Genetic Factors 



423 



flattish types grow more rapidly in width. In all such instances, form 

 changes as size increases. Where a particular race is genetically small, 

 its fruit shape at maturity will be different from one which is genetically 

 larger but has the same shape genotype. This fact complicates a study 

 of the inheritance of shape in cases where the parents differ in both shape 

 and size. 



In types where shape changes during development the dimensional 

 relations, if plotted logarithmically, are allometric, the points falling 

 along a straight line the slope of which measures the relative growth rate 

 of the two dimensions (Fig. 5-8). It is this relative rate which the genes 

 control, for if two races of Lagenaria differing in the slope of this line 



6O-1 

 50- 



40 

 30 



20- 



O 



z 

 u 



10 



8 



6- 



— r~ 

 4 



-j— 

 6 



-i 1 — 



8 10 



20 30 



WIDTH. MM 



Fig. 19-3. Segregation of relative growth rates. Allometric growth of length to width 

 of fruits in an F 2 from a cross between a rather elongate and a rather flat variety of 

 cucurbits. The two F 2 classes resemble in general the parental forms. The Fi was 

 like the elongate type. What is segregating is the character of the relative-growth 

 line. (From Sinnott.) 



are crossed, the trait which segregates in F 2 is the steepness of this slope 

 (Fig. 19-3), the value of k in the allometric equation (p. 105; Sinnott, 

 1958). 



It should be remembered that what is being controlled is not simply 

 the relationship between two dimensions but between all the dimensions 

 that make up the organic pattern. In the "bottle" gourd, for example, the 

 length of the axis during growth is related to the width of the upper, 

 sterile lobe; of the isthmus; and of the lower, fertile lobe. Relative to the 

 axis, the lower lobe grows fastest, the upper next, and the isthmus least. 

 The form of the whole structure thus changes in a precise and predict- 

 able fashion. 



This concept of shape inheritance may be illustrated most simply by 



