14 



GENETIC VARIATIONS 



Changes in the Operation of Single Genes. Mutations 



Changes in the action of single genes, known as gene mu- 

 tations, are still more important in bringing about changes 

 in characteristics than are the alterations in grouping of 

 genes and chromosomes described in the preceding chapter. 

 There it was shown that breakage and injury in chromo- 

 somes are often accompanied by a change in the action of 

 particular genes situated near the points of breakage, so 

 that chromosome breakage produces gene mutations. 



But great numbers of gene mutations were known and 

 studied in detail before it was discovered that such changes 

 may result from breakage or injury to the chromosomes. 

 In such an organism as Drosophila, the study of inheritance 

 is based largely on gene mutations; all the recessive char- 

 acters, and many of the dominant ones, are the result of 

 gene mutations that have occurred since Drosophila became 

 an object of experimentation in genetics.^ Only the so-called 

 "wild type" or "normal" characters are not the result of 

 recent mutations. Whether the mutations resulted from 

 breakage or injury to chromosomes is not known; the ques- 

 tion is one that is under investigation at the present time. 



For a change in inherited characteristics to be identified 

 as a gene mutation, certain conditions must be fulfilled. It 

 must be possible to show that the change is not the result of 

 ordinary recombinations, such as occur in Mendelian in- 



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