146 GENETICS IN RELATION TO AGRICULTURE 
firmly and shrinks uniformly to form a round seed. Like the 
seeds of round races those of wrinkled peas are also round at the 
height of development, but in peas of such varieties the sugar is 
very incompletely transformed into starch. Consequently in ripening 
and drying they give up more water proportionally than round 
races and do not shrink uniformly. As a result they become 
very much wrinkled at maturity. This difference in the starch grains 
of the wrinkled pea is not only a matter of less complete trans- 
formation of sugar into starch, but is also associated with less perfect 
production of starch grains as shown in Fig. 66. Thus in the round races 
the starch grains are numerous and are large and entire. They show 
practically no subdivision of the grains. But in the wrinkled peas the 
grains are not only less numerous, but they show fissures which give them 
an appearance like that of the compound starch grains of some species 
of plants. This appearance is probably due to the fact that actual 
breaking down of starch grains occurs in wrinkled peas during ripening 
so that the grains remaining are in a partial stage of disintegration. In 
the hybrid between a round and a wrinkled pea, however, the condition 
of the starch grains is intermediate between that of the two parents. 
The grains are intermediate not only in number and shape but also in 
the degree of disintegration they display. In the contrasted pair of 
characters, round vs. wrinkled seed in peas, the dominance of round is, 
therefore, merely a superficial character expression. Actually the basic 
phenomena involved, 1.e., the transformation of sugar into starch, show 
an intermediate condition in the hybrid. The superficial character ex- 
pression of this intermediate condition happens to be the same as that 
of the strict parental round condition, so that dominance here is merely 
dependent on superficial resemblance. We may well hesitate, therefore, 
in our judgment as to the completeness of dominance in any case until 
it has been examined with considerable care. 
Sometimes the application of more precise character measurements 
will suffice to detect a difference between the homozygous and hetero- 
zygous character expression. This is shown for the case of miniature 
vs. long wings in Drosophila. In miniature-winged flies the wings reach 
about to the tip of the abdomen, whereas in the long-winged flies they 
extend considerably beyond the abdomen. The long-winged condition 
is dominant, to the eye completely, and there is absolutely no difficulty 
in segregating the long-winged flies of an F, population from those which 
have miniature wings. Nevertheless Lutz has shown that when biomet~ 
rical methods are employed the length of wings of heterozygous flies com- 
pared with the length of legs is shorter than that for flies homozygous for 
the long-winged factor. The difference in character expression in this 
case is slight but it can be demonstrated by the employment of precise 
methods of measurement. 
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