356 GENETICS 



natural conditions practically all of them die without repro- 

 duction, so that the stock is kept free from mutated charac- 

 teristics. 



A large proportion of the mutations that occur cause In- 

 deed a complete failure of development, unless the mutated 

 gene is accompanied by a normal gene of the same type. 

 Such lethal mutations occur more freqeuntly than any other 



type. 



Mutations as Partial Inactivation of Genes: There oc- 

 cur many gene mutations whose effects, both special and 

 general, are so slight that from an examination of the indi- 

 viduals affected by them It is difficult to determine whether 

 they are harmful or not. Even In such cases, however, the 

 gene after mutation has usually a less marked or Intense 

 effect than it had before mutation. This Is Indeed a rather 

 general characteristic of mutated genes; thus by any muta- 

 tion of the "white-eye" gene located at 1.5 In the X-chromo- 

 some of Drosophila, there is caused a change In eye-color 

 from a deep red to some lighter color, with less pigment. 

 Such peculiarities have led to the suggestion that the effect 

 of the mutation is partly to inactivate the gene. This sug- 

 gestion has been confirmed for many mutations by certain re- 

 sults of observation and experlment.^^ These are as fol- 

 lows: 



In order that a recessive mutation shall be manifested, it 

 is ordinarily necessary that both the genes of the pair shall 

 be mutated, so that there Is no unmutated gene of that type 

 present. But sometimes in one of the two chromosomes of 

 the pair the gene is entirely lost, either by removal of a piece 

 through radiation, or by some other means. Such a chromo- 

 some is said to show a deficiency for the gene In question. In 

 such a case therefore but one of the mutated genes is pres- 

 ent, although there is also no normal dominant gene of that 

 pair present (figure 70, B). In such cases, where only one 

 mutated gene Is present. It was discovered by Mohr that the 



