424 Morpho genetic Factors 



inscribing the lengthwise profile of an organic form like that of a fruit 

 in a series of rectangular coordinates, as D'Arcy Thompson ( 1942 ) has 

 done with various structures, and then seeing how other forms may be 

 derived from it by deforming these coordinates in a particular fashion 

 (Fig. 19-4). A change such as might be produced by a single gene dif- 

 ference is evidently not a localized one but involves, at least to some 

 degree, the pattern as a whole. 



The objection may be raised that in some cases a single dimension does 

 seem to be inherited, as in vine height in peas. In a strongly polarized 

 organ like the stem, height may be changed with little reference to stem 

 diameter. In such cases height seems to be a direct expression of size in 



Fig. 19-4. Geometric modification of fruit shape. The fruit at the upper left has its 

 longitudinal profile inscribed in a grid of equal coordinates. This may be changed to 

 various other shapes by changing these coordinates. ( From Sinnott. ) 



which the major effect of the genes is channeled in one particular dimen- 

 sion, that of the polar axis. Ear length in maize, one of the first characters 

 to be analyzed in terms of multiple genes, is the major dimension of a 

 polarized organ and thus strongly affected by any genes that control 

 total amount of growth. Here, however, ear width is also involved to 

 some extent, and there are differences in the relation of length to width 

 in various races. Such a case may perhaps be regarded as intermediate 

 in genetic control between one in which genes for shape express them- 

 selves in a weakly polarized structure (like a fruit), thus producing a 

 wide range of patterns, and one in which growth is essentially limited 

 to the polar dimension. 



