62 GENETICS 



from the males. The individuals seem as it were to be more 

 female than those containing but two X's. Bridges calls 

 them super-females. 



From these experiments we discover that when two nor- 

 mal sets of autosomes are present, lack of an X is fatal; the 

 presence of one X causes the production of a male; two X's 

 cause the production of a female; three X's give rise to an 

 abnormal "super-female." 



One further fact should be mentioned. The males con- 

 taining a single X but no Y, while well formed, are sterile; 

 they do not produce functional sperm. Thus the chromo- 

 some Y is not entirely without function; it is necessary for 

 the production of normal male germ cells. 



Still other combinations of chromosomes can be produced 

 by mating together in various ways the different types of 

 individuals just described. In the following list are given 

 the various combinations that have been produced, with the 

 nature of the individuals that they yield. 



AAXY, normal male. 

 AAXX, normal female. 

 AAXXY, typical female. 

 AAXXYY, typical female. 

 AAXO, male, sterile. 

 AAYO, does not develop. 

 AAXXX, abnormal, "super-female." 



All of these results show that changing the number of X's 

 present is what changes the sex, while changing the number 

 of Y's does not change the sex. 



In all these cases there are present two sets of autosomes, 

 AA, in addition to the X's and Y's. Have these autosomes 

 any effect on sex? What would happen if the numbers of 

 autosomes were changed? To this question we now turn. 



The Effect of Changing the Number of Sets of Auto- 



