131 



THE AMERICAN NATURALIST 



[Vol. LIV 



Further evidence that this test is fairly reliable is fur- 

 nished by the fact that in matings of heterozygote to het- 

 erozygote affected offspring appeared in litters before 

 five normal young had been born in 84 per cent, of the 

 cases, while in 22 of the 32 matings the recessive appeared 

 in the first litter. The complete data are given in Table II. 



TABLE II 



3 5 

 32 



For present purposes we have adopted the symbol N to 

 represent a factor for normality; the recessive, palsied 

 animal is therefore nn. 



1. Ratio of Palsied Offspring when two Heterozygotes 

 are mated. — As there appears to be no need of presenting 

 the detailed data of individual matings, the combined re- 

 sults of mating heterozygous animals together are given 

 in the left hand side of Table III. Of the total number 



TABLE III 



of offspring alive when born (that is, when found) 183 

 were normal and 63 palsied, an almost exact three-to-one 

 ratio. We, therefore, feel safe in our assumption that 

 the palsied condition is based on a single unit factor dif- 

 ference. The question might be raised as to whether the 

 rather large number of offspring "born dead" might not 

 represent a disproportionate number of palsied young. 

 This does not, however, seem probable for a number of 



