LIFE CYCLE OF HYDATINA SENTA ils 
genes, representing a certain proportion of male-producers. For 
this explanation could not account for intermediate F, in some 
eases, and an F, higher than both parent lines in other cases. 
If we assume that many genes participate in the production of 
the cycle, many of the results so far obtained are easily explained. 
If we think of these genes for male-producers as being all alike, 
equipotent, and additive in their effects, so that six genes pro- 
duce twice as many male-producers as three genes; we should 
then have to assume, in order to explain the crosses described in 
my former paper, that the percentage of male-producers is pro- 
portional to the number of genes for which the line is heterozygous. 
This explanation seemed plausible when it was thought that vigor 
and the life cycle were correlated; for Mm corn it seems probable that 
vigor is dependent on the degree of heterozygosis. The crux of 
this explanation is found in the results of inbreeding. If the per- 
centage of male-producers is proportional to the number of genes 
for which the line in question is heterozygous, inbreeding, by 
reducing the number of genes for which the line is heterozygous, 
should rapidly reduce the proportion of male-producers. This 
it does not do. Inbreeding results in a line that includes practi- 
cally the same proportion of male-producers as the line from which 
it was derived. Even twice inbreeding, or inbreeding a line itself 
the result of inbreeding, does not certainly show a reduction in the 
percentage of male-producers. 
It can not be assumed, therefore, that the genes for the propor- 
tion of male-producers are all alike and effective in proportion to 
their numbers. Instead, we may assume that the life cycle is 
dependent on a number of genes not all alike, some being more 
effective than others, and some combinations producing more 
male-producers than other combinations, even when these combi- 
nations involve the same number of genes. That something 
akin to segregation of these representatives of the cycle occurs, 
is made probable by the fact that crosses between the same par- 
thenogenetic lines are not equal with respect to the proportion of 
male-producers. That the genes are not alike nor additive in 
their effects is shown by the fact that a cross may result in a higher 
proportion of male-producers than in both parent lines combined. 
