138 PHYSICAL BASIS OF HEEEDITY 



An extension of the principle of agreement of linkage 

 groups and chromosomes (if they are thought of only as 

 a linear order of genes) is found in the case of ^'duplica- 

 tion'^ described by Bridges, where a short series of linked 

 genes appears to lie at one end of the regular series, dupli- 

 cating their number for this region of the chromosome. 

 Obviously this is not to be looked upon so much as an 

 exception to the principle but rather as a special case due 

 to an accidental change in the mechanism. The number 

 of linkage groups is not changed, but one of them has 

 its genes duplicated for a short part of its length. 



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