SEX-LINKED LETHALS IN DR060PHILA 97 



THE SECOND LETHAL FACTOR 



In an experiment with certain stock which had been inbred 

 for three years, a pair produced: 



73 females and 16 males ( = 5:1) 



These numbers represent the total output of this pair, or at 

 least all the flies that were produced from one bottle. (A) 

 Twenty-two of the seventy-eight females were mated to white 

 miniatm'e (of which two pairs produced nothing). (B) Twenty- 

 nine of the seventy-eight females were mated to eosin vermilion 

 males (of which six pairs produced nothing). (C) Twenty- 

 four females of the seventy-eight were mated to eosin miniature 

 males (of which fom' pairs produced nothing). The experi- 

 ments with white and with eosin should give similar results, since 

 white (w) and eosin (w^) are allelomorphs. 



The sixteen males were mated to eosin miniature, white 

 miniatiu'e, and eosin vermilion females, but every one of these 

 males proved sterile. 



The output of the three lots of females mated to the respective 

 males mentioned above is shown in A, B and C of table 10. 

 Under each column. A, B, C, the progeny is provisionally classi- 

 fied, first, into those that give approximately, a 1:1 ratio, and 

 then those that gave approximately a 2:1 ratio (or higher). A 

 horizontal line separates these two classes. 



The classification into these two groups may appear arbitrary, 

 since the ratios in the two classes come very near to each other 

 in some cases. For the present it will suffice to explain that 

 the 2:1 ratio is expected if one lethal is present, the 1:1 ratio if 

 no lethal is present. 



From table 10 one can get no clue as to the location of the 

 lethal factor that is assumed to be upsetting the normal sex- 

 ratio, but by breeding the females to a male that carries the 

 same sex-linked factors as their fathers, the location of the lethal 

 factor is revealed. 



