INTRODUCTORY. 



23 



percentage of cross-overs) will be less than the diagram indicates, 

 because the diagram has been corrected for double crossing-over. 



Diagram I has been constructed upon the basis of all the data sum- 

 marized in table 65 (p. 84) for the first or X chromosome. It shows the 

 relative positions of the gens of the sex-linked characters oi Drosophila. 

 One unit of distance corresponds to i per cent of crossing-over. Since 

 all distances are corrected for double crossing-over and for coincidence, 

 the values represent the total of crossing-over between the loci. The 

 uncorrected value obtained in any experiment with two loci widely- 

 separated will be smaller than the value given in the map. 



It may be asked what will happen when two factors whose loci are 

 more than 50 units apart in the same chromosome are used in the same 

 experiment? One might expect to get more than 50 per cent of cross- 

 overs with such an experiment, but double crossing-over becomes dis- 

 proportionately greater the longer the distance involved, so that in 

 experiments the observed percentage of crossing-over does not rise 

 above 50 per cent. For example, if eosin is tested against bar, some- 

 what under 50 per cent of cross-overs are obtained, but if the distance of 

 bar from eosin is found by summation of the component distances the 

 interval for eosin bar is 56 units. 



In calculating the loci of the first chromosome, a system of weighting 

 was used which allowed each case to influence the positions of the loci 

 in proportion to the amount of the data. In this way advantage was 

 taken of the entire mass of data. 



The factors (lethal i, white, facet, abnormal, notch, and bifid) which 

 lie close to yellow were the first to be calculated and plotted. Ihe 

 next step was to determine very accurately the position of vermilion 

 with respect to yellow. There are many separate experiments which 

 influence this calculation and all were proportionately w^eighted. Then, 

 using vermilion as the fixed point the factors (dot, reduplicated, 

 miniature, and sable) which lie close to vermilion were plotted. The 

 same process was repeated in locating bar with respect to vermilion 

 and the factors about bar with reference to bar. The last step was to 

 interpolate the factors (club, lethal 2, lemon, depressed, and shifted), 

 which form a group about midway between yellow and vermilion. Of 

 these, club is the only one whose location is accurate. The apparent 

 closeness of the grouping of these loci is not to be taken as significant, 

 for they have been placed only with reference to the distant points 

 yellow and vermihon and not with respect to each other; furthermore, 

 the data available in the cases of lemon and depressed are very meager. 



The factors which are most important and are most accurately 

 located are yellow, white (eosin), bifid, club, vermilion, mmiature, 

 sable, forked, and bar. Of these again, white (eosin), vermilion, and 

 bar are of prime importance and will probably continue to claim first 

 rank. Of the three allelomorphs, white, eosin, and cherry, eosin is 

 the most useful. 



