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GENETICS: C. B. BRIDGES 
Proc. N. a. S. 
THE MUTANT CROSSVEINLESS IN DROSOPHILA 
MELANOGASTER'' 
By Calvin B. Bridges 
Department of Zoology, Columbia University 
Communicated by T. H. Morgan, September 30, 1920 
The Origin oj Crossveinless. — A recessive sex-linked mutant called small- 
eye appeared, the locus of which was found to be very close to that of 
fused (locus of fu, 59.5). To determine definitely the relation of the two 
loci, a forked (locus of /, 56.5) small-eye (symbol, sy) female was crossed 
to a fused male, and five of the Fi wild- type females were mated singly 
to forked small-eye brothers. In F2, no crossovers between small-eye and 
fused were obtained. While this result confirms the closeness of the 
loci, it was decided to raise more F2 cultures until one or more cross-overs 
appeared which would show the position of the new locus with reference 
to the known loci forked and fused. Accordingly, from one of the five 
F2 cultures (No. 10,846) four wild-type females, of the same constitution 
as the original Fi female, were mated to forked small-eye brothers. There 
appeared two cross-overs (wild-type) that showed that the locus of small- 
eye is to the left of that of fused (and about 0.2 unit distant, i.e., at 59.3). 
In one of these four cultures it was noticed (No. 10,988, Dec. 21, 1919) 
that about half the males did not have either anterior or posterior cross- 
veins. All females had normal crossveins. Evidently crossveinless 
(cv) is a sex-linked recessive. The other three sister cultures were then 
examined, and no crossveinless flies were present. 
The males that hatched after the mutant character was noticed were: 
cv f sy 12, fu 11, cv fu S, f sy 9. Since approximately half the males 
showed the crossveinless character, the mother is known to have been 
heterozygous for the mutant gene. The crossveinless character emerged 
in association with forked more often than with fused, which means that 
the mutant gene was in the forked small-eye chromosome of the mother. 
She had received this chromosome from her father — the Fi forked small- 
eye male used in raising culture 10,846. But three of her sisters Hkewise 
received forked small-eye sperm from this same male, and, as we have 
seen, none of these three sisters received the crossveinless gene. It follows 
that the mutation occurred relatively late in the ontogeny of the testis 
of the male in question, perhaps as late as the maturation stage. 
The Locus of Crossveinless. — Of the thirty-five males in which the 
linkage relations of crossveinless to forked, small-eye, and fused were 
noted, 12, or 34.3%, were crossovers between crossveinless and forked. 
In view of the small numbers, this value indicated only that the locus 
of crossveinless is to the left of that of forked and probably at least as 
* Contribution from the Carnegie Institution of Washington. 
