138 THE SECOND-CHROMOSOME GROUP 



distinguished from the wild tjrpe, and because of the lethal action of 

 the truncate gene this can not exceed two-thirds of the flies. 



If some of these modifiers could themselves produce a moderate 

 truncation, then the percentage would pass beyond 67, but there is 

 evidence that the modifiers are unable to produce truncation in the 

 absence of the chief gene. 



TRUNCATE LETHAL. 



Since at least one-third of the flies must look like wild flies rather 

 than tnmcates, their non-appearance in the highly selected stock is to 

 be accounted for by the assumption of a lethal gene which is carried 

 in the II chromosome, homologous to that carrying the truncate 

 gene. This condition has not been proved by testing, but the action 

 of autosomal lethals is well understood and a similar case of an auto- 

 somal lethal giving an apparently pure-breeding stock when balanced 

 against an autosomal dominant which is itself lethal when homozygous 

 (beaded) has been demonstrated by Muller. 



It is now clear what is the meaning of the early history of the 

 truncate stock which was for so long a puzzle. The original truncate 

 fly was heterozygous for the dominant truncate gene. In a cross to 

 wild- type sisters only 10 per cent of the offspring showed the truncate 

 character, though half of them carried the truncate gene. This low 

 power of expression of the character was due to the lack of effective 

 mf)difiers, most of which were recessive. 



The first period of inbreeding and selection resulted in the collect- 

 ing of whatever modifiers were present, and by the approach to homo- 

 zygosis allowed more of the recessive ones to show their effect. But 

 by such means the proportion of truncate could not be advanced 

 beyond 67 per cent and was not advanced much beyond 50 per cent. 



The limit of 67 per cent imposed by the lethal nature of the primary 

 truncate gene must have been broken down by the occurrence (Febru- 

 ary 1911) of the lethal mutation in the chromosome homologous to 

 that carrying the primary truncate gene. The second period of selec- 

 tion then increased the percentage of truncates by increasing the preva- 

 lence of this lethal in the stock. The limit which this second selection 

 approached was the condition in which every second chromosome 

 which was not carrying the truncate gene should carry the lethal. By 

 this means all the non-truncate flies would be eliminated except those 

 saved by crossing-over. If the lethal were 20 units away from trun- 

 cate, measured along the second chromosome, then 10 per cent of not- 

 truncate flies should survive. The very low productivity of the 

 truncate stock observed at this time was the result of the action of the 

 two lethals — all homozygous truncates as well as all homozygous 

 lethals dying as early zygotes; also the modifiers tending to produce 

 infertility when homozygous. While the proportion of truncates was 



