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 {21} 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 |Figure.|Symbol.| Locus. | Date found.|Found by.| | |affected. | | | | | | +------------+----------+-------+-------+--------+------------+---------+ |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 | w^e | 1.1 | Aug. 1911 |Morgan. | |Bifid |Wings | B | b_i | 6.3 | Nov. 1911 |Morgan. | |Reduplicated|Legs | | | 34.7 | Nov. 1911 |Hoge. | |Lethal 1 |Life | | l_1 | 0.7 | Feb. 1912 |Rawls. | |Lethal 1_a_*|Life | | l_1a | 3.3 | Mar. 1912 |Rawls. | |Spot* |Body-color| 14-17 | y^s | 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 | l_m | 17.5 | Aug. 1912 |Wallace. | |Lethal 2 |Life | | l_2 | 12.5+/- | Sept. 1912 |Morgan. | |Cherry |Eye-color | 9 | w^c | 1.1 | Oct. 1912 |Safir. | |Fused* |Venation | D | f_u | 59.5 | Nov. 1912 |Bridges. | |Forked* |Bristles | E | f | 56.5 | Nov. 1912 |Bridges. | |Shifted* |Venation | F | s_h | 17.8 | Jan. 1913 |Bridges. | |Lethal sa |Life | | l_sa | 23.7 | Jan.