ARE RECESSIVES LOSSES OF GENES? 75 



ducing gene is absent and if color depends on the pres- 

 ence of this gene it is difficult to explain the presence 

 of these colored hairs. 



A mutant race of Drosophila is called vestigial (Fig. 

 10) because only vestiges of the wings are present, but if 

 the larvae are reared at a temperature of about 31 °C. the 

 rudiments are quite long and in extreme cases may be 

 almost as long as the wings of the wild type. If the gene 

 for producing long wings is absent, how can a high tem- 

 perature bring it back again? 



There is another highly selected race of Drosophila in 

 which the eyes are absent in most individuals, but small 

 eyes may be present in other individuals (Fig. 30). As 

 the culture °;ets older more and more of the flies have 

 eyes and the average size of the eyes is larger. It is not 

 probable that the gene changes as the culture gets older 

 and if it were absent in the eyeless flies that first hatch, it 

 is not likely that the age of the culture could bring back 

 the missing gene. Moreover, if this were the case, flies 

 from the older culture should produce offspring more of 

 which had eyes or larger eyes than the average of the 

 race, but this does not happen. 



In still other recessive mutant types the loss of the 

 character itself is by no means obvious. A black rabbit 

 is recessive to the gray wild type. The black rabbit has 

 actually more pigment than has the gray rabbit. 



There are dominant genes that produce pure white 

 individuals. The white leghorn race of fowls is due to 

 such a factor. Here the argument is reversed, and it is 

 said that there is present in the wild type jungle fowl a 

 gene that suppresses white plumage. When this suppress- 

 ing gene is lost the bird is then able to develop white 

 plumage. Logical as this argument may appear, the as- 

 sumption of the presence of factors of this sort in the 

 wild bird seems far fetched, and in the light of the occur- 



