482 The Determination of Sex 



possibility is the correct one. According to this evidence, the 

 X chromosome contains one or probably a nmiiber of female- 

 determining genes, whereas numerous male-determining genes 

 are located in the autosomes (or at least the balance of the sex- 

 determining genes in the autosomes is male-determining) ; the 

 ratio of the sex chromosomes to the autosomes is the important 

 consideration. 



In a normal female, there are two X chromosomes and two sets 

 of autosomes so that the ratio is 1 : 1, but in the normal male 

 only one X chromosome is present and the ratio is 1 : 2. Appar- 

 ently the male-tendency genes in the autosomes are not so ef- 

 fective as the female-tendency gene or genes which are in the 

 X chromosome for a 1 : 1 ratio produces a typical female, and 

 the normal male appears only when there are twice as many sets 

 of autosomes as X chromosomes. Intermediate between these 

 normal individuals are the intersexes which Bridges has de- 

 scribed. These forms can be considered as modified triploids and 

 have three sets of autosomes but only two X chromosomes. Con- 

 sequently the ratio of X chromosomes to sets of autosomes is 

 1 : 1.5. With respect to their ratios these flies are intermediate 

 between females and males, and in their external appearance 

 they are complex mixtures of both female and male characters. 

 They are not all alike and vary from some that have mostly 

 male characters to others that are largely female-like. Very few 

 lack the sex comb, a structure normally found on the forelegs of 

 males. Intersexes are mixtures of phenotypically female-like 

 and male-like parts although they are not composed of genetically 

 male and female parts, for all the cells of an intersex have the 

 same genetic constitution. Apparently during development each 

 structure can develop only as in a female or as in a male and 

 cannot be intermediate. Apparently each fly starts to develop as 

 a male and subsequently switches over and develops as a female. 

 The earlier this switch occurs, the more female-like will be the 

 fly. If they develop late in ontogeny, male characters are less 

 and female characters more likely to be normal. Intersexes are 

 sterile. 



If an individual possessing three sets of autosomes has only 

 one X chromosome, its ratio is 1 : 3. Such flies are supermales. 

 Their viability is poor and they are highly sterile. At the other 

 end of the list are the superfemales with three X chromosomes 



