ARE RECESSIVES LOSSES OF GENES? 81 



There is, nevertheless, a slight difference between the 

 character produced by two recessive genes in this region 

 and one recessive and the notch "absence." This differ- 

 ence might seem to be due to one real absence (notch) and 

 one recessive not being equivalent to two recessive genes, 

 but further consideration shows that the two situations 

 are not quite comparable owing to the absence of other 

 genes in the lost notch piece. These genes are present in 

 the double recessive type, and the slight differences in the 

 result in the two cases may be referable to the presence 

 or absence of these other genes. 



In the preceding case it has not been possible to show 

 by cytological evidence that a piece of the X-chromosome 

 is absent in the notch mutant — its absence is deduced 

 from the genetic evidence alone. In the next case, how- 

 ever, an actual absence has been demonstrated. 



Occasionally one of the small fourth chromosomes is 

 lost (haplo-IV, Fig. 29). This chromosome carries, in 

 certain mutant stocks, recessive genes. It is possible to 

 make up an individual that has a recessive gene — eyeless 

 for example — in its single IV-chromosome. Such individ- 

 uals show the characteristics of the eveless stock but are, 

 as a class, more extreme than when two eyeless genes are 

 present. This difference may be due to the absence of the 

 other genes in the absent chromosome. 



A different relation arises in the case of translocation, 

 so called by Bridges, which means (from genetic evidence) 

 that a piece of a chromosome has become detached and re- 

 attached to some other chromosome. It perpetuates itself, 

 and, owing to the genes that it carries, introduces a com- 

 plication into the genetic results. For example, a piece 

 of the normal X-chromosome in the region of the ver- 

 milion locus became attached to another X-chromosome 

 (Fig. 46b). A female with vermilion genes in each of its 

 X-chromosomes and the transposed piece attached to one 



