CYTOLOGY AND MEN DELI AN HEREDITY 



185 



selected for study because the}^ contained 

 two A'-chromosomes, one from each 

 parent. Some of them had white eyes, 

 indicating a loss or mutation of the domi- 

 nant W originally present in the father's 

 A"-chromosome. When these and other 

 females were tested by further crosses, the 

 chromosomes in the salivary glands re- 

 vealed the conditions shown in Fig. 130. 

 These chromosomes, it will be recalled. 

 (Fig. 67), are synapsed pairs, each of them 

 representing two parental homologues in 

 intimate lateral union. Since the union 

 is so precise, band for band, dissimilarities 

 in the two can be clearly analyzed. 



The three cases in the diagram includ(> 

 deletions in the same general region of the 

 paternal A'-chromosomes, and by compar- 

 ing these with the normal maternal chro- 

 mosomes with which they are in synapsis 

 it can be seen that where the band indi- 

 cated by an arrow is present in the lower 

 (paternal) longitudinal half of the chromo- 

 some pair the eyes of the fly are red, in- 

 dicating the presence of W, whereas the 

 eyes are white when this band is lacking 

 in this half. From this it is concluded 

 that the gene W was removed in the por- 

 tion deleted in two of the three cases, 

 leaving the recessive w to function alone 

 and produce white eyes. This gene affect- 

 ing eye color is accordingly assigned to a 

 small region occupied by this band. This 

 region represents considerably less than 1 

 per cent of the length of the chromosome. 

 In this manner a large number of genes 

 have now been located with various de- 

 grees of accuracy in the chromosomes of 

 Drosophila. 



Fig. 131. — Diagram comparing the linkage map 

 of the X-chromosome of Drosophila melanogaster 

 with the salivary-gland chromosome map for some 

 of the genes that have been located. Explanation 

 in text. {Adapted from T. S. Painter and (). 

 Mackensen.) 



SI. 5 sd 



33.0 V 



27.7 Iz 



20.0 ct 



13.7 cv _ 



