INHERITANCE 35 



appear in the proportion of one Albino to three pigmented. In 

 the F 2 and in the later generations pied animals may be had 

 and the color pattern both fixed and modified by selection 

 (Castle, '12, '12a, and Castle and Phillips, '14\ 



In the spring and autumn when the hair is renewed, the color 

 pattern of the hooded rat is often outlined in the Albino by a 

 differential growth of the new hair. 



4. Inheritance, (a) Brain weight. The inheritance of brain 

 weight in the reciprocal crosses Norway X Albino has been 

 studied by Hatai ('15). The relative brain weight in the adult 

 domesticated albino rat is from 12-15 per cent less than in the 

 wild Norway. The FI and F 2 hybrid offspring from Norway 

 X Albino crosses possess brain weights intermediate between 

 those for the respective parents. There is no evidence of segre- 

 gation in the hybrid brain weight data and the pigmented ex- 

 tracted rats possess brain weights similar to those of the non- 

 pigmented. 



(6) Suprarenal weight. The inheritance of suparenal weight 

 hi the reciprocal crosses Norway X Albino has been studied by 

 Donaldson, J. C. ('23). In the FI generation, pigmented, the 

 weight of the suprarenals is less than in the wild gray, while 

 in the subsequent generations, F 2 -F 4 , the weight in the extracted 

 Albinos was less than in the extracted grays, but never as low as 

 in the pure Albinos. 



(c) Body weight. The inheritance of body weight has been 

 studied by Ibsen ('22) but the results are not yet complete. 



(d) Mutations. Mutants studied at The Institute since 1906 

 are ruby-eyed dilute grays: a color variety of the Norway rat 

 that was obtained near the University of Pennsylvania in 1916 

 (Whiting and King, '18). 



Through crossing with the albino strain, two other new color 

 varieties of rats, fawn and sepia, were also produced. 



In the colony at The Institute, Hatai obtained albino mu- 

 tants from captive gray Norways (Hatai, '12), and King ('23) 

 has obtained cream mutants, the recessive of the black eyed 

 yellow, from a strain of blacks extracted from the Norway X 

 Albino cross, which had bred true since 1916. 



