock now containing it before Im i arose in that chromosome 
by mutation. 
4. This remarkable genetic situation, wherein both types of homo- 
zygotes are prevented from appearing by the action of lethal factors 
lying in opposite chromosomes, may be termed a condition of 'bal- 
anced lethal factors.' Surprising as it may seem, it appears that 
such a condition is no mere 'freak of nature,' and that it is apt to arise 
wherever dominant mutant factors exist which either have some nat- 
ural survival value, or have, like beaded, been artificially selected for. 
This conclusion is based upon the following experimental results and 
theoretical considerations. 
(a) In an attack upon this question, an investigation was undertaken 
by the writer to determine how generally dominant mutant factors of 
Drosophila are lethal when homozygous. There were nine dominant 
mutants known (excluding the intensifiers of beaded and truncate), 
and they were distributed equally among the three large chromosomes. 
The viability of the three sex-linked dominants was, of course, already 
known; the remaining six dominants were tested. In all, it was found 
that three of the nine are not lethal, one (in chromosome III) is semi- 
lethal, and five (one in chromosome I, two in II and two in III) are com- 
pletely lethal when homozygous. Thus it is a phenomenon of common 
occurrence in Drosophila for dominant mutant factors to be lethal when 
homozygous. 
(b) It would be very far fetched to assume that the natures of the 
characters produced by dominant factors differ as a class from those 
produced by recessives. We must therefore believe that lethals are 
very frequent among recessive factors also. It should be noted, how- 
ever, that in the case of recessive factors a lethal action prevents or 
greatly hinders their discovery, whereas with factors dominant in re- 
spect to some visible character, a lethal effect, when homozygous, does 
not interfere with their being detected; for this reason it is quite in 
accord with expectation that a much smaller proportion of lethals has 
actually been found among the recessives than among the dominant 
mutants. The evidence for the frequency of origin of recessive lethals 
is not entirely by analogy, however, for in the case of sex-linked factors 
the discovery of lethals is easier, owing to their effect upon the sex ratio, 
and here a considerable number of lethals has in fact been found, by 
