THE ORIGIN OF GYNANDROMORPHS. 



No. da, 6 2 , c. April 1914. E.M.Wallace. Plate 3, figure 4 (colored drawing). 



Parentage. The mother was a white-eosin compound female, carrying the 

 genes for yellow and white in one X and eosin in the other. The father was 

 an ebony male, used to show the lack of elimination of the third chromosome. 



Description. The mosaic was entirely female. The right side of the thorax, 

 the right wing, and the right legs were yellow in color, while the rest of the 

 female, including all of the head and abdomen, was gray. Both wings were 

 of the same size and there was no size inequality in bristles or other parts. 

 There was no sex-comb on the yellow right foreleg. 



Explanations. On the view that this gynandromorph arose from a bi- 

 nuclealed egg, it must be assumed that one of these nuclei must have contained 

 two yellow white bearing X's that arose through equational non-disjunction; 

 the other nucleus contained (as the offspring showed) a yellow white chromo- 

 some. The former nucleus fertilized by a Y sperm gave the yellow parts 

 of the fly (not including right side of head, which is gray red) ; the latter nucleus 

 fertilized by wild-type X sperm (from the ebony male) gave the left side of 

 the fly, including all of head and abdomen. 



Left side. 



Right side. 



W 



y w 



If the mosaic had arisen from a yellow white egg fertilized by the X sperm 

 of the ebony male (whose X chromosome carried only wild-type genes) it 

 would have been easy to explain the case as simple elimination were it not 

 that the yellow parts were unmistakably female, which is impossible without 

 the additional hypothesis of a succeeding somatic non-disjunction. It was 

 next supposed that the mechanism of the production of the mosaic had been 

 double somatic non-disjunction, that the two daughter wild-type X's had gone 

 into the same cell, giving rise to the wild-type left-side female parts, and that 

 the two daughter yellow white X's had both been included in the other cell, 

 which gave rise to the yellow female parts on the right side. On this hypothe- 

 sis the offspring (disregarding ebony) should correspond to those of a pure 

 yellow white female or of a pure wild female. In fact, however, the offspring 

 correspond to those of the original zygote when the mosaic was mated to a 

 yellow white brother: yellow white females, 106; yellow white males, 103; 

 wild-type females, 117; wild-type males, 107; yellow males, 2; white male, 1. 



A possible escape from this dilemma is to suppose that the non-disjunction 

 took place after the first division and that the normal cell was the one which 

 gave rise to the germ-cells. This mosaic would then be triregional the ab- 

 domen and gonads heterozygous for yellow and white and representing the 

 original zygote, the right side of the thorax pure yellow white, the left side of 

 the thorax and the head pure wild-type. 



Another type of explanation is that in the normal XX zygote somatic muta- 

 tion to yellow occurred in the wild-type chromosome, so that the yellow part 

 contains a mutant yellow X and the maternal yellow white X. Or somatic 

 deficiency for the yellow locus occurred in the wild-type X, so that the yellow 

 parts are haploid for yellow, and like the normally haploid male show yellow, 

 while these parts are female because the sex gene is situated in some other 

 part of the X than the yellow-deficient region. 



