No. 635] 



WHITE SPOTTING IN MICE 



473 



mits the formation of pigment on the average, in about 

 85 per cent of the dorsal surface, although such mice may 

 vary from solid colored dorsally (0 per cent, white) to 

 100 per cent, white dorsally. 



There are certain other combinations of these genes 

 possible, and by experiment these other combinations 

 have been found to produce other spotted types. Some 

 of these types have been found to be indistinguishable 

 somatically from piebald, although differing from pie- 

 bald in genetic constitution. The experimental evidence 

 bearing on the appearance, constitution and variability 

 of such additional types follows. 



Type "A" Spotting 

 When black-eyed white is crossed with self approxi- 

 mately half of the progeny are spotted and half arc self 

 or exhibit at most only a small white ventral spot. From 

 such a cross in the present experiments 76 young have 

 been born, of which 40 were spotted and 36 were self col- 

 ored (Table II, Cross 2). Little found that the self (or 

 nearly self) young from sueh a cross were ordinary 

 heterozygotes between self and piebald. The spotted 

 young he found to be due to unions of gametes carrying 

 black-eyed white and piebald (Ws) with self gametes 

 (wS) producing the double heterozygote WwSs. He 

 called such spotted mice Type "A". The somatic ex- 

 pression of this combination of genes has produced 

 spotted mice varying from to 45 per cent, of white dor- 

 sally (Fig. 1, broken line, and Fig. 3, dotted line). The 

 distribution as regards amount of white dorsally is seen 

 from this figure to resemble closely the distribution of 

 piebald in range, while the mode of Type "A" is at 5-0 

 per cent, and the mode of all piebalds is at the same point. 

 The mean of all Tvpe "A"s raised (83 in number) was 

 7.9 ± .6 per cent, white dorsally with a standard devia- 

 tion of 8.4 ± .4. This is very close to the mean of all 

 piebalds (9.7 per cent, white) and almost identical with 

 the mean of piebalds extracted from crosses of piebalds 

 with self (8.05 ± .3). Moreover, Tvpe "A"s which have 



