THE GENETIC CONTROL OF PATTERN 201 



compare such a formulation to the ideas of evocators and compe- 

 tence. 



Another method by which a fundamental pattern can be modified in 

 its expression is by effects on relative growth rates. Sinnott and Dunn^ 

 have described some good cases in squash plants, in which there are 

 different genetically determined fruit shapes. In some cases the dif- 

 ferent shapes are determined very early, in the fruit primordium, 

 which then grows uniformly in all directions. But in other cases, the 

 fruit primordia may have the same shape, and the differences between 

 the fully grown fruit may be due to different growth rates in different 

 directions. Similar effects on relative growth rate are exerted by whole 

 groups of genes acting together in the trisomies of Datura, each of 

 which produces fruit of a characteristic shape. The genes responsible 

 for facial conformation in man presumably work in the same way. 

 Goldschmidt- pointed out that even such a striking pattern-abnormality 

 as brachydactyly in man can be explained as a result of abnormal 

 relative growth rates. In brachydactyly, the second row of phalanges 

 develops very late and grows very slowly so that it is extremely small in 

 the adult and may in extreme cases be reabsorbed. 



3. Genes which Affect the Fundamental Pattern 



In all the cases we have described above, the action of the genes can 

 be described as modifying an underlying pattern which is essentially 

 unaltered. Particular elements in the pattern may be expanded or con- 

 tracted, but the changes are quantitative rather than qualitative. One 

 can, however, find examples of genes whose effects seem to be more 

 radical. Again, we are not dealing with a sharp division of genes into 

 t;wo categories, but with a gradual increase in the importance of the 

 gene effects. A clear-cut distinction between the two sorts of genes 

 could only be made if we had a definite idea of what constitutes a 

 qualitative change in a pattern; this is a task for topology, but so far 

 the mathematicians have not provided a scheme suitable for application 

 to biological material. 



{a) Disruption of the Pattern. —Ks a first group of genes with radical 

 effects on patterns we may consider the numerous genes which prevent 

 any pattern being formed. They break down the old pattern but do not 

 substitute any other definite arrangement for it. For instance, the 

 short-tailed gene in mice^ causes, when homozygous, a profound dis- 

 turbance of all the anatomical relations in the posterior end of the 



1 Sinnott and Dunn 1935. ^ Goldschmidt 1927. ' Chesley I935- 



