314 



AN INTRODUCTION TO MODERN GENETICS 



genes concerned, but he later demonstrated the correctness of the more 

 orthodox view that the selection had merely accumulated minor faaors 

 affecting the extent of the pattern. He crossed rats with extremely 

 small pigmented areas with rats from a normal unselected stock, and 

 recrossed the hooded segregates into the same normal line; after several 



© o o 



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-3 



Fig. 130. Selection in Rats. — The hooded rat has very variable pigmentation. 

 Four of the arbitrary grades used for measuring the pigmentation are shown 

 above. Belov^ are the results of selection for twenty generations of inbred lines; 

 hollow circles selection for increased pigmentation, full circles selection for 

 decreased pigmentation. (Data of Castle.) 



generations of such crossing, the hooded gene from the low pigmented 

 line had been got into a normal genotypic milieu and again produced a 

 normal, and thus higher, degree of pigmentation. Thus the low pig- 

 mentation in the selected line was due only to the accumulation of 

 modifiers tending to reduce pigment. 



4. Inbreeding and the Production of Pure Lines^ 

 Inbreeding is the breeding together of related organisms. Their 

 ^ General references: East and Jones 1919. 



