SEX-LINKED HEREDITY 



283 



While in the birds the chromosomal condition has never been 

 completely worked out, on account of inherent technical difficulties 

 likely to be overcome any day, the case of the currant moth, Abraxas, 

 has been thoroughly analyzed. The sex linkage follows the poultry 

 plan and the gametes of the male have been found to be all alike, while 



Fig. 72. — Reciprocal cross to that shown in Figure 71. P, parents, unbarred 

 male, barred female; F I( barred males, unbarred females (crisscross inheritance); 

 F 2 , barred and unbarred birds equally numerous in both sexes. (From Castle.) 



those of the female are of two types, one containing an X-chromosome 

 (W-chromosome) and the other a Y-chromosome (Z-chromosome). 

 The striking parallelism between the reversal in sex-linked heredity 

 and in the visible reversal of chromosome composition in these two 

 groups of animals (Drosophila and man, on the one hand, and the 

 butterflies, moths, and birds, on the other) offers one of the most 

 cogent proofs of the validity of the chromosome theory of heredity, 

 which we have already come to rely upon and shall have further occa- 

 sion to make use of later on. 



