. The physical basis of heredity . Dry Wet Fia. 11.—Relation of normal to abnormal abdomen as shown by classes of Fi flies. Drysignifies conditions that make for normal; wet for abnormal. In this case both the heterozygous and the parentalabnormal type may show normal abdomen like the MENDELS FIRST LAW 33 wild type. The abnormal type is treated as the dominantalthough only when the conditions are favorable to itsappearance is the hereditary phenomenon seen. Inanother case (duplicate legs) only the homozygous formmay show the duplications (in a special environment).The following scheme (Fig. 12
. The physical basis of heredity . Dry Wet Fia. 11.—Relation of normal to abnormal abdomen as shown by classes of Fi flies. Drysignifies conditions that make for normal; wet for abnormal. In this case both the heterozygous and the parentalabnormal type may show normal abdomen like the MENDELS FIRST LAW 33 wild type. The abnormal type is treated as the dominantalthough only when the conditions are favorable to itsappearance is the hereditary phenomenon seen. Inanother case (duplicate legs) only the homozygous formmay show the duplications (in a special environment).The following scheme (Fig. 12) represents this relation,reduplication of legs being treated as a recessive:. Fig. 12.—Relation of normal to duplicate legs. There are still other relations that affect the dominanceof characters. For example, there may be internal fac-tors, which when present, determine that a character shallbe dominant over its allelomorph, or recessive to it. Inthis connection might be mentioned what has been calledreversal of dominance. An example from Davenportwill illustrate what is meant. In a certain strain of fowlsthere is a tendency for the toes to be united by a web atthe base. Crossed to birds with normal feet, no birdswith united toes (syndactyls) appeared in F^. The F^birds inbred gave in Fo only about 10 per cent, of syndactylbirds. It would appear that the latter character is reces-sive, and that the recessive type overlaps largely thedominant heterozygous type. Davenport interpreted, however, the syndactyl as thedominant type, because two syndactyls may give nor-mals, but no true normals give syndactyls. In otherwords, he defines the dominant type as the one
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