CHAPTER VI 

 LINKAGE RELATIONS IN MENDELISM 



Thus far Mendelian experiments have been- considered in which the 

 different pairs of factors segregate independently, and it has been shown 

 that such cases may be explained very simply on the assumption that 

 different pairs of chromosomes carry independent factors. However, 

 there are several different species of plants and animals in which the 

 number of known factor differences exceeds the number of pairs of 

 chromosomes. Since it is reasonable to believe that only a small 

 proportion of the possible number of factorial differences in any species 

 has been analyzed, the conclusion appears justifiable that the number 

 of factors in any species of plant or animal greatly exceeds the number of 

 pairs of chromosomes; in fact our present evidence leads us to believe 

 that the number of hereditary units in any organism must reach into the 

 thousands. If the chromosome view of heredity is valid, therefore, 

 each chromosome must carry a very great number of factors. In the 

 present chapter it is proposed to discuss that class of Mendelian phe- 

 nomena which depend upon factors which tend to remain together during 

 segregation rather than to undergo independent assortment. Assuming 

 that such factors are borne by the same chromosome, it will be shown 

 how the chromosome mechanism provides an adequate physical basis 

 for all the relations exhibited by such factors. Linkage and factor 

 coupling are terms applied to that type of inheritance in which the 

 factors tend to remain together in segregation. Linkage of factors is 

 definitely an exception to one of the principles which Mendel laid down, 

 namely, that of independent character segregation. Nevertheless by 

 common consent the term Mendelism has been extended to include all 

 phenomena of inheritance based on the unit factor hypothesis. For a 

 long time only a few cases of linkage were known, and these were regarded 

 in effect as anomalies. But the advocates of the chromosome theory of 

 heredity have zealously prosecuted the study of linkage because of the 

 many ways in which linkage relations parallel chromosome behavior. 

 Moreover as the number of definitely recognizable factors within a 

 species increases it becomes more and more important to determine the 

 relations which the factors display among themselves. Linkage relations 

 among factors, therefore, are of primary importance, and have been the 

 direct means of giving us a clear and illuminating picture of the consti- 



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