54 SEX-DETERMINATION 



The qualitative-quantitative differentiation of the sex- 

 chromosomes has actually been observed in every stage of 

 development from their behaviour at prophase, their relative 

 size at metaphase, and their behaviour at meiosis. 



The differentiation of the sex-chromosomes would be 

 followed by important genetic effects. Since mutations 

 within a chromosome can only be tested in different com- 

 binations when they can be freely distributed by crossing- 

 over, suppression of crossing-over prevents mutations 

 occurring in the Y from being so tested. Since crossing- 

 over does not occur, the Y cannot undergo any structural 

 change by means of interchange of parts. The Y-chromo- 

 some, therefore, during its evolution, would come to lose its 

 effectiveness in the matter of sex-determination, and its 

 place would be taken by the autosomes interacting with 

 theX. 



