We propose to give in a series of papers an account of the mutant races of Drosophila and the linkage shown in their inheritance. In this paper we shall consider only the members of the first chromosome, describing a large number of new mutants with their linkage relations and summarizing to date all the linkage data relating to the first chromosome. In later papers we propose to consider the members of the second, third, and fourth chromosomes.

The list at the top of page 21 gives the names of the factors dealt with in this paper. They stand in the order of their discovery, the mutant forms reported here for the first time being starred.

In each experiment the percentage of crossing-over is found by dividing the number of the cross-overs by the sum of the non-cross-overs and the cross-overs, and multiplying this quotient by 100. The resulting percentages, or cross-over values, are used as measures of the distances between loci. Thus if the experiments give a cross-over value of 5 per cent for white and bifid, we say that white and bifid lie 5 units apart in the X chromosome. Other experiments show that yellow and white are about 1 unit apart, and that yellow and bifid are about 6 units apart. We can therefore construct a diagram with yellow as

the zero, with white at 1, and with bifid at 6. If we know the cross-over values given by a new mutant with any two mutants of the same chromosome whose positions are already determined, then we can locate the new factor with accuracy, and be able to predict the cross-over value which the new factor will give with any other factor whose position is plotted.

The sex-linked factors of Drosophila.

Gen. Part affected. Figure. Symbol. Locus. Date found. Found by.
White Eye-color 11 w 1.1 May 1910 Morgan.
Rudimentary Wings A r 55.1 June 1910 Morgan.
Miniature Wings 7-8 m 36.1 Aug. 1910 Morgan.
Vermilion Eye-color 10 v 33.0 Nov. 1910 Morgan.
Yellow Body-color 5 y 0.0 Jan. 1911 Wallace.
Abnormal Abdomen 4 A′ 2.4 July 1911 Morgan.
Eosin Eye-color 7-8 we 1.1 Aug. 1911 Morgan.
Bifid Wings B bi 6.3 Nov. 1911 Morgan.
Reduplicated Legs 34.7 Nov. 1911 Hoge.
Lethal 1 Life l1 0.7 Feb. 1912 Rawls.
Lethal 1a* Life l1a 3.3 Mar. 1912 Rawls.
Spot* Body-color 14-17 ys 0.0 April 1912 Cattell.
Sable* Body-color 2 s 43.0 July 1912 Bridges.
Dot* Thorax 33 ± July 1912 Bridges.
Bow* Wings C Aug. 1912 Bridges.
Lemon* Body-color 3 lm 17.5 Aug. 1912 Wallace.
Lethal 2 Life l2 12.5± Sept. 1912 Morgan.
Cherry Eye-color 9 wc 1.1 Oct. 1912 Safir.
Fused* Venation D fu 59.5 Nov. 1912 Bridges.
Forked* Bristles E f 56.5 Nov. 1912 Bridges.
Shifted* Venation F sh 17.8 Jan. 1913 Bridges.
Lethal sa Life lsa 23.7 Jan. 1913 Stark.
Bar Eye-shape 12-13 B′ 57.0 Feb. 1913 Tice.
Notch Wing N′ 2.6 Mar. 1913 Dexter.
Depressed* Wing G dp 18.0 April 1913 Bridges.
Lethal sb Life lsb 16.7 April 1913 Stark.
Club* Wings H cl 14.6 May 1913 Morgan.
Green* Body-color May 1913 Bridges.
Chrome* Body-color Sept. 1913 Bridges.
Lethal 3 Life l3 26.5 Dec. 1913 Morgan.
Lethal 3a Life l3a 19.5 Jan. 1914 Morgan.
Lethal 1b* Life l1b 1.1- Feb. 1914 Morgan.
Facet* Eye fa 2.2 Feb. 1914 Bridges.
Lethal sc Life lsc 66.2 April 1914 Stark.
Lethal sd Life lsd May 1914 Stark.
Furrowed Eye fw 38.0 Nov. 1914 Duncan.

The factors are located preferably by short distances (i.e., by those cases in which the amount of crossing-over is small), because when the amount of crossing-over is large a correction must be made for double crossing-over, and the correction can be best found through breaking up the long distances into short ones, by using intermediate points.

Conversely, when a long distance is indicated on the chromosome diagram, the actual cross-over value found by experiment (i.e., the percentage of cross-overs) will be less than the diagram indicates, because the diagram has been corrected for double crossing-over.