196 GENETICS 



diagrammatically in Figure 44, 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 sym- 

 bols 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 determines the somato- 

 plasm in each case, while the long dotted arrows show 

 what possible recombinations of germplasms can be 

 made. Child No. 4 is an "extracted recessive" derived 

 from dominant parents, but with one recessive grand- 

 parent on each side. It is a case of "atavism," or 

 taking after the grandparent. 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, 

 for the ancestral character or characters in this type 

 of hereditary behavior, as said before, reappear only 

 after a lapse of many generations. 



