244 



NATURE 



[February 23, 1922 



grey male. Fertilised by a Y-sperm, the two 

 kinds of eggs give XXY yellow females and YY 

 individuals (which die). Thus, of the four kinds 

 of individuals expected, half the females (XXX) 

 and half the males (YY) die, and a sex ratio 

 of I : I remains. It has been stated above that 

 XXX females appear at rare intervals. These 

 are grey and are recognisable as XXX in- 

 dividuals by certain stigmata, and have been 

 shown in sections to possess the three X's. 



(4) Drosophila, and presumably other animals 

 belonging to the XX-XY type, are so constituted 

 that they can develop with one X or with two 

 X's, provided the other chromosomes are present 

 in duplex. In short, sex determination has been 



?^ 





regulated along these lines. Failure to obtain 

 similar situations in the case of the other chromo- 

 somes led us to suppose that an individual lacking 

 one or both members of a pair could not "come 

 through " ; but we had no actual proof that this 

 was the explanation of their absence. Neverthe- 

 less, it was anticipated that it might be possible 

 for individuals lacking one or containing three of 

 the very tiny IV-chromosomes (Fig. 9) to survive. 

 Recently Bridges has found such individuals, and 

 we now realise that they must have been rather 

 frequently met with in the past, but were not 

 recognised as such. A fly with only one of the 

 IV-chromosomes is small, pale, hatches late, has 

 small, slender bristles and a dark trident. The 



4« 



XX 





Fig. g. 



wings are blunt and slightly spread, and the eyes 

 large and roundish. If a female, it is expected to 

 contain two kinds of mature eggs {i.e. eggs after 

 the polar body has been extruded) — one kind with, 

 the other kind without, a IV-chromosome. The 

 ^%^ with one IV-chromosome gives a normal 

 result when fertilised. The ^^^ without a IV- 

 chromosome, if fertilised by a sperm carrying a 

 recessive IV-chromosome character, produces an 

 individual {6 or 9) showing the recessive char- 

 acter of the father, because the single IV-chromo- 

 some of this individual came from the father that 

 carried the recessive in question. A male that has 

 only one IV-chromosome in its cells produces two 

 kinds of sperm, one with IV and one without IV. 

 Mated to a normal female, the results are in 

 NO. 2730, VOL. 109] 



principle the same as above. A male and a 

 female, each with only one IV-chromosome, when 

 mated, might be expected to give some individuals 

 (25 per cent.) without a IV. None such appear, 

 and the ratios show that they die. 



Individuals with three IV 's are also known. 

 Their characteristics are the opposites of those 

 shown by the haplo-IV's. For example, they are 

 dark with a very faint trident, long-bristled, and 

 have small, smooth eyes. Their wings are long 

 and narrow. Females of this kind produce two 

 kinds of eggs, one kind with two IV 's, the other 

 kind with one IV. Mated to a normal male, with 

 a IV-chromosome recessive character, such 

 females produce daughters and sons of two kinds, 

 namely, one kind with three IV 's, like the mother, 

 and the other kind normal. 



If these males and females, triploid for IV, are 

 mated, the recessive character appears in only 

 4 per cent, instead of the Mendelian 25 per cent, 

 of the offspring, as would be expected when one 

 recessive and two dominant characters are in- 

 volved. 



Many combinations between triploids and hap- 

 loids are possible, and unique ratios are expected. 

 These have also been worked out. Cytological 

 preparations of triplo- and haplo-IV's show in 



one case three small chromosomes, and in the 

 other only one. 



(5) Complete triploid individuals having three of 

 each kind of chromosome have recently been 

 found by Bridges (Fig. 10). The triploid flies are 

 larger and coarser than normals, and also have 

 large, rough eyes. Their eggs, as shown by 

 genetic tests, contain all possible combinations of 

 chromosomes, behaving as though non-disjunction 

 takes place independently in each set of three. 



Amongst the offspring of a triploid female 

 (mated to a normal male) there is one class that 

 has three II's, three Ill's, and three IV 's, but with 

 two X-chromosomes. This individual is an inter- 

 sex, more like a male than a female. There 

 is another class that has three II's, three Ill's, 

 but only two IV 's. It also is an intersex, but 

 more like a female. 



Thus sex itself, in this animal, is shown to be 

 an expression of a balance between the X-chromo- 

 somes and the rest of the chromosomes. The 

 results show that the differentials which determine 

 sex are not confined to the sex-chromosomes alone. 

 Some appear to be in the II- and Ill-chromo- 

 somes, and others in the IV-chromosome. 

 (To be continued.) 



