THE PROGENY OF HOODED RATS TWICE CROSSED 



WITH WILD RATS. 



In 1914 Castle and Phillips published a report on breeding experi- 

 ments with hooded rats, in which it was shown that the hooded color 

 pattern — itself a Mendelian recessive character in crosses with the 

 entirely colored (or "self") coat of wild rats — is subject to quantitative 

 variation, and that different quantitative conditions of the hooded 

 pattern are heritable. (Compare fig. 36, plate 7.) It was also shown 

 that by repeated selection of the more extreme variations in the hooded 

 pattern (either plus or minus) it is possible gradually to modify the racial 

 mean, mode, and range as regards these fluctuations, without eliminat- 

 ing further fluctuation or greatly reducing its amount. We concluded 

 that the unit character, hooded color pattern, is a quantitatively vary- 

 ing one, but were at that time unable to decide whether the observed 

 variability was due simply and exclusively to variation in a single 

 Mendelian unit factor or partly to independent and subsidiary modify- 

 ing Mendelian factors. 



Since publication of the above I have been engaged in further experi- 

 ments designed to show which of the alternative explanations is the 

 correct one, and these are now sufficiently advanced to indicate definite 

 conclusions. Previous experiments had shown that when a race of 

 hooded rats, whose character has been modified by selection (either 

 plus or minus), is crossed with wild rats, the extracted hooded animals 

 obtained in F 2 as recessives show regression toward the mean condition 

 of the recessive race before selection began. This result suggested 

 that the regression observed might be due to removal by the cross of 

 modifying factors, which selection had accumulated in the hooded race. 

 If this view was correct, it was thought that further crossing of the 

 extracted hooded animals with the same wild race should result in 

 further regression, and that if this further regression was not observed 

 a different explanation must be sought for the regression already noted. 



The entire experiment has accordingly been repeated from the 

 beginning, with the same result as regards regression in the first F, 

 generation, but with no regression of the same sort in a second F 2 con- 

 taining twice-extracted hooded animals. So far from observing further 

 regression as a result of the second cross with wild rats, we have unmis- 

 takable evidence that the movement of the mean, mode, and range of 

 the hooded character has been in the reverse direction. So the hypothe- 

 sis of modifying factors to account for the regression and for the pro- 

 gressive changes observed under selection becomes untenable. 



In repeating the experiment of crossing hooded rats of our selected 

 races with wild rats, great care has been taken to employ as parents 

 individuals of the greatest racial purity and to inbreed the offspring 



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