32 SEX-DETERMINATION 



haplo-diploid type. The genie balance concept of sex- 

 determination cannot be applied since the ratio of male- 

 determining to female-determining genes is the same both 

 in the haploid male and the diploid female. Formal explana- 

 tion is possible by means of the hypotheses advanced by 

 Goldschmidt (1920) and by Schrader and Sturtevant (1923). 

 The former suggests that precocious activity by the male- 

 determining genes in the diploid dose and prior to reduction 

 predispose all eggs to a male pattern of sexual differentia- 

 tion and that this is then overridden in fertilized eggs by 

 the delayed activity of two sets of female-determining 

 genes. But this 'turning-point' hypotheses is not gener- 

 ally accepted. The algebraic sum hypothesis of Schrader 

 and Sturtevant, helpful and ingenious as it is, is not suitable 

 for general application. 



Sciara. Metz (1938) and his colleagues studied the genetic 

 and cytological aspects of sex-determination in the dipteran 

 fungus-gnat Sciara coprophila over a long period of years 

 and have recorded much that is remarkable. A given female 

 produces offspring predominantly of one sex. Among the 

 families in which most of the progeny are females there are 

 two types of females, female-producers and male-producers, 

 indistinguishable on inspection and in respect of the be- 

 haviour of their chromosomes. They differ genetically, 

 however. The female-producers may be designated XX^, 

 being heterozygous for a gene in the X, and the male- 

 producers as XX, being homozygous for the recessive allele 

 of this gene. Males are genetically XOiAA in somatic con- 

 stitution and produce one type of sperm, XXA. A given 

 female produces the same kind of offspring, whether sons 

 or daughters, irrespective of the origin of the male to which 

 she is mated. 



The mode of inheritance of certain mutant characters is 

 peculiar. Metz (1927) used an autosomal recessive mutant 

 truncate wing. Truncate $ x wild-type ^ gave none but 

 wild-type. Some of the F^ families were mostly daughters, 

 others mostly sons. F.i o x homozygous truncate ? gave 

 all truncate, daughters in some families, sons in others. But 



