182 
GENETICS: A. H. STURTEVANT 
Proc. N. A. S. 
In the first paper (1913) in which an attempt was made to construct a 
chromosome map, 2 the statement occurs (p. 49): 
"Of course there is no knowing whether or not these distances as drawn represent 
the actual relative spatial distances apart of the factors.... we have no means of knowing 
that the chromosomes are of uniform strength, and if there are strong or weak places, 
then that will prevent our diagram from representing actual relative distances..." 
Later than this 3 definite evidence for the existence of genes modifying the 
amount of crossing over was reported, and in the ''Mechanism of Men- 
delian Heredity" 4 the situation was discussed as follows (pp. 67-68): 
"It is not supposed, however, that the per cent of crossing over represents precisely 
the distance between the factors, for it may be that crossing over is more likely to 
take place in one region than in another. In that case the distances between factors 
in this region calculated from the amount of crossing over between them, would be 
relatively greater than the actual distance.... Sturtevant has found definite factors 
which alter the amount of crossing over in the chromosomes, and these factors actually 
do affect the amount of crossing over differently in the different regions however, 
....the order of the factors remains unchanged." 
The three papers mentioned above were all published before Detlefsen 
says he began his experiments, as was also Bridges, 5 demonstration that 
frequency of crossing over in the second chromosome changes with the age 
of the female. Since that time the question has been discussed again and 
again, and several cases of inherited linkage variations have been thor- 
oughly analyzed. 6 The conclusions to be drawn from such evidence have 
been well stated by Morgan, 7 in a chapter devoted entirely to variationsin 
linkage (p. 125) : 
"It is to be understood, then, that when we substitute the idea of distance for crossing 
over values the term is not used in an absolute sense, but in a relative sense, and that 
it depends always on the conditions of the experiment. That the genes do stand at 
definite levels in the chromosome, and that in this sense they are definitely spaced, 
seems reasonable in the light of all the evidence bearing on this point; but even if they 
are so spaced that crossing over is a function of their distance from each other in the 
series, any influence that determines how often interchange between homologous pairs 
will take place would give the appearance that the actual distances themselves have 
changed." 
As will be evident from the quotations given, the chromosome maps are 
intended to show the actual sequence of the loci, and the relative amounts 
of crossing over between them. The intervals between adjacent loci are 
not to be taken as necessarily proportional to the actual spatial distances 
between them — though that distance is evidently one of the elements con- 
cerned. When the amount of crossing over changes from any cause it is 
evident that we are dealing with a new system, and the intervals of the 
"normal" map will no longer be applicable (though in all cases so far in- 
vestigated the sequence of loci shown in the "normal" map is unchanged). 
The "normal" map itself is based on the average results obtained under 
conditions in which no recognized disturbing factors (genetic or environ- 
mental) were known to be present. This map represents the results to be 
