HABROBRACON 3I 



eye-colour is mated with a wild-type male (black eye- 

 colour) the daughters are black-eyed and the sons orange- 

 eyed. 



But it was found that in matings in which the orange-eyed 

 mother and the black-eyed father were from the same stock 

 or from stocks closely related through inbreeding, black- 

 eyed sons appeared. By genetical experiment and by cyto- 

 logical examination these were shown to have received a 

 chromosome set from both the father and the mother, having 

 20 altogether and being heterozygotes. 



The hypothesis elaborated to account for these 'biparental 

 males' rests on the assumption that the normal female is a 

 heterozygote in respect of two sets of multiple sex alleles or 

 differential chromosome segments which for purposes of 

 discussion may be designated X^ and X^ respectively. There 

 are two kinds of the normal haploid male, X^ and X^. The 

 'biparental male' is aX^iaA or aX^iaA, with the same 

 genie balance as the ordinary haploid male X^:A or X^:A. 

 The normal diploid heterozygous female is X^:A/X^:A. 

 So that the male is N^ or N^, the female NVN^, and the 

 'biparental male' N^/N^ or N^/N^. 



The fact that inbreeding yielded a marked increase in the 

 production of 'biparental males' resisted satisfactory ex- 

 planation for a long time (Bostian, 1934; Whiting, 1935; 

 Snell, 1935). Ultimately Dordick( 1 937) was able to show by 

 a number of ingenious experiments that the low incidence 

 of biparental males (i such to 9 biparental females in the 

 ordinary laboratory stocks) was due to the conversion by 

 gene action of the biparental male into a female, the gene or 

 genes concerned being resident not in the X but in another 

 chromosome, the so-called Z. Thus in Habrobracon the 

 existence of multiple sex-differentials in different chromo- 

 somes was disclosed and the notion of genie balance shown 

 to apply. 



Dreyfus and Breuer (1944) found in another parasitic 

 wasp Telenomus fariai a special chromosome mechanism 

 which makes inbreeding compatible with a method of sex- 

 determination resembling that in Habrobracon. 



In the Iceryini the sex-determining mechanism is of the 



