190 PHYSICAL BASIS OF HEEEDITY 



binatioiis of the two sexes, and it is probable that g^Tian- 

 dromorphs may arise in more than one way, but in Droso- 

 pliila it can be demonstrated that the great majority of 

 gynandromorphs result from dropping out of one of the 

 sex-chromosomes at some early division of the fertilized 

 ^gg. The demonstration is made possible by using sex- 

 linked characters that are known to be carried by the sex- 

 chromosomes. For example : Yellow body color in Droso- 

 pJiila is due to a recessive gene carried by the X-chromo- 

 some. Its allelomorph (wild type) lies also, of course, in 

 the normal X-chromosome. If yellow is crossed to wild, 

 and a bilateral gynandromorph should arise, it may be 

 yellow on the male side (as seen in the yellow wings and 

 yellow hairs over half the body) and wild type on the 

 female side (Fig. 87). 



Since the male characters arise when only one sex- 

 chromosome is present, it must be the yellow-bearing 

 chromosome in this case that gives the male side. Since 

 the female characters arise when two X's are present, 

 both must be present in the female side, which will here 

 be the wild type, since the gene for wild type domi- 

 nates the yellow-producing gene. The gynandromorph 

 must have arisen, therefore, at a very early nuclear divi- 

 sion in the egg in which one daughter X-chromosome failed 

 to pass into one of the daughter nuclei. The diagram 

 (Fig. 88) shows how such a result might be supposed to 

 have come about. 



The diagram indicates that one daughter chromosome 

 X' (bearing the gray gene) has failed to become incor- 

 porated in its proper nucleus, which is therefore left with 

 only one X. From this nucleus the nuclei of the male half 

 are produced, while from the XX nucleus the nuclei of the 

 female half arise. That both of these nuclei, the XX and 

 the X nucleus contain other chromosomes derived from 

 both parents has been shown by making one of the original 

 parents homozygous for some recognizable autosomal 



