64 THE THEORY OF THE GENE 



zygous. Rarely, however, an individual survives. The 

 mutant, yellow coat-color in mice, is lethal as double 

 dominant, -as is also the mutant gene for black-eyed white 

 in mice. In all types of this sort, pure breeding stock can- 

 not be produced (except by "balancing" the dominant 

 with another lethal). They produce, in each generation, 

 individuals like themselves and some other type (the 

 normal allelomorph) in equal numbers. 



The short-fingered or brachydactyl type in man is a 

 striking dominant character whose inheritance is well 

 known. It will hardly be questioned that it arose as a 

 dominant mutant that established itself in certain fami- 

 lies. 



All the stocks of Drosophila have arisen as mutants. 

 In the cases that I have given the mutant first appeared 

 as a single individual. In several other cases, however, 

 the new mutant type first appeared in several individuals. 

 In such cases the mutation must have appeared early in 

 the germ-track, so that several eggs or sperm-cells came 

 to carry the mutated element. 



At other times a quarter of all the offspring from a 

 pair are mutants. These mutants are recessives, and the 

 evidence shows, in such cases, that the mutation had 

 occurred in an ancestor, and, being a recessive, it did not 

 appear on the surface until two individuals each having 

 the mutated gene met. A quarter of their offspring are 

 then expected to show the recessive character. 



Closely inbred stocks are expected to give this sort of 

 result more often than outbred stocks. If outbred, the 

 recessive gene may be distributed to a large number of 

 individuals before two such individuals meet by chance. 



It is probable that there are many concealed recessive 

 genes in the human germ-material, since some defective 

 characters recur oftener than expected by independent 

 mutation. When their pedigrees are traced they often 



