OTHER METHODS OF SEX-DETERMINATION 227 



kinds of males, X Y m and X Y era . The latter are rare, one 

 ont of 73 sons. 2 Whether crossing-over also occurs in the 

 female cannot be determined from Winge 's data, since he 

 gives no cases of X e X M females. Moreover, he represents 

 one type of female as X and implies that the X -chromo- 

 some is lacking in certain genes. Two pairs of genes are 

 necessary to show crossing-over when two X's are pres- 

 ent. In fact, Winge represents an X e that has crossed over 

 to a Y m as X without representing the reciprocal allelo- 



_m . — x _m 



m II \/ m 



non cross over crossover 



Fig. 132. 

 Diagram illustrating the possible relations of an attached X-chro- 

 mosome to crossing-over between the autosomal portion of this 

 compound chromosome and the autosome (the male of the latter) 

 here called Y. 



morphic change. The full formula should represent one 

 of the X's containing the genes M and e, and the Y, in 

 this case, as containing the genes m and E. After cross- 

 ing-over the X would then contain E and M and the Y 

 would contain e and m, as shown in Fig. 132. The X-chro- 

 mosome after crossing-over is not X , but X ME , and the 

 Y-chromosome Y me . If m and e are dominant over M and 

 E, the results would be as recorded, except that another 

 cross-over is expected, namely, X ME . If the part of the X 

 to the left of the M contains the sex-determining genes 

 (the heavier part of the X in the figure) the absence of 

 this cross-over in the experiment might be explained as 

 due to the proximity of M to the X-component. 



Aside from these questions of interpretation, the re- 



2 In another experiment 4 cross-overs out of 68 sons are recorded. 



