l6o GENETICS 



together small exchange ratios are close together, and that 

 the larger the exchange ratio between two genes the farther 

 they are apart. Such maps of the chromosomes of Dro- 

 sophila are shown in figure 38. 



The position of a gene on the map or in the chromo- 

 some is known as its locus. The locus of any gene is defined 

 by its distance in certain units of length from one of the 

 ends of the chromosome, the upper or "left" end in the 

 maps being that selected as the point of reference. Each 

 unit is such a chromosome length as gives, for two genes 

 at opposite ends of it, exchanges in one per cent of all cases 

 (thus it is a trifle less than the distance in the X- 

 chromosome from the gene yellow to the gene white, which 

 exchange is about 1.5 per cent of all cases). To designate 

 the locus of a gene, it is common to specify the chromo- 

 some in which it occurs (I, II, III or IV) and its distance 

 in units from the upper end of the chromosome. Thus, 

 in the X-chromosome, which is number I, the gene white 

 is at the locus I, 1.5; the gene miniature at I, 36.1. The 

 gene purple is at II, 54.5 ; the gene sepia at III, 26.0, and 

 so on. 



Such maps when carefully made can be depended on as 

 showing correctly the order of the genes, and approxi- 

 mately their relative distances apart. The relative physical 

 distances apart, as would be seen under the microscope, 

 differ somewhat from those shown on the map, because the 

 frequency of exchange is somewhat greater in some parts of 

 the chromosome than in others. Also, the frequency of the 

 breaks and exchanges in the chromosomes is known to be 

 affected by a number of other conditions besides the dis- 

 tances separating the genes. Yet on the whole the relations 

 shown by the map are sufficiently near to those actually 

 occurring to make the maps of great value. They are ex- 

 tremely useful in helping to understand and predict the 

 course of inlieritance. From them can be determined the 



