No. 635] 



WHITE SPOTTING IN MICE 



491 



spotting, which is expressed in the presence of but one 

 dose of piebald. 



On this view, self animals which, when bred to pie- 

 balds, produce animals with small amounts of ventral 

 spotting, must be heterozygous for a recessive gene for 

 such spotting. Moreover, the piebald parents must also 

 be heterozygous in the same gene. This is required by 

 the evidence. 15 pairs of parents producing young with 

 some white gave a total of 151 young, of which 33, or 21 

 per cent., showed some white. This is nearly a ratio of 

 three perfect selfs to one with some white, and if the 

 cause of the small spotting is a gene, both piebald and 

 self parents must have been heterozygous for it. The 

 presence of such a gene in piebald mice is difficult of 

 demonstration, since when bred together piebald mice 

 produce only piebalds, with their characteristic dorsal 

 and ventral spotting, which obscures the action of any 

 genes for spotting of the ventrum only. 



A summary now partially completed shows that the 

 peculiarity of small belly spotting in 1 'self" mice does 

 not breed true. Matings of 4 'belly spot" X "belly spot" 

 have produced 70 young of which 57 were graded as self 

 (viz., having either no white spotting or less than 12 per 

 cent, white ventrally) while 13 were clearly piebald. In- 

 dividual tests showed that all belly spotted mice bred 

 were heterozygous for piebald, so this ratio is probably 

 a deviation from a 3 : 1 ratio. Of the 57 "selfs," 41 had 

 some ventral spotting like the parents, while 16 were true 

 selfs without any white hairs. The appearance of these 

 true selfs, and the fact that all belly spotted mice tested 

 were heterozygous for piebald indicate that the assumed 

 gene for belly spotting is only expressed by mice which 

 are heterozygous for piebald. The total distribution 

 from the matings just referred to resembles somewhat a 

 1:2:1 ratio, which would be expected if selfs hetero- 

 zygous for piebald show by reason of an imperfection in 

 dominance, a small amount of ventral spotting. The 

 distribution tabulated on this assumption follows: 



