Other Explanations . 355 



or genes, independently produce a given character or modify it 

 in some way so as not to destroy its identity. Plm^al genes in- 

 clude duplicate genes. 



One of the real difficulties with the theory of polymeric genes 

 is that it treats a quantitative character as a unit when often it 

 is a very complex phenomenon. Can plant height, for example, 

 be treated as one thing, when the height of a plant depends 

 upon both the length of the internodes and their number? If 

 ten genes acted in such a way that each added one centimeter 

 to the length of each of five internodes, such genes would be 

 duplicate genes; and if their action was cumulative, and no 

 gene was dominant over its allele, we would have a typical case 

 of polymeric genes. The height of a plant, after the amount 

 due to residual heredity was subtracted, would then be propor- 

 tional to the number of genes present. Although such a situ- 

 ation is abstractly possible and may be found in some plants, 

 it seems unlikely to occur very often. It seems more likely 

 that one gene might add five centimeters to the lowest inter- 

 node, a second increase each internode by one centimeter, and a 

 third increase the number of internodes from five to six. In 

 addition, genes that increase or decrease the amount added by 

 the first two genes might also be present, as might a gene for 

 dwarfing that reduces the number of internodes from five to 

 three. Different degrees of dominance might be shown by dif- 

 ferent genes. In other words, the second scheme, although more 

 complicated, may coincide more closely with other situations 

 we have in genetics. 



Some interesting plural genes for body size in mice have 

 been described by Castle, who found that a majority of the com- 

 mon mutant genes in mice influence the size of the body, either 

 increasing or decreasing it. The gene for brown coat color (b) 

 is one of the genes that increases weight, body length, and tail 

 length (Table 20). Interestingly, the gene for brown in rats 

 and rabbits also increases body size. As Castle says, ''It seems 

 probable, therefore, that in mammals generally production of 

 brown instead of black pigment in the integument permits 

 greater growth in other body structures." In mice, the genes 

 for dilution (d) and yellow (A^), which also affect pigmenta- 

 tion, bring about an increase in body size, but yellow can act 

 only in a heterozygous condition for it is lethal when homozy- 



