VARIATION OF CHROMOSOMES 159 



(as might have been expected), but shows vermilion and 

 sable, because in this case the two recessive genes for 

 vermilion and for sable dominate the single normal allelo- 

 morphs. But a female having two such duplicated chro- 

 mosomes {i.e., tetraploid for the genes of certain regions 

 of the sex-chromosome) is now wild type in appearance, 

 because the two dominants dominate the two recessives. 

 Such a female crossed to a vermilion sable male gives wild- 

 type sons and vermilion sable daughters, which is criss- 

 cross inheritance in an opposite sense from that ordinarily 

 met with in DrosopMla. 



A second instance discovered by Bridges, but not yet 

 reported, seems best explained on the assumption that a 

 piece taken from the second chromosome has become 

 attached to the middle of the third chromosome. This 

 condition makes possible the linkage of mutant characters 

 to genes in both the second and the third chromosome at 

 the same time. The second chromosome that lost a piece, 

 and the third chromosome that gained the piece (both were 

 of course in the same cell), have been easily kept together 

 in the same stock ever since, because in those cases where 

 they become separated through assortment every zygote 

 that receives the deficient (2nd) chromosome dies unless 

 the same zygote has received the third chromosome with 

 the duplicate piece. 



The preceding results show that chromosomes may 

 not only gain genes by the attachment of pieces 

 (duplication), but also that chromosomes may lose 

 pieces ( deficiency) . 



Other instances of deficiency have been reported by 

 Bridges which can be explained either as total losses of 

 certain regions, or due to their inactivation. Unless the 

 lost pieces happen to have been retained as in the last 

 case, the distinction between these possibilities is difficult. 

 A study of one ease has shown that no crossing over takes 

 place in the region of deficiency, although the rest of the 

 chromosome was little or not at all affected. As a result 



