CHAPTER VI 

 LINKAGE 



Mendel's results involving two or more pairs of char- 

 acters led to the conclusion that distribution of the mem- 

 bers of one pair of genes is independent of the distribution 

 of the members of other pairs. This process may be 

 called free or independent assortment, and is what is 

 expected if each pair of genes is carried by a different 

 pair of chromosomes. If this rule held for all pairs of 

 characters then there could be no more pairs that assorted 

 independently than there were pairs of homologous 

 chromosomes. On the other hand, if the chromosomes 

 carry the genes we should anticipate from what we have 

 found out concerning the individuality of the chromo- 

 some, and from what we know concerning the large 

 number of inherited characters, that many of these fac- 

 tors must be carried in the same chromosome. H this 

 is true, then Mendel's second law can have only a very 

 limited appKoation. 



As our information about the mode of inheritance of 

 characters has widened, the number of cases in which free 

 assortment does not occur has steadily increased. Many 

 characters have been found to keep together in successive 

 generations. This tendency to keep together rather than 

 to assort freely is called linkage. The most extreme cases 

 are those where characters hold together completely; 

 at the other extreme are those that show only a slightly 

 greater probability of holding together than of assorting 

 freely. Between these extremes all intermediate degrees 

 of linkage are found. For the sake of simplicity, cases of 

 complete linkage will be dealt with in this chapter; the 

 others will be taken up in the next chapter. 



If a fly (Drosophild) with two recessive mutant charao- 



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