Vol. 6, 1920 
GENETICS: J. A. DETLEFSEN 
663 
both these regards, while differing from each other. Cut^ is now the 
only cut mutant used for linkage determinations involving this locus. 
Additional linkage data upon the cross veinless cut distance was ob- 
tained by raising nine F2 cultures from the cross of cross veinless to cut^. 
The crossover value was found to be 6.7 on the basis of the 2,163 flies 
(table 3). 
The locus of crossveinless is at 13.7, as calculated on the basis of the 
data of the three preceeding tables, and on the assumptions that the locus 
of ruby is at 7.5 of cut at 20.0, and of vermilion at 33.0. 
Description of the Crossveinless Character; Homology with Crossveinless 
in D. virilis. — The somatic changes produced by the crossveinless gene 
seem to be restricted to the entire absence of the posterior crossvein and 
the almost complete absence of the anterior crossvein. There is a slight 
trace only of the anterior crossvein, though on casual inspection it seems to 
extend outward from the Ill-longitudinal vein about half way to the IV*^. 
However, what is seen is largely a sense-organ that is normally present 
near the mid-point of the anterior crossvein and that is not affected by 
the crossveinless mutation. Examination of crossveinless in D. virilis 
showed that the sense organ is unaffected there also, but that the cross- 
vein is not reduced in length or thickness as much as in D. melanogaster. 
(See figures of the crossveinless mutant in Weinstein's paper preceding 
this.) 
The similarity of the characters is parallelled by a similarity of position 
on the maps of the two X-chromosomes. It seems highly probable that 
the two mutants are homologous, though, as Weinstein's discussion 
brings out, this cannot yet be accepted without reservations. 
IS CROSSING OVER A FUNCTION OF DISTANCED 
By J. A. DetIvEFsen 
Laboratory of Genetics, Illinois Agricultural Experiment Station 
Communicated by C. B. Davenport, September 24, 1920 
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 correspond to or represent, roughly perhaps, the actual 
conditions in the chromosome. The basis for this attractive and suggest- 
ive 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. 
