208 Basis of Sex Determination 



experiments between the expected and observed result 

 cannot well be an accident. The fact that the inheri- 

 tance of sex-linked characters in man follows the same 

 laws as in Drosophila is a strong argument in favour of 

 the assumption that in man, also, sex is determined by 

 two kinds of spermatozoa. 



Morgan and his students discovered no less than 

 thirty-six sex-linked characters in Drosophila, and each 

 behaved in a similar way to the red and white eye 

 colour in regard to sex-linked inheritance, so that the 

 chromosome theory of sex determination rests on a safe 

 basis. That sex is merely determined by the number 

 of X chromosomes, not by the Y chromosome, is proved 

 by the facts that the Y chromosome may be completely 

 absent as in Protenor and that Bridges 1 has found a 

 type of female Drosophila with a chromosome formula 

 XXY whose sex was not affected by the supernu- 

 merary Y. 



3. On the basis of all these experiments and theories 

 it is comparatively easy to explain a number of pheno- 

 mena concerning sex ratios which before had been very 

 puzzling. In bees it had been shown many years ago 

 by Dzierzon that the males develop from unfertilized 

 eggs while the females, queens and workers, develop 

 from fertilized eggs. This is intelligible on the assump- 

 tion that the unfertilized egg contains only one X 

 chromosome while the spermatozoon carries into the 



3 Bridges, C. B., Genetics, 1916, i. f i. 



