GENETICS — MORGAN 353 



analysis of the relation of bands and gene location. Thus, whether 

 or not the bands are the actual genes, the evidence is clear in show- 

 ing a remarkable agreement between the location of genes and the 

 location of corresponding bands. The analysis of the banded struc- 

 ture has confirmed the genetic evidence, showing that when certain 

 alterations of the order of the genes takes place there is a corre- 

 sponding change in the sequence of the bands which holds for the 

 finest details of the bands. 



The number of chromosomes in the salivary nuclei is half that of 

 the full number (as reported by Heitz), which Painter interprets as 

 due to homologous chromosomes conjugating (pi. 1, fig. 1). More- 

 over, the bands in each of the component halves show an identical 

 sequence which is strikingly evident when the halves are not closely 

 apposed. It has been suggested by Bridges and by Koltzoff that 



SCALE I— 5A^— I 





<^ X '^ 



FiGDRB 6. — Above to right the four pairs of chromosomes of Drosophila in the metaphase 

 stage from a cell of the ovary. The two smallest chromosomes are in the middle of the 

 group. The same chromosomes from the .'salivary gland are drawn below to the same 

 scale (after Bridges from the Journal of Heredity). 



homologous chromosomes have not only united but that they have 

 each divided two or three times, giving in some cases as many as 

 16 or 32 strands (fig. 6). The bands may then be said to be com- 

 posed each of 16 or 32 genes; or, if this identification of the bands 

 as genes is questioned insofar as the genes are concerned, the bands 

 are multiples of some kind of unit of which the chromosomes are 

 composed. ^ 



A few examples may serve to illustrate the way in which the 

 banded chromosomes confirm the genetic conclusions as to occasional 

 changes that have taken place in the serial order of the genes. In 

 figure 7 the right half of chromosome 3 from the salivary gland is 

 represented. In part the two components are fused, in part are 

 separate. In the lower part of the figure a reversed piece of one 

 component is present (terminal inversion). Like bands conjugate 



