5 



RELATION OF GENETIC SYSTEM TO 

 CHARACTERISTICS 



Autosomes and Y-Chromosomes 



In the two preceding chapters the X-chromosomes have 

 been dealt with because of the great advantages they offer 

 for experimental study. It was seen that they affect develop- 

 ment in many ways, and that in a given species there are 

 many different types of X-chromosomes in the different in- 

 dividuals, causing them to show different characteristics, 

 structural and physiological. 



Is the X-chromosome typical in these relations? Shall we 

 find similar effects in the other chromosomes? 



In addition to the X-chromosomes, there are the auto- 

 somes, which are present as a rule as several or many pairs. 

 In man there are 23 pairs of autosomes (figure 19) : in 

 Drosophila there are 3 pairs (figure 7). There is further 

 in many species the single Y-chromosome. The autosomes 

 and the Y-chromosome are distributed from parent to off- 

 spring in characteristic ways, differing from the distribu- 

 tion of X-chromosomes. If they affect characteristics, these 

 differences should appear in a different system of inheri- 

 tance. We shall examine first the autosomes. 



The Autosomes: Typical Mendelian Inheritance 



We have already seen that the autosomes play a role in 

 development, in the fact that they help to determine the sex 



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