PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 7 JULY 15, 1921 Number 7 
LINKAGE VARIATION AND CHROMOSOME MAPS 
By A. H. Sturtevant 
Carnegie Institution otf Washington 
Communicated by T. H. Morgan, March 10, 1921 
A recent paper in this journal by Detlefsen 1 is introduced as follows: 
"There is a well intrenched concept of recent genetics that hereditary factors or 
genes may be given fairly definite loci on chromosome maps and that these maps corre- 
spond to or represent, roughly perhaps, the actual conditions in the chromosome. The basis 
for this attractive and suggestive view is the premise that the distance between two 
genes is necessarily proportional to the percentage of crossing over which these two 
genes show, other things being equal. If the distance which gives one per cent of 
crossovers is used as an arbitrary unit of measurement, then it follows that distances 
on the chromosome may be calculated in terms of this unit. It has seemed to me for 
some time that the antecedent in this hypothetical proposition contains a more or 
less gratuitous assumption. We do not know that the distance which gives 1% (or n%) 
of crossovers is a fixed unit. Stated differently, we do not know how constant the 
percentage of crossing over may be between two genes to which we give a fixed distance, 
i.e., our arbitrary unit of measurement may itself prove to be a variable. It may be 
possible for the distance which gives 1% of crossing over to differ in different females 
of the same population, or differ between stocks. In order to throw some light on 
these questions I began a set of experiments in 1916 " 
Detlefsen then gives an account of an experiment in which crossover 
values for the white and miniature loci (in the X-chromosome of Droso- 
phila melanogaster) ranging from 0% to about 33% have been obtained. 
Crosses between "high" and "low" lines are taken to indicate that a num- 
ber of genetic factors influence the percentage of crossing over. The paper 
closes as follows : 
"In view of these considerations it would perhaps be simpler to conclude that linkage 
is not a function of distance, i.e., crossing over is not necessarily proportional to dis- 
tance. The distance between two genes may remain fairly constant, but the amount 
of crossing over depends upon numerous hereditary factors." 
One unfamiliar with the literature of the subject would probably infer 
from Detlefsen's paper that the possibility of inherited linkage variations 
had not been taken into account by those concerned in constructing chro- 
mosome maps. In point of fact, the matter has not only been taken into 
account, but has been often discussed in the literature, as the following 
references will show. 
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