718 



SCIENCE 



[N. S. Vol. XXXVI. No. 934 



SPECIAL ABTICLES 



THE EXPLANATION OF A NEW SEX RATIO IN 

 DROSOPHILA 



Extraordinary sex ratios have appeared at 

 three different times in our stocks of Droso- 

 phila. Quackenbush described the first case.' 

 Miss E. Eawls met with another case. Her 

 results are now in press." A third case has 

 quite recently appeared in one of my other 

 cultures not related to the last. During the 

 past summer Miss Eawls turned over some of 

 her stock to me. At that time some females 

 were producing two females to one male, and 

 other females equal numbers of both sexes. 



If sex is determined by a factor in the sex 

 chromosomes it seemed probable that some 

 change had occurred in this chromosome. 

 Several possibilities suggested themselves and 

 were tested by means of the following crosses. 

 I mated virgin females (red eyes) of Eawls's 

 stock, in pairs, to white-eyed males. All the 

 offspring had red eyes. Some of the F^ fe- 

 males gave the 2:1 ratio. When these fe- 

 males were bred to white-eyed males again 

 the following results were obtained: 



Bed 2 



Red c? 

 2 



White ? 

 452 



White t? 

 374 



The unusual ratio is evidently due to the 

 almost complete disappearance of the red- 

 ayed males, equality in all four classes being 

 the normal expectation for this cross. On the 

 face of these returns it seemed likely that 

 some lethal factor must be contained in the 

 single sex chromosome of the lost males. The 

 lethal portion of this chromosome is derived 

 from the red-eyed grandmother that gave the 

 abnormal sex ratio. If this is the correct ex- 

 planation, then, as the following analysis 

 shows, all the red-eyed females in the last re- 

 sult should give a 2 : 1 ratio irrespective of the 

 male to which they are bred. This, in fact, is 

 the case. Similarly, the white-eyed females 

 should give the usual 1 : 1 ratio ; and this also 

 proved true. The only doubtful point is the 



^Science, 1910. 

 " Biol. Bulletin. 



case of the two red-eyed males. If the lethal 

 factor contained in the chromosome in ques- 

 tion should occasionally " cross over " from the 

 red factor, then a red-producing chromosome 

 would result, which, if it went into a male, 

 should give a normal male. To test this these 

 males were united to normal females and gave 

 normal sex ratios. The daughters of these 

 were then tested individually and all have pro- 

 duced normal ratios. The explanation holds. 

 Conversely, there are expected a few white fe- 

 males due to crossing over that contain the 

 lethal factor. The chance of obtaining one is 

 approximately 1 in 200 times. As yet this 

 test has not been carried out. The formulas 

 which illustrate the relation just described 

 are as follows: 



Let X = the ordinary sex chromosome and 

 X the sex chromosome that carries the lethal 

 factor. The factor for red eye, R, and its 

 allelomorph for white eyes, W, are carried by 

 the sex chromosomes. The original female 

 that gave a 2 : 1 ratio would have the formula 

 •S^ — Rx and the white-eyed male WZ — 

 Then 



Bed 2 EX — B.X 



White (? WX~ 



MX WX = red $ 



Sx WX = red ? 



MX = red c? 



Of the two kinds of red females we are con- 

 cerned here only with RxWX. If she is mated 

 to a white-eyed male the results are: 



Mx —WX 

 WX 



WX WX = white ? 

 Mx WX = red $ 



WX == white c? 



Mx =— 



In both cases the male with the lethal fac- 

 tor does not develop, or dies. He does not, 

 therefore, appear in the results except in those 

 rare cases (the two cases above) where an in- 

 terchange takes place between the sex chromo- 

 some in the female Rx — WX, so that there 

 results RX—Wx. 



