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R. Ruggles Gates. 
way. They are nearly all classed as Mendelian dominants or 
recessives in inheritance, though in a few dominance is variable and 
subject to environmental influence. Morgan (1919), states that 
only 12 are dominant out of over 150. These dominants have each 
appeared in a single individual. Mutants from recessive genes, on 
the other hand, usually came to light in about 25% of the offspring 
of a pair, showing that the pair were both heterozygous for the new 
factor, which must have arisen at least one generation earlier. 
Whether both members of a pair of chromosomes undergo the 
change simultaneously is unknown, since if it occurred in the egg 
one chromosome will be extruded into the polar body, and if in the 
spermatocyte the chances are small that more than one sperm 
from the same spermatocyte will function. Still, as far as it goes, 
the 12 single mutant individuals suggest that perhaps a single 
sperm and hence a single chromosome of»a pair has the new factor. 
In the case of CEnothera lata and semilata , which arise through an 
irregular heterotype division, evidence is available, for these 
mutants sometimes occur in pairs, suggesting that the change 
occurred in a pollen mother cell and that the two resulting 
8-chromosome pollen grains both functioned. 
Drosophila Mutations. 
The evidence for mutations in animals was rather scanty until 
Morgan took up the study of the fruit fly, Drosophila melanogaster, 
about 1910. The rapid breeding and easy handling of these flies 
in large numbers, makes it an ideal form for genetic experiments, 
and combined with this is the advantage that it has only four 
pairs of chromosomes. It is not, therefore, surprising that in the 
last decade, Morgan and his pupils have accumulated a mass of 
breeding data, closely analyzed and correlated, which is unequalled 
in any other organism. With plants growing only one seed- 
generation a year, it would probably require 150 years to produce 
an equal number of generations. 
The Drosophila work has therefore given us a look into the 
constitution of the germ plasm such as no annual-breeding plant 
or animal could furnish in a lifetime. The Mendelian behaviour, 
sex-linked inheritance, and other features are similiar to those 
found in many other organisms, so there can be no doubt of the 
wide applicability of the conceptions of mutations and the germ 
plasm derived from these experiments. In many respects they are 
in accord with those derived from the (Enothera work. Although 
the simple Mendelian mutations in the latter are comparatively 
