208 BIOCHEMICAL SYSTEMATICS 



low probability. The same interpretation may be expected to hold for 

 the surprisingly large number of complex loci which affect, quantita- 

 tively, anthocyanin synthesis. Alston (1959) has discussed certain im- 

 plications of the existence of such loci in a large proportion of plant 

 species studied, and it is pertinent to note that homologies among such 

 complex loci are considered likely to be rare. 



The foregoing discussion serves to provide a perspective from 

 which to view certain systematic investigations involving the flavo- 

 noid pigments or simpler phenols. Several illustrations have been 

 selected which disclose that a single gene may alter several bio- 

 chemical components of a plant (in one further case, the P*" allele of 

 Impatiens balsamina governs not only the amount of anthocyanin in 

 the stem, sepals, and petals, but in addition has a different qualitative 

 expression in each plant part). Despite such examples, it seems im- 

 proper to conclude, in the absence of genetic criteria, that when re- 

 lated anthocyanins and flavonols occur together or when similar 

 glycosides of anthocyanins and flavonols occur together, the same 

 enzyme (or gene) is necessarily implicated. In one such situation cited, 

 involving Lathyrus odoratus (Harborne, 1960a), this assumption was 

 made after examining a number of varieties but without benefit of 

 genetic studies. In a previous genetic study Beale (1939) reported 

 that the genes affecting anthocyanidin type did not influence flavonol 

 composition in Lathyrus odoratus. 



Just as cases are known in which one gene governs several bio- 

 chemical differences, there are instances in which several different 

 genes may affect the same biochemical character. It seems to be 

 established that numerous gene effects are highly indirect, the 

 primEiry gene effect remaining completely unsuspected. 



Gene mutations affecting relatively late stages in the flavo- 

 noid biosynthetic pathway appear to be far more frequently detected 

 than those affecting an early step. This assumption is based on the 

 rarity of cases in which a gene is known to inhibit the total synthesis 

 of aU flavonoids (either as a dominant or recessive). Most "white" 

 mutants involve the anthocyanins and in such mutants other types 

 of flavonoids may still be produced. Roller (1956) who studied the 

 flavonoids of certain white-flowered varieties of over forty species, 

 found other types of flavonoids present in practically every instance. 

 From two to six flavonoids were present, as a rule, with flavonols 

 most frequent. The infrequency of cases involving mutations inhibit- 

 ing total flavonoid synthesis was also noted in the discussion follow- 

 ing a recent paper on anthocyanin genetics by Harborne (1960b). 



On a priori grounds, one may predict that the earlier 



