GYNANDROMORPHISM 27 



Thus the addition of a Y to XX has no discernible effect 

 upon the sex-differentiation of the individual. The X-bear- 

 ing sperm which usually takes part in the creation of a 

 female here contributes to the origin of a male if it fertilizes 

 a Y-bearing egg. Sex is thus determined by the sex- 

 chromosome distributing mechanism which operates at the 

 time of karyogamy. 



It is now established that the Y-chromosome is not com- 

 pletely inert. Part of it is inert but there is a portion which 

 carries genes and which is homologous gene by gene with 

 the X-chromosome. Both Bridges and Stern (1927) have 

 identified 'fertility' genes in the Y and Sturtevant showed 

 that the reason why the males of a 'bobbed' stock do not 

 exhibit this recessive sex-linked character is that there is a 

 dominant normal gene in the Y. 



Gynandromorphism. The essential feature of the con- 

 dition of gynandromorphism is the presence in one and the 

 same individual of a species in which sex-dimorphism is the 

 rule, of sharply defined regions of the body some of which 

 show the characters of the male, others of which display the 

 characters typical of the female. A gynandromorph is a sex- 

 mosaic in space. A 'lateral' gynandromorph has one half of 

 the body, the left or the right, including the reproductive 

 organs, completely male in its characterization while the 

 other half displays the typical female characterization; in an 

 anterior-posterior gynandromorph the anterior half of the 

 body has the characterization of one of the sexes, the 

 posterior half those of the opposite sex. The sex-mosaic can 

 be much less regular than this, however, most of the 

 regions of the body displaying the characters of one 

 sex and only a relatively small area exhibiting those of the 

 other. 



Time came when this phenomenon of gynandromorphism 

 could with great advantage be investigated in Drosophila 

 melanogaster . The genetic and cytological analysis of this 

 fly came to be very advanced. The spacial relationships in 

 the different chromosomes, both autosomes and sex-chro- 

 mosomes, of several hundreds of genes were quickly 



