RELATION OF GENETIC SYSTEM 

 TO CHARACTERISTICS 



X-Chromosome as Type 



In the preceding chapter it has been seen that differences 

 in sex are produced by altering the chromosome combina- 

 tions present in the fertihzed egg, and this has brought to 

 light some of the methods by which the chromosomes act on 

 development and characteristics. We now examine other 

 effects of altering chromosomes. 



We shall have to examine the effects of altering all the 

 different classes of chromosomes; the X-chromosome, the 

 Y-chromosome, and the different autosomes. Each of these 

 three groups (X, Y, and autosomes) give rise, as we shall 

 see, to a different type of inheritance. The simplest relations, 

 from the experimental point of view, are presented by the 

 X-chromosomes. We therefore deal first with these. 



Effects on Characteristics Produced by Altering 

 the X-Chromosomes. Sex-linked Inheritance 



The X-chromosomes take such a course in passing from 

 generation to generation that it is possible to follow the 

 descendants of a particular X-chromosome (that present in 

 the original male parent, for example), knowing in which 

 individuals they are present, in which they are absent. Fur- 

 thermore, in certain individuals (males, in organisms of 

 Group I) there is but a single X-chromosome instead of a 



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