238 M. R. IRWIN 



and its presence or absence in several species is genetically determined. One 

 way in which anthocyanin may be modified is by the degree of oxidation of 

 the prime ring. According to Beale (1941) in the two genera Lathyrus and 

 Streptocarpus, the hydroxyl group is at position 4' in the pelargonidin type, 

 at positions 3' and 4' in the cyanidin types, and at 3', 4', and 5' in the delphin- 

 idin types. The more oxidized pigments are usually dominant to the less 

 oxidized types. Thus flowers with genes AB and -46 will be of the delphinidin 

 type of pigment, those with aB of the cyanidin type, and those with ab of the 

 pelargonidin type. 



These and other extensive chemical studies on the anthocyanin pigments 

 genetically modified in various ways are dramatic examples of the specifici- 

 ties of gene effects. The analogy drawn above between the various genes and 

 students working on a complicated synthesis becomes a little more clear in 

 relation to flower pigments, since considerable information is available as to 

 what some of the genes accomplish. 



A further example of the effect of many genes upon a character is that of 

 eye color in Drosophila melanogaster. Between twenty-five and thirty genes 

 are known to modify the brownish-red color of the wild-type eye. There ap- 

 pear to be two independent pigments, brown and red, concerned in the devel- 

 opment of the wild-type eye, each of these being affected by specific genes. 

 Certain components of the brown pigment are diffusible from one part of the 

 body to another, and hence are more readily subjected than others to chem- 

 ical analyses. 



The details of these analyses are presented in other review articles (Beadle, 

 1945; Ephrussi, 1942a, 1942b). Briefly, dietary tryptophan is converted to 

 alpha-oxytryptophan by a reaction controlled by the wild-type allele of the 

 vermilion gene (?). This substance is oxidized further to kynurenine (the so- 

 called i'+ substance). By virtue of the activity of the normal allele of the cin- 

 nabar gene, kynurenine is further oxidized to the or substance, which may 

 be the chromogen of the brown pigment (Kikkawa, 1941). The production of 

 either brown or red eye pigment can be blocked by genes at the white eye 

 locus, thus indicating that such genes act on a common precursor of the red 

 and brown pigments. 



Mention should be made of the relation between the original designation 

 of certain of the genes for eye color and their presently known effects. Thus, 

 the eyes of flies with the mutant alleles bw bw are brown. But it is now known 

 that this pair of alleles, instead of being concerned with the production of 

 brown pigment, restricts the development of red pigment and thus we see 

 only the brown color. Similarly, the four gene pairs whose mutants modify 

 the red coloration do so by virtue of their effect on the brown pigment, not 

 upon the red. 



Wheldale (1910) proposed four decades ago that genetic characters were 

 the resultant of a series of reactions, and that if a break in the chain occurred, 



