622 
GENETICS: H. J. MULLER 
various observers. Following up, now, our original inference regarding 
the high frequency of lethals among recessive mutants, it should further 
be pointed out that since recessive mutants as a class are much more 
numerous than dominant mutants, recessive lethals also should arise 
much oftener than dominant ones. 
(c) It now remains to join these two results together in one conclusion. 
Suppose that a race already contains a dominant mutant factor which 
is favored by selection. Firstly, as shown in (a), this is likely to be 
lethal when homozygous. Then, as shown in (b), it is likely that a 
recessive lethal will some time, in some line of individuals, arise by 
mutation in the opposite chromosome. Since the first factor is being 
selected for, and the presence of this second lethal will cause the pro- 
duction of a smaller proportion of individuals not showing the desired 
dominant character, the line containing the second lethal in addition 
will tend to be selected. Thus a condition of balanced lethal factors 
will automatically become established, just as it did in the case of beaded 
wings. Factors which prevent crossing over, or balancing lethals which 
cross over less frequently with the desired dominant, will of course also 
be favored by selection, provided they occur, for the less crossing over 
there exists between the two balanced factors, the more perfect is the 
balance and the greater is the proportion of individuals showing the 
advantageous character. The frequency of factors like C is however 
unknown, although they have been discovered in nearly a dozen stocks 
of Drosophila. In the case of the beaded stock, the presence of C seems 
to be just a happy coincidence, as it probably existed there before 
lin 1 aiose. But, however that may be, it is evident that the present 
case is but a special instance of a general class of cases of balanced lethal 
factors that will probably confront the geneticist in increasing numbers.^ 
5. What will be the distinguishing characteristics of races in this 
condition? (a) In the first place, crosses of these varieties to other 
races will result in the production of hybrids of two types, according to 
which of the chromosomes of the balanced pair they receive. Thus, 
flies of beaded stock crossed to normal give 50% beaded and 50% 
normal in Fi. In cases where one of the lethal containing chromosomes 
is not dominant to the chromosomes of the foreign race in any 'visible' 
factor, one of these hybrids (in our case the normal) will appear to 
breed true, while the other will show segregation in subsequent genera- 
tions. By introducing other mutant factors besides into the balanced 
chromosomes of beaded stock, these results were made more striking 
and made to apply to a number of different characters at once. 
(b) Still more unusual results can' be and were obtained by crossing a 
