208 THE SECOND-CHROMOSOME GROUP 



A summary of the linkage data involving arc is given in table 55. 

 The locus of arc on the basis of these data is 6.7 units to the left of 

 speck, which is its base of reference, or 98.4 units from star. 



The calculation of the locus of arc made by Muller (1916) on the 

 basis of relatively few flies agreed with this position. 



VALUATION OF ARC. 



Arc is of second rank as a working character; its demerit comes from 

 the fact that the character is occasionally simulated in the wings of 

 flies of not-arc stocks. The occurrence of such mimics in a critical 

 experiment involving arc would lead to confusion in the classification 

 and to seemingly impossible results. The opposite error — failure to 

 distinguish the character in flies really arc — occurs rarely or possibly 

 never. In all other respects arc is of first rank; in viability and habit 

 it is excellent; its locus is especially convenient, since it is situated in 

 the gap between curved and speck near the right end of the known 

 chromosome, and since the arc-speck interval of 5.9 is, like black 

 purple, long enough to avoid the high probable errors due to small 

 percentages of crossing-over, but short enough to avoid all danger of 

 double crossing-over within it, and likewise to afford a concise " caliper" 

 region in studies on coincidence or linkage. 



GAP. 



(Text-figure 76.) 



ORIGIN OF GAP. 



The first appearance of the character called "gap " was in an Fi mass- 

 culture from the cross of black by arc (July 10, 1912, culture B 42, 

 table 51). Several of the black flies of that culture showed a break 

 or gap in the fourth longitudinal vein between the posterior cross-vein 

 and the wing-tip. This gap varied from nearly all of this section (see 

 fig. 76) to a mere weakening of the vein. The black color, which nor- 

 mally in black flies forms a heavy band on each side of the veins, was 

 likewise absent from this region. From B 42 the stock of black arc 

 was derived, and this stock occasionally showed the gap character. 

 Little attention was paid to it until it turned up again in a cross 

 involving black arc (March 19, 1913). 



INHERITANCE OF GAP. 



A gap black arc female was then crossed back to a stock black arc 

 male which did not show gap. In Fi there appeared 27 gap to 55 not- 

 gap offspring (M 37). It was not known whether this denoted that 

 the male had been heterozygous for gap, which is recessive, or that the 

 gap character was dominant. In either case the character gave little 

 promise of usefulness, since obviously some of the flies really gap were 



