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THE AMERICAN NATURALIST [Vol.LVI 



male of Drosophila there is only one X-chromosome, 

 though there is present a Y-chromosome that can be dis- 

 regarded, since the evidence from non-disjunction of the 

 X-chromosome shows that it has very little effect upon 

 sex or characters. These individuals with only one X- 



chromosome likewise show a complex of characters that 

 are different from those shown by the individuals with the 

 normal two X-chromosomes. Among these characters 

 are gonads and genitalia of a type that we call male. The 

 haplo-X individual is also smaller, has smaller bristles, 

 is less viable, hatches later, and differs in other details 

 from the 2-X type that we call female. Each of these dif- 

 ferences likewise corresponds to a character for which 

 the balance of the genes in the X is different from that in 

 the group as a whole. The absence of one X leaves in 

 action an unbalanced set of genes which produces male 

 characters. The X-chromosome is a chromosome that is 

 internally unbalanced by an excess of genes that we may 

 call female-producing. 



