OPERATION OF THE GENETIC SYSTEM 6 1 



also a Y. Will such an individual be a male or a female? 

 The results show that it is a female. 



This result shows at once certain important things about 

 sex determination, (i) The presence of Y (found normally 

 in males) is not sufficient to produce a male. (2) It is the 

 presence of two X's, not the absence of Y, that yields a 

 female. (3) The same sperm AY that would normally pro- 

 duce a male, yields a female if the ovum contains two X's. 



When the ovum AO, without X, unites with a sperm con- 

 taining X (that is, AX), we obtain a fertilized egg AAXO, 

 that has but one X, and no Y (figure 20, D). What sort of 

 an individual will this be? 



Observation shows that this combination produces a 

 male, although it has no Y. Certain other important points 

 are thereby demonstrated. (4) The Y-chromosome is not 

 required in order to produce a male. (5) It is the lack of 

 one X that gives origin to a male. (6) The sperm that 

 contains X, which normally produces a female, yields a 

 male if it enters an ovum containing no X. (7) When the 

 usual autosomes are present, two X's give a female, one 

 X a male, whether the Y-chromosome is or is not present in 

 addition. 



Certain other combinations are produced. When an ovum 

 AO, lacking X, unites with a sperm AY, containing Y but 

 no X, the fertilized eggs AAY do not develop. (8) This 

 shows that the presence of one X is necessary for develop- 

 ment. 



When an ovum AXX, with two X-chromosomes, unites 

 with a sperm AX, that also contains an X, the resulting in- 

 dividual AAXXX contains three X's, but only two sets of 

 autosomes. The results are as follows: Most such individ- 

 uals with XXX do not develop. A few develop, but are ab- 

 normal; development is disturbed by the presence of the 

 additional X. The abnormality consists In an accentuation 

 of the structural peculiarities that distinguish the females 



