Sex Determination (/) 



97 



marcated in such flies, sometimes front and 

 hind halves and other times right and left 

 sides are of different sex. On the hypothesis 

 that in Drosop/iila, as in the grasshoppers 

 mentioned, XX makes for female and X for 

 male, it would be predicted that in gynan- 

 dromorphs each of the diploid cells in the 

 female part would contain XX, and each of 

 those in the male part X. If this is correct, 

 then approximately half-and-half gynanders 

 could originate in the following way. The 

 individual starts as a zygote containing 

 3AA -f XX, that is, as a female, and the 

 zygotic nucleus divides mitotically to produce 

 two nuclei. But, on this occasion, the first 

 mitosis is abnormal. While one daughter 

 nucleus is normal, containing 3AA + XX, 

 the other daughter nucleus is defective, con- 

 taining 3AA + X, because one of the X's 

 it should contain failed to be included in this 

 nucleus, degenerated, and was lost. Subse- 

 quently, however, nuclear division is normal, 

 cells produced following mitosis of the XX 

 nucleus and its descendents giving rise to 

 female tissue, and cells derived from the X 

 nucleus giving rise to male parts. In this case 

 the gynander would have about half its body 

 male and half female. If, however, the X 



FIGURE 13-1. D. melanogaster gynandromorph 

 whose left side is female and right side is male. 

 {Drawn by E. M. Wallace.) 



were lost at some later mitosis, a correspond- 

 ingly smaller portion would be male. This 

 would explain gynanders containing one 

 quarter or less of the body as male. 



What evidence can we obtain that this may 

 sometimes be the correct interpretation? Let 

 us make use of an X-linked gene which pro- 

 duces a phenotypic effect over a large portion 

 of the body surface, that is, one affecting the 

 size and shape of the bristles and hairs. This 

 gene is forked, two of its alleles being /^^* 

 and /. Homozygotes (females) and hemi- 

 zygotes (males) for/^-"' have bristles and hairs 

 of normal length and shape, those for /have 

 these shortened, split, and gnarled; /''^V/ 

 heterozygotes have bristles and hairs slightly 

 abnormal in these respects, showing a "weak 

 forked" phenotype. If a cross is made so 

 that the female offspring are heterozygotes as 

 just indicated, the following predictions can 

 be made regarding the phenotype of gyna-nders 

 among the siblings. All gynanders having 

 the origin postulated will be weakly forked 

 in their female parts; on the other hand, the 

 male parts will have either normal or strongly 

 forked bristles and hairs depending upon 

 whether the X carrying / or /^^^ respectively, 

 was the one lost. The results actually ob- 

 tained exactly confirm these expectations. 

 Moreover, such a gynander may lay eggs if 

 its posterior part is female, but it is sterile 

 if its posterior half is male, as expected from 

 XO males. 



While most Drosophila gynanders can be 

 explained in this manner, there are some 

 which originate another way. With extreme 

 rarity, an abnormal egg is produced after 

 meiosis, which contains not one but two 

 haploid gametic nuclei. Now, because poly- 

 spermy takes place in insects, that is, because 

 more than one sperm can enter an egg (al- 

 though only one is used in fertilization), one 

 of the two haploid egg nuclei may be ferti- 

 lized by an X-carrying sperm, the other by 

 a Y-carrying one. Resultant individuals 

 would be approximately half-and-half gynan- 



