376 



LINKAGE PHENOMENA 



linked in the sex chromosome. This is in fact true and 

 we may now proceed to examine this matter more closely. 

 Since the vinegar fly has 2 X's in the female and one X 

 and one Y in the male, it is evident that the female can 

 be pure (i.e., have the same genes in both chromosomes) 



'1 



B-* 



»-V 



-b 



Fig. 110. — Diagram to show crossing-over of genes. I shows 

 the arrangement of genes in the 2nd pair of chromosomes of the hy- 

 brid before crossing-over has occurred. II shows the recombinations 

 after crossing-over has occurred. Segregation of the chromosomes in I 

 will give gametes like the parents; of the ones in II will give recom- 

 binations. In this particular instance crossing-over occurs in about 

 17 % of the gametes. B=gray body; b -black body; V-Iong wings; 

 v -vestigial wings. 



for genes carried in the X chromosomes, but that the 

 male cannot, because he has only one X and the Y has 

 not been shown to carry any genes. An illustration or 

 two will make the matter clear. 



Red-eyed Female X White-eyed Male Fly (Fig. 

 111). — The gene for eye-color in this cross is located in 

 the X chromosome. Red is dominant to white. The 

 red-eyed female produces eggs all of which have one X 

 chromosome containing the gene for red-eye. Half the 

 sperms have an X with the gene for white-eye and half 

 have the Y chromosome. When " white " X-sperms fer- 

 tilize the " red " X-eggs, red-eyed females result ; because 

 the red is dominant to the white. When Y-sperms fer- 

 tilize " red " X-eggs, red-eyed males result, because the 

 only X present has a " red " gene. 



