228 DONALD F. JONES 



series of alleles that produce characteristic color patterns and intensities of 

 colors in different parts of the plant such as culm, leaf sheath, leaf blade, 

 glumes, anthers, silks, cob and pericarp, and endosperm. They may be con- 

 sidered either as genes located so closely together that they never show- 

 crossing over, or compound genes with multiple effects. Without going into 

 the evidence for or against these two hypotheses, it is obvious that compound 

 genes can have an important part in heterosis if they control growth proc- 

 esses. More information is needed on the specific effect of compound genes. 



In Godetia a series of multiple alleles has been described by Hiorth (1940) 

 that is often cited as an illustration of an interaction between alleles produc- 

 ing an effect analogous to heterosis. Actually these are color determiners that 

 control pigment production in different parts of the flower quite similar to the 

 A, P, and R loci in maize. Each allele has a different manifestation, and all 

 tend to accumulate color in the heterozygotes. 



The familiar notation of a chromosome as a linear arrangement of loci, 

 each of which is the site of a single gene with one effect function, is probably 

 an oversimplification of the actual condition. It is difficult to see how an 

 organism could have originated in this way. It is more likely that a chromo- 

 some is an association of primitive organisms of varying types and functions. 

 These primitive organisms found it to be an advantage in the evolutionary 

 process to become associated in some such process as the colonial organisms 

 now exhibit. This association has undergone very great modification and 

 ramifications, but the compound genes may be vestigial structures of such 

 an association, differing greatly in size, arrangement, and function. Many of 

 them still retain some independence, and when removed from their normal 

 position in the chromosome could function as plasmagene or viroid bodies. 



These compound genes may undergo mutation and possibly recombina- 

 tion or reorganization within themselves, but crossing over takes place for 

 the most part only between these compound structures. Compound genes 

 also arise by unequal crossing over and duplication of loci are shown by the 

 Bar eye gene in Drosophila and others of similar type. 



In addition to compound or multiple genes, there are single genes with 

 multiple effects. Many of these are important in growth processes and are 

 illustrated by chlorophyll production in maize studied by H. L. Everett 

 (1949). One major gene is essential for the production of carotene. In the 

 recessive condition the seeds are pale yellow in color, in a normal, dark yel- 

 low seeded variety. The young seedlings grown from these pale yellow seeds 

 are devoid of chlorophyll. The recessive allele is therefore lethal. By using the 

 pale yellow endosperm as a convenient marker and crossing with a number of 

 standard field corn inbreds, it has been found that these inbreds differ widely 

 in their normal chorophyll mechanism. Many of them have genes that can 

 restore normal chorophyll production without restoring the production of 

 carotene in the seed. Other genes restore chlorophyll production only partial- 



