164 PHYSIOLOGICAL GENETICS 



above. But it ought to !><• added thai the destructive effect 

 upon the development of practically the whole organism sets 

 aside sucli dominant mutations from what is ordinarily called a 

 gene mutation. It will he wise at present to be cautious aboul 

 drawing conclusions from such cases as Minutes, Deltas, etc. 



Dunn and Coyne (1935, 1937) have carried the same problem 

 •a little further. r l ney selected Minutes that show an increasing 

 series of retarding effects upon development, the descending 

 series being Mw — Mb — Mz — Mh — Ml 2 , which meant an average 

 delay in the Larval period from 3 to 1 days. These Minutes were 

 combined with mutant genes reducing the eye size, under the 

 assumption that a decrease of eye size means a change in the rela- 

 tive growth rate between eye and body. In these combinations 

 with the Lobe mutant, the eye size was actually reduced, and in 

 addition the grade of the reduction paralleled the foregoing series 

 for the growth-retarding effect of the Minutes. Apparently, in 

 this case the interaction effect of Minute and Lobe is clear: it is 

 the growth-retarding influence of the Minute reaction that 

 interferes with eye development of Lobe, pointing to a similar 

 effect of both "genes," which becomes additive. 



A corresponding piece of work has been accomplished with 

 the eye-color mutants of Drosophila, though no actual quantita- 

 tive work has been published thus far. The many genes for eye 

 color situated at different loci have been bred in many combina- 

 tions showing their interaction. In a general way, it may be 

 said that combinations of two colors are at least as light as the 

 lighter one and frequently lighter. Vermilion and garnet, for 

 example, give together a yellowish red. Other combinations of 

 two colors appear white. (The same has also been found by 

 Whiting, 1934, in Habrobracon.) Attempts at further analysis 

 in terms of genie products have been made by Wright (1932), 

 Crew and Lamy (1932), Schultz (1935), and Ephrussi and 

 Beadle (1937). Wright noticed that the double recessive of 

 scarlet and brown, neither of which changes the eye color very 

 considerably, is white. Scarlet and vermilion produce together 

 a color like vermilion. From which it might be expected that 

 vermilion and brown together would produce white, which is the 

 case. Wright thinks that the vermilion and scarlet genes are 

 concerned with two different links in the same reaction chain, 

 whereas brown may have to do with a qualitatively different 



