64 



The Source of Variability 



Pleiotropy is the condition in which one allele has a direct effect 

 on more than one trait. Thus in Drosophihj the allele causing ves- 

 tigial wings also causes other slight morphological differences from 

 normal flies and in addition causes lower fecundity (Dobzhansky, 

 1955); the allele scabrous produces both bulging eyes and addi- 

 tional sets of certain body bristles; and the allele giant produces 

 both larger size and slower development (Patterson and Stone, 

 1952). 



Allometry, or heterogonous growth, is the condition in which 

 different parts of the organism grow at different rates so that at 

 different size stages of the individual the body proportions are not 

 the same. An excellent example of this phenomenon occurs in hu- 

 mans. In the human embryo the head is very large compared with 

 the trunk, but as the individual grows the trunk increases in size 



Infant 



Adult 



Fig. 24. Allometry in the human head and trunk. The effects of differences 

 m the relative growth of parts of the body are strikingly brought out by a 

 comparison of the proportions of infant and adult. (From Simpson, Fittendrigh, 

 and Tiffany, Life: M Introduction to Biology, Harcourt, Brace and Company, 

 1957. Adapted, with permission, from Etkin, College Biology, Thomas Y. 

 Crowell Co.) 



