ORIGIN OF MUTANT CHARACTERS 65 



show relatives or ancestors with the same mutant charac- 

 ter. Human albinos furnish, perhaps, the best example of 

 this sort. In many cases they come from stocks both of 

 which carry the recessive gene, but it is always possible 

 that a new gene for albinos may have been produced by 

 mutation. Even then it cannot come to expression until 

 it meets another like gene. 



Most of our domesticated animals and plants show 

 characters that behave in inheritance in the same way as 

 do the mutants whose origin is established. There can be 

 no reasonable doubt that many of the characters have 

 arisen by sudden mutations, especially in cases where the 

 domesticated types have come from inbred pedigree 

 stocks. 



It is not to be inferred from the preceding examples 

 that the production of mutants is peculiar to domesticated 

 races; for this is not the case. There is abundant evi- 

 dence that the same kinds of mutations occur also in 

 nature. Since most of the mutants are weaker or less well- 

 adapted types than the mid type, they disappear before 

 they are recognized. In cultivation, on the other hand, 

 the individual is protected, and the weaker types have a 

 chance to survive. Moreover, domestic forms, especially 

 those reared for genetic purposes, are carefully scruti- 

 nized, and our familiarity with them accounts for the 

 detection of many new types. 



A study of the occurrence of mutations in the stocks of 

 Drosophila has brought to light a curious and unexpected 

 fact. The mutational change takes place in one member 

 only of a pair of genes — not in both at the same time. 

 It is difficult to imagine what kind of an environmental 

 effect could cause one gene in one cell to change, and not 

 the other identical gene. Hence it may seem that the cause 

 of the change is internal rather than external. This ques- 

 tion will be further discussed later. 



