HEREDITARY TYPE OF CHARACTER STUDIED. 45 



gray individuals, represents an extreme condition. Numerous cases, 

 both in animal breeding and in experimental heredity, present a 

 striking difference in the character when the dominant genes are 

 homozygous and when they are heterozygous. 



In this connection the following statements might be quoted : 



" The failure of many characters to show complete dominance raises doubt 

 as to whether there is such a condition as complete dominance." (Morgan, 

 Sturtevant, MuUer, and Bridges, 1915.) 



Baur in his book (1914) emphasizes the same point: 



"Man hat dieser Dominanzerscheinung, die durchaus keine allgemeine 

 Kegel ist, vielfach iibertrieben, grosse Bedeutung zugeschrieben und von 

 einer ' Dominanzregel ' gesprochen. Das ist ganz verkehrt. Eine irgendwie 

 allgemein giiltige Dominanzregel giebt es nicht und sehr haiifig ist eine 

 Dominanz nur scheinbar." 



The same opinion is expressed by East (1916) : 



" But as a matter of fact, absolute dominance is rare. A heterozygous gene 

 very seldom produces an effect identical with that of homozygous genes." 



Many examples might be used to illustrate this relation. One of us 

 (Wriedt) studied a case of ''short ears" in sheep which behaves as a 

 dominant character (Wriedt, 1914, 1916, 1919 ; Ritzman, 1916) . Recently 

 we have been able to see the offspring of two short-eared sheep. In 

 some of these the external ears are entirely absent. These are probably 

 the homozygotes. 



One of the most striking cases of this kind is found in the mutant 

 character Star in Drosophila melanogasier (s. ampelophila), found by 

 Dr. Bridges. Star has been very much used in the experimental 

 analysis in Dr. Morgan's laboratory in the past few years and an 

 account of the character is now in press. Dr. Bridges has kindly per- 

 mitted us to refer to the case, which in many respects furnishes us with 

 very interesting parallels to the character we are here deaUng with. 

 Star is a dominant eye character. When heterozygous the gene for 

 Star causes a slight irregularity of the ommatidia of the eyes, giving 

 the eye surface a shinmiering aspect. This trifling malformation is of 

 no practical importance and does not affect the viabiUty of the indi- 

 viduals at all. When homozygous, however, this gene is lethal. Flies 

 homozygous for Star die without exception. 



The mutant character Star, here chosen as an example, is not unique 

 in this respect. In fact, 5 of the known 9 dominant mutants in Dro- 

 sophila are completely lethal when homozygous, 1 is semilethal, and 

 only 3 are not lethal when homozygous (Muller, 1917). Another well- 

 known example is the yellow body-color in mice (Cu6not, 1915, 1918; 

 Castle and Little, 1910). 



Just as Star in Drosophila, so the brachyphalangy here described is 

 of no practical importance at all in heterozygous individuals. But it 



