CHAPTER 2 

 GENETICS 



1. General. 2. Inbreeding. 3. Coat color. 4. Inheritance: (a) Brain weight: 

 (b) Suprarenal weight; (c) Bodj' weight; (d) Mutations. 



1. General. See References. 



2. Inbreeding. Inbreeding of albino rats from males and fe- 

 males of the same litter, accompanied by the selection of the 

 most vigorous litters, has been carried on for 50 generations by 

 Dr. King. If we allow for man 3^ generations to a century, 

 then this is equivalent to what has occurred in man since the 

 year 423 A. D. Using the data for the first 25 generations. 

 King has examined the effects of inbreeding on the growth and 

 variability in body weight (King, '18, '18 a, '18 b and '19), and 

 also the effect on fertility, constitutional vigor and on the 

 sex ratio, comparison in all cases being made with the data 

 for the stock rats in the colony at The Institute. It appears 

 that although the general form of the growth curve for 

 body weight is unmodified, and the sex differences are un- 

 affected, yet the inbred rats (especially the males) are heavier, 

 while the variability in body weight is less than in the stock 

 rats. The inbred rats attain puberty earlier, live longer and 

 have larger litters than do the stock rats with which they were 

 compared. The sex ratio is not modified by inbreeding, but 

 it was possible to select two groups from the inbreds in one of 

 which the sex ratio was high and in the other low. 



The general conclusion reached by King is that inbreeding in 

 the rat, when accompanied by selection, may be practised with- 

 out giving rise to any ill effects (King '23). 



3. Coat color. Genetic studies on the Norway ^'at have been 

 concerned mainly with the inheritance of coat color. The gray 

 coat of the wild Norway is dominant in crosses between the 

 wild gray and the Albino. The Albinos in the F2 generation 



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