78 The Thiio) and Fourth Generation 



the chromosome b, the sperm/. Tracing back 

 the source of the chromosomes thus step by 

 step, it is evident that the two chromosomes 

 in the cells of this particular individual whose 

 ancestry is diagrammed were derived one from 

 a great-grandmother, one from a great-grand- 

 father, and they might in a similar way be 

 traced back much farther. If now a given 

 chromosome contains a set of determiners for 

 particular characters we can readily see why 

 this individual bears striking resemblance to 

 two of its great-grandparents. This, of course, 

 is a crude illustration of the sort of thing that 

 inheres in the notion of the individuality of 

 the chromosomes. It must not be taken for 

 granted that all of this has been proven; it is 

 merely a working, hypothesis rendered probable 

 by recent investigations and the experience of 

 breeders. 



The first successful attempt to connect 

 specific characters with particular chromosomes , 

 came with the discovery that in certain insects 

 there is one chromosome in the male called the 

 odd or x-chromosome which does not pair with 



