162 GENETICS 



F. THE KINDS OF GRAY RABBITS 



Each of the apparent kinds of gray rabbits indicated 

 in the foregoing table may be made up of various geno- 

 types. For instance, there are thirty-two different 

 genotypes, each of which is pheno typically a gray 

 rabbit. The zygotic formula for each of these thirty- 

 two possibilities is displayed in the next table, and it 

 will be seen that these range all the way from rabbits 

 homozygous in all their variable characters (No. 1) 

 to those homozygous in none (No. 32). 



The progeny of these various types of gray rabbits 

 when inbred will consequently vary from the pure gray, 

 as in No. 1, to a gray from which sixteen possible 

 types of young may be expected as in No. 32. 



Up to the time when Castle's paper upon the factor 

 hypothesis 1 was published in 1909, nine genotypic 

 kinds of gray rabbits had been obtained in his experi- 

 ments, whose genotypic formulae correspond to the 

 following numbers in the list: 1, 3, 6, 10, 13, 20, 22, 

 28, 29. 



5. LETHAL GENES 



Among mammals, as shown by Cuenot and confirmed 

 by Little, homozygous or pure yellow mice are un- 

 known although yellow individuals have long been ex- 

 ploited by fanciers. In other words, all kinds of yel- 

 low mice behave as if heterozygous or simplex with 

 respect to yellow color, for when any two yellow mice 



1 "Studies of Inheritance in Rabbits." Carnegie Institution 

 Publications, No. 114, 1909. W. E. Castle in collaboration with 

 Walter, Mullenix and Cobb. 



