148 GENETICS 



of this apparently inconsistent hereditary behavior is 

 perfectly simple in the light of the Mendelian ratios, 

 as shown diagrammatically in Figure 48, in which the 

 circles represent the blue-eyed and the squares the 

 brown-eyed character. 



This figure also illustrates what typically occurs in 

 the formation of Mendelian monohybrids of the first 

 and second filial generations. The squares are 

 symbols for the dominant characters, while the circles 

 are symbols for the recessive characters. When the 

 two are superimposed, the circle recedes from view, 

 The large outside figures indicate the somatoplasm, 

 therefore the phenotype. The small inclosed figures 

 indicate the germplasm, therefore the genotype. The 

 short dotted arrows indicate what it is that deter- 

 mines the somatoplasm in each case, while the long 

 dotted arrows show what possible recombinations of 

 germplasms can be made. Child No. 4 is an " ex- 

 tracted recessive" derived from dominant parents, 

 but with one recessive grandparent on each side. It 

 is a case of "atavism," or taking after the grand- 

 parent. Notice that atavism can occur only by 

 alternative inheritance. 



To quote Davenport: "In the majority of cases 

 atavism is a simple reappearance in one fourth of the 

 offspring of the absence of a character due to the 

 simplex nature of the character in both parents." 



An illustration of reversion would be the reappear- 

 ance of the ancestral jungle-fowl pattern in domestic 

 poultry or of the slaty blue color of the ancestral 

 rock-pigeon among buff and white domestic pigeons, 



