The results of the different experiments are remarkably uniform. There can be no doubt that the cross-over value is independent of the way in which the experiment is made, whether any two recessives enter from the same or from opposite sides. TABLE 18.--_Linkage of vermilion, sable, and bar with balanced viability._ +---------------------+---------+---------+---------+---------+-------+ | | ------- | --+---- | ----+-- | --+-+-- | Total.| +---------------------+---------+---------+---------+---------+-------+ | Wild-type | 755 | 110 | 140 | 4 | | | Vermilion | 734 | 92 | 151 | 1 | | | Sable | 724 | 97 | 131 | 4 | | | Bar | 845 | 87 | 126 | 4 | | | Vermilion sable | 608 | 80 | 123 | 3 | | | Vermilion bar | 800 | 95 | 129 | 1 | | | Sable bar | 665 | 81 | 107 | 1 | | | Vermilion sable bar | 641 | 74 | 108 | 3 | | | +---------+---------+---------+---------+-------+ | Total | 5,772 | 716 | 1,015 | 21 | 7,524 | | Percentage | 76.7 | 9.53 | 13.49 | 0.28 | | +---------------------+---------+---------+---------+---------+-------+ In table 18 the data from each of the four separate experiments have been combined in the manner explained, so that viability is canceled to the greatest extent. The amount of each kind of cross-over appears at the bottom of the table. The total amount of crossing-over between vermilion and sable is the sum of the single (9.53) and of the double (0.28) cross-overs, which value is 9.8. Likewise the cross-over value for sable bar is 13.49 + 0.28 (= 14), and for vermilion bar is 9.53 + 13.49 (= 23). By means of these cross-over values we may calculate the coincidence involved, which is in this case 0.0028 x 100 --------------------------------- = 20.8 0.0953 + 0.0028 x 0.1349 + 0.0028 This value shows that there actually occurs only about 21 per cent of the double cross-overs which from the values of the single cross-overs are expected to occur in this section of the chromosome. This is the result which is to be anticipated upon the chromosome view, for if crossing-over is connected with loops of the chromosomes, and if these loops have an average length, then if the chromosomes cross over at one {44} point it is unlikely they will cross over again at another point nearer than the average length of the loop. The calculation of the locus for sable gives 43.0. DOT.