106 



CHAPTER 8 



some balance as the typical basis of sex de- 

 termination in Drosophila. 



What is the relationship between \-auto- 

 some balance and tra, the sex-transforming 



gene'.' Sex is determined by X-autOSome 

 balance when the individuals carry tra + , 

 which they normally do. When tra is homo- 

 zygous, however, the balance view does not 

 apply and 2X + 2A sets produces a male. 



Gynandromorphs 



On relatively rare occasions, abnormal Dro- 

 sophila appear with some of their parts 

 typically male and the remainder, typically 

 female. Such individuals are said to be 

 mosaic for sex traits; sex mosaics are also 

 called gynandromorphs or gynanders (Fig- 

 ure 8-5). The male and female parts are 

 clearly demarcated in such flies, sometimes 

 the front and hind halves, at other times 

 the right and left sides are of different sex. 

 The sharp borderline between male and 

 female parts in an insect gynander is due 

 to the relatively small role that hormones 

 play in insect differentiation, so that each 



figure 8-5. D. melanogaster gynandromorph 



whose left side is female and right side is male. 

 (Drawn by E. M. Wallace.) 



body part is formed according to the geno- 

 type it contains. In view of the preceding 

 discussion, one would predict that the dip- 

 loid cells in the female part of a gynander 

 contain XX and those in the male part X, 

 the chromosome number being otherwise 

 normal. If this prediction is correct, then 

 approximately half-and-half gynanders could 

 originate as follows: the individual starts as 

 a zygote containing 3AA -+- XX — that is, as 

 a female. The first mitotic division of the 

 zygotic nucleus is abnormal — one daughter 

 nucleus contains 3AA + XX and is nor- 

 mal, the other daughter nucleus contains 

 3AA + X and is defective, because one of 

 the X's failed to be included in this nucleus, 

 degenerated, and was lost. However, sub- 

 sequent nuclear divisions are normal — 

 cells produced following mitosis of the XX 

 nucleus and its descendants giving rise to 

 female tissue, and cells derived from the X 

 nucleus giving rise to male parts. In this 

 case the gynander has about half its body 

 male and half female. If, however, the X is 

 lost at some later mitosis, a correspondingly 

 smaller portion of the body will be male, 

 explaining gynanders one quarter or less 

 male. 



We can test whether this explanation is 

 sometimes correct by making use of an X- 

 linked gene which produces a phenotypic 

 effect over a large portion of the body sur- 

 face; that is, a gene that affects the size and 

 shape of the bristles and hairs. Such a gene 

 is forked, two of its mutant alleles being 

 / 346 and /. In homozygotes (females) and 

 hemizygotes (males), / :: "' produces bristles 

 and hairs of normal length and shape; / 

 causes them to be shortened, split, and 

 gnarled. The f' ih f heterozygotes have 

 bristles and hairs slightly abnormal in these 

 respects, showing a "weak forked" pheno- 

 type. If a cross is made to produce female 

 offspring that are f" ib /f heterozygotes, the 

 following predictions can be made regarding 

 the phenotype of the gynanders occasionally 



