232 The Nature of Gene Mutations 



the expected percentage. That the environment is a factor in 

 the viability of a mutant was also shown by Timofeeff-Ressov- 

 sky, who found that certain mutants would be much less viable 

 than the wild type at certain temperatures but would be as viable 

 and sometimes more viable at other temperatures. He showed 

 also that not only is the environment important but the other 

 genes as well. For example, at 24 to 25° centigrade, miniature 

 flies are only 69 per cent as viable as the wild type whereas the 

 bobbed mutants are 85 per cent as viable; flies that are both 

 miniature and bobbed, however, are 97 per cent as viable as 

 the wild type or more viable than either mutant type alone. 



Sterility 



Some mutations tend to make one or both sexes sterile. In 

 Drosophila melanogaster, the gene rudimentary produces a wing 

 abnormality and also affects egg development so that the females 

 are highly sterile, only occasionally producing any eggs at all in 

 the ovary. Similarly the genes fused and morula produce ste- 

 rility in the female in addition to affecting the wing veins and 

 the facets of the eye. 



Recessive Nature of Mutations 



In Table 4 is a list of some of the genes that have been dis- 

 covered in Drosophila melanogaster, arranged according to the 

 four chromosomes on which they are located. Following each 

 gene is its symbol. All these genes have arisen as mutations 

 from the wild-type fly and almost all are believed to be gene 

 mutations although a few, including bar eye and hairy wing, 

 are the result of the reduplication of a very small segment of a 

 chromosome. If the symbol of a mutant gene begins with a 

 lower-case letter, the mutant is recessive to the wild type; but 

 if it begins with a capital letter, the mutant is dominant. A 

 glance over the list shows that most of the mutant genes are 

 recessive to the wild type. A far greater number of recessive 

 than dominant mutants have been discovered in Drosophila 

 melanogaster, and they also have been observed in other species 

 of fruit fly and in a number of other animals. In organisms in 

 which there is no definite wild type, as in many plants, most 

 of the mutants that have been discovered are recessive to the 



