I OS PHYSIOLOGICAL GENETICS 



(Crew-Lamy, 1932). Another case in rodents is described by 

 Burrows (1934). An unknown number of dominant dominigenes 

 makes the heterozygote of Agouti and black intermediate instead 

 of Agouti. 



Such eases, according to Schultz (1935), do not seem to be rare 

 in Drosophila. He found in 37 combinations of a heterozygote 

 of one eye-color mutant with another homozygous mutant five 

 cases of conditioned dominance, as the phenomenon is sometimes 

 called, without being able to establish a rule connecting this 

 phenomenon with the process of pigmentation, which we described 

 on page 33. A few similar facts have also been known for 

 rodents where recessive albinism and spotting may become 

 incompletely recessive in the presence of other genes (Wright, 

 1927; Cuenot, 1911), and Snell (1931) described a case that he 

 classifies as a change of dominance in an individual, in which 

 at first dilution appeared dominant, whereas later the normal 

 recessiveness was restored. The case is, however, not completely 

 clear. Kikkawa (1934) described for Drosophila virilis an 

 enhancing action of the gene confluent upon the dominance of 

 plexus. But here another wing-venation gene is involved, and 

 therefore the case might also be regarded from a different angle. 



In some other cases, dominigenes were found after crossing: 

 Harland (1932, 1934) found that a recessive mutation of cotton 

 behaved as an incomplete recessive if crossed with different varie- 

 ties which are supposed to possess dominigenes for this character. 



A comparable case had been known for Drosophila. Morgan 

 and Sturtevant (1929) found that in hybrids of D. melanog aster 

 and simulans the genes Bar and Lobe are no longer dominant. 

 Furthermore, the expression of Delta from melanogaster is less 

 extreme, but the expression of Delta from D. simulans is more 

 extreme in the hybrid than in the pure species. 



A parallel case in mammals has been described by Green 

 (1936) for the short-tailed mice, the genetics of which we men- 

 tioned before. He backcrossed heterozygotes of this dominant 

 mutant to a normal strain; the resulting heterozygotes showed 

 less dominance of the abnormality. The same heterozygotes 

 were outcrossed to a different species {baetrianus) , and still more 

 normal tails resulted. In the first case, the introduction of one, 

 in the second of more than one, dominigene is assumed. To the 

 same class of phenomena belongs also Fisher's (1935) recent 



