SHORTER ARTICLES AND CORRESPONDENCE 



THE BEARING OF THE SELECTION EXPERIMENTS 

 OF CASTLE AND PHILLIPS ON THE 

 VARIABILITY OF GENES 



Castle and Phillips have recently reviewed the results of six 

 years' work in which they selected for and against "hoodedness" 

 in rats. 1 In "hooded" or "piebald" rats only part of the coat 

 is pigmented ; the area of dark (versus white) coat varies greatly 

 in different animals, but tends, in those of medium grade, to 

 cover the head, shoulders and middle of the back, like a hood. 

 Starting with a strain which was probably hybrid, although of 

 unknown ancestry, and selecting during thirteen generations 

 for a larger extent of colored coat ("plus" selection), they suc- 

 ceeded in obtaining animals with a greater and greater area of 

 pigmentation. The average, the mode, and the extremes were 

 raised. Conversely, selection for less pigmentation ("minus" 

 selection) was accompanied by a gradual but decided and 

 continual diminution in the dark area. "Return" selection 

 also succeeded ; that is, plus selection was effective even in a line 

 which was already lighter than the average on account of a 

 previous minus selection, and. vice rasa, minus selection caused 

 a lightening of a strain that had been made exceptionally dark 

 by a prior plus selection. 



Certain crosses proved that more than one factor affecting 

 hoodedness is involved in the difference between the different 

 races. Therefore the production of animals of desired grade by 

 selection may perhaps be explained as a mere sorting out, into 

 different lines of descent, of different combinations of the various 

 factors for hoodedness originally present in the heterozygous 

 ancestors. It is the opinion of Castle and Phillips, however, that 

 this explanation will not suffice to account fully for the continued 

 efficacy of selection in their experiments, and they believe it 

 probable that a factor or factors for hoodedness are underpin'..' 

 variation of a fluctuating nature. 



1 Castle and Phillips, "Piebald Rats and Selection, An experimental test 



Mondelirm crosses." Published by the Carnegie institution of Washing- 

 ton. See also Castle's "Pure Lines and Selection" in American Breeders' 

 Magazine, 1914. 



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