PIEBALD RATS AND MULTIPLE FACTORS 



E. C. MacDOWELL 

 Station for Experimental Evolution, Cold Spring Harbor, L. I. 

 Introduction 



The experiments of Castle and Phillips ( :14) with pie- 

 bald rats afford the largest mass of recorded data on the 

 influence of selection in mammals. For 17 generations, 

 the area of pigmentation on their hooded rats has been in- 

 creasingly modified. In one line (the plus race) the pig- 

 mentation has been extended; in the other line (the minus 

 race) it has been reduced. When rats from the plus or 

 minus race are crossed with fully pigmented rats, such as 

 the normal wild, or the Irish variety, the hooded pattern 

 behaves as a simple Mendelian recessive, disappearing in 

 the first generation and reappearing in one fourth of the 

 offspring in the second generation. These results lead to 

 the conclusion that hoodedness appears when a certain 

 germinal unit, or factor, is in a zygote in a homozygous 

 condition. Besides this, Castle concludes that the factor 

 determining hoodedness fluctuates, and, in accord with its 

 fluctuations, the amount of hooding varies. It follows 

 that the selection of extreme grades of hoodedness results 

 in the simultaneous selection of extreme variations of the 

 factor. Moreover, Castle ( :16, p. 722) finally concludes 

 that the selection of these extreme grades of hoodedness 

 influences the direction in which the factor for hoodedness 



These conclusions bear on one of the most generally in- 

 teresting and vital questions before biologists. If, be- 

 sides deciding which individuals shall mature and repro- 

 duce, selection can influence the direction in which the 

 units of inheritance, or factors, vary, there can be no 

 question but that 



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