GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 393 



the offspring are intermediate, the mean or average height being 22 inches. 

 Connecting the tops of the columns in the graph results in an approxima- 

 tion to a normal frequency curve. Many of the quantitative characteristics 

 determined by polygenes exhibit such a normal distribution. The greater 



Pollen (male nucleus) 



Proportionate 

 number of 

 Fi individuals 



6- 

 4 



2 



y' 



/ 



/ 



/ 



s 



\ 



\ 



\ 



N 



Dwarf variety 

 10" 



aabb 



® 



AaBb 

 22" 



Ovule (egg nucleus) 



AABB 

 34" 



AABb 

 28" 



AaBB 

 28" 



AaBb 

 22" 



34" 28 



Height in incfies 



Ratio 1 : 4 



22" 



16" 



10" 



AABb 

 28" 



AAbb 

 22" 



AaBb 

 22" 



Aabb 

 16" 



AaBB 

 28" 



AaBb 

 22" 



aaBB 

 22" 



aaBb 



16" 



AaBb 

 22" 



Aabb 

 16" 



aaBb 

 16" 



aabb 

 10" 



: 6 : 4 : 1 

 FIG. 17.8. Quantitative inheritance based upon multiple genes (polygenes). 



the number of pairs of genes involved the closer the approximation to the 

 typical bell-shaped curve. For sake of simplicity we have assumed that 

 the difference in height between the two varieties depends upon but two 

 pairs of genes; obviously such differences might be dependent upon three, 

 or four or more pairs of genes. 



Polygenic characteristics are very common, though the genetic basis of 



