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, i.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 z 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. 



