276 



NATURE 



[March 2, 1922 



vestigial male of pure stock, the Fg offspring will be 

 of four kinds, in the proportions given below : — 



Non cross-overs 



i>*On ClU33-UVCi3 



Black vestigial Grey long 



41*5 per cent. 41 '5 per cent. 

 83 per cent. 



Cross-overs 

 Black long Grey vestigial 



8-5 per cent. 8-5 per cent. 



17 per cent. 



In this experiment 17 per cent, of crossing-over occurs 

 in the Fj female. As before, the relation of these 

 facts to the chromosomes is illustrated by the rods 

 in the centre of the diagram. The two pairs of elements 

 (genes) involved are indicated by the letters inside the 

 rods. 



Many examples of linkage and crossing-over are 

 known at the present time. Linkage is said to be 

 strong when, as in the yellow-white case, crossing-over 

 takes place in a small proportion of cases. Linkage 

 is said to be weak when crossing-over takes place 

 frequently. Crossing-over may be less than i per cent., 

 or even not take place at all (complete linkage), as in 

 the case of the black vestigial male given above. It 

 may take place in nearly 50 per cent, of the individuals 

 of a back-cross, which means that about half of the 

 flies show linkage, and half show crossing-over. This 

 would be, of course, numerically the same result as 

 when the two pairs of characters involved freely assort. 

 A case of this kind could not, in fact, by itself alone 

 be distinguished from a case where the pairs are carried 

 by different chromosomes. It may appear, therefore, 



NO. 2731, VOL. 109] 



incorrect to speak here of Unkage, and this would be 

 true were there no other evidence showing that the 

 two characters involved are in the same chromosome. 

 But whenever a number of other characters are known 

 in the same group the linkage of the two characters 

 giving 50 per cent, of crossing-over can still be shown, 

 for if each of the characters is found to be linked to a 

 third one they must be linked to each other. 



In Drosophila there are more than one hundred 

 sex-linked characters. If their linkage relations are 

 studied in series an important result comes to light. 

 This may be illustrated by the following example. 

 It has been stated that crossing-over takes place in 

 I '5 per cent, of cases between yellow colour and white 

 eyes. There is another eye character, called echinus. 



Fig. 13. 



that gives 5-5 per cent, of crossing-over with yellow 

 (Fig. 14). If, now, the position of echinus is represented 

 as 5*5 units of distance from yellow, then its " distance " 

 from white must be either 5"5-f-i'5 =7'0, if it lies to 

 the "north" of yellow, or else 5-5- 1-5 = 4-0 if it Hes 

 to the " south." In fact, when the experiment is 

 made, the percentage of crossing-over between white 

 and echinus is found to be 4-0. 



There is another sex-linked character, ruby, that 

 gives 7*5 per cent, crossing-over with yellow. If it 

 lies to the north of yellow it must give with echinus 

 7'5 + 5'5 = 13 J or if to the south of yellow, 7-5 - 5*5 = 2-0. 

 It is found to give 2 per cent, of crossing-over. Hence, 

 lying south of yellow, it should give with white 6*o, 

 and this is what is found. 



Such a method of analysis can be followed step by 

 step until the whole of the sex-chromosome is plotted. 

 This procedure has a twofold significance. First, if 

 a new mutant character is found, its " linkage-group " 

 is first made out ; then its " distance " from any one 



