Different Types of Sex Determination 475 



only one F and possibly many hemizygous m, which then are epistatic 

 to the F. The same is true for the homozygote: an FF is not acting 

 diflFerently from one dominant F, and a number of homozygous m are 

 present. Whiting calls this "complementary sex determination." If we 

 would express this scheme in terms of the former discussion, we would 

 have to say that here a unique condition obtains: the M and F 

 determiners are not located in different chromosomes, but both within 

 the X-chromosome; further, they are completely linked here; and the 

 F/M balance is such that F has a high potency which is additive in 

 compounds but not in homozygotes; the M potency is such that it has 

 no action in the heterozygous condition ( recessiveness of m) but an 

 additive one in hemizygotes and homozygotes, so that MM ( = m o, 

 or mm, or ma ma mb mb) is epistatic over F or FF. If this remarkable 

 scheme turns out to be correct, we could conclude that here also the 

 F/M balance system is at work, but, by changing place so as to be 

 confined in an X-chromosome, it permits the variant of sex deter- 

 mination by haploidy. This might be called a new type of sex deter- 

 mination, but I prefer to consider it only an extreme variant of the 

 general scheme. Thus the general genetic theory of sex determination 

 is, in spite of all variants, unified and rather simple. 



