LIFE PROCESSES IN CAPTIVE GRAY RATS 57 



litters contained sixty-two young of which eight were still- 

 born. Of the fifty-four living young, twenty-five were curly 

 and twenty-nine were normal. The proportion of normal and 

 mutant young was thus near the 1 to 1 ratio to be expected 

 when one parent is a heterozygous dominant. Normal rats 

 from these litters, when inbred, produced only normal young. 

 Breeding the curly rats inter se gave eighty-nine litters with 

 a total of 544 young of which twenty-eight were stillborn. As 

 many of the young died shortly after birth, coat character- 

 istics could be determined for only 266 individuals, of which 

 181 were curly and 85 normal. Among the curly rats there 

 were 82 males and 99 females, so the mutation is autosomal 

 and not sex linked. The number of curly rats in these litters 

 was below the number (199) to be expected, but doubtless 

 there was an excess of curly rats among the individuals that 

 died young, since these mutants showed the same lack of 

 vitality and vigor that has been found in other mammals in 

 which morphological variations have appeared. 



From the breeding tests described above it is evident that 

 curly is a mendelian dominant differing from smooth coat by 

 a single gene. Rats heterozygous for curly show the mutant 

 characteristics in a somewhat less degree than do homozy- 

 gotes. In them the period when the coat is barely distinguish- 

 able from that in normal grays is longer, and in older animals 

 the wave in the hair is less pronounced than it is in homozy- 

 gotes of like age. 



A strain of curly mutants was developed for study and for 

 experimental use. In the early generations curly rats were 

 much inferior to normal grays in fertility and in vigor, and 

 mortality among the young was excessive. After sufficient 

 mutants were available to make possible a rigid selection of 

 breeding stock there was a marked increase in fertility, mor- 

 tality among the young decreased, and adult individuals com- 

 pared favorably with normal grays in size and vigor. 



Structural modifications that produce curly or wavy hair 

 occur in many mammals, including man. When the hair be- 

 comes very curly or wooly, the gene responsible has been 



