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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