in the same organism (Figure 4-13 (a) and (b)). On genetic test, the 

 Y-chromosome cannot be considered entirely inert. Aside from a short 

 region, which contains one or more standard genetic loci, elements on 

 the distal ends of the long and short arms are essential for male fertility. 



1 1 10 Li. 



I 1 ± 



(°) IH NO K 



X hD hC hB hA XR 



' < lO/x 



(b) Y^ ~y C ^ ^ K ^ NO ^ Y^ 



lC lB lA sA sB 



Figure 4-13. Diagrammatic Representation of the X and Y Chromo- 

 somes of Drosophila melanogaster: (a) X chromosome with heteropycnotic 

 regions shaded: IH, junction of isopycnotic and heteropycnotic regions; 

 HA — hD, main segments of Xh with the "paranucleolar bodies" set off to 

 each side of the nucleolus organizer; K, kinetochore; NO, nucleolus organizer; 

 XR, genetic right limb of X. (b) Y chromosome with heteropycnotic seg- 

 ments shaded: small triangles indicate uncertain points of subdivision; C, 

 main secondary constriction of long arm; K, kinetochore; l-^-l*^', main seg- 

 ments of long arm; NO, nucleolus organizer; s^, s", main segments of short 

 arm; Y'^ long arm; Y**, short arm. (From Cooper, K. W., 1959. "Cyto- 

 genetic Analysis of Major HeterochromaticElements (Especially Xh and Y) 

 in Drosophila melanogaster, and the Theory of Heterochromatin," Chroino- 

 soma, 10, Fig. 9, p. 541, and Fig. 25, p. 543.) 



In the plants, Trillium and Paris, and in the newt, Tritiirus, exposure 

 to low temperature for some period of time results in the differentiation 

 of the chromosomes during division with respect to staining capacity. 

 This reaction was termed differential reactivity by Darlington and La 

 Cour. Certain specific regions of specific chromosomes are understained 

 relative to the rest, that is, they are negatively heteropycnotic (Figure 

 4-14 (a) and (b)). Wilson and Boothroyd (1941, 1944) have also 

 shown, at least in Trillium, that the pattern of these unstained regions is 

 a species characteristic and that they represent areas which are under- 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 91 



