DIFFERENCES BETWEEN X AND Y 



359 

 but is 



1923). The X chromosome is usually larger than the 1. 

 occasionally smaller (as in Drosophila melanogaster) . 



3. Organisms in which the Y chromosome is represented by two 





fcf 9ll<:^ 9lcr 9 



8 8 8 |i |: |: 



5+ + 3<Z+X2 



ftlihsis 



'cT 



54*x+z*>f-fz 



I 



I 



f 

 I 



F 



f 

 I 



f 

 I 



> 



1^^ 



f 



Fig. III. — Diagram showing how fragmentation in a chromosome 

 carrying sex factors makes the sex chromosomes distinguishable 

 in the heterozygous sex and how crossing-over between the point 

 of breakage and the sex-factors destroys the distinction. I, the 

 unfragmented type with 54 -f X + Y chromosomes in the 

 female. II, breakage of the Y chromosome into two components, 

 " X " and " Z," " X " containing the F factor or factors. 

 Crossing-over in this female in the X and Y chromosomes 

 between the breakage and the F factor will give the diplotene 

 configuration shown and yield males with a fragmented X chro- 

 mosome, i.e., type III. The female is the sex heterozygote in 

 the Lepidoptera. ^".^5. The change cannot be simple frag- 

 mentation, since it leads to reduplication of the centromere. 



or more fragments. The mode of origin of this type from the first 

 is made clear by Seller's work on Phragmatohia ftdiginosa (1925). 



