+----------+-------------------------+---------------+-------+----------+ | | | | | | | | Non-cross-overs. | Cross-overs. | | | | | | | | | |Reference.+-----------+-------------+-------+-------+ Total.|Cross-over| | | | | | | | value. | | | Wild-type.|Yellow sable.|Yellow.| Sable.| | | | | | | | | | | +----------+-----------+-------------+-------+-------+-------+----------+ | | | | | | | | | 31 I | 108 | 51 | 58 | 56 | 273 | 42 | | 49 I | 265 | 175 | 161 | 169 | 770 | 43 | | +-----------+-------------+-------+-------+-------+----------+ | Total | 373 | 226 | 219 | 225 | 1,043 | 43 | | | | | | | | | +----------+-----------+-------------+-------+-------+-------+----------+ In these tables the last column (to the right) shows for each culture the amount of crossing-over between yellow and sable. These values are found by dividing the number of cross-overs by the total number of individuals which might show crossing-over, that is, males only or both males and females, as the case may be. Free assortment would give 50 per cent of cross-overs and absolute linkage 0 per cent of cross-overs. Except where the percentage of crossing-over is very small these values are expressed to the nearest unit, since the experimental error might make a closer calculation misleading. The combined data of tables 8 and 9 give 686 cross-overs in a total of 1,600 individuals in which crossing-over might occur. The females of table 8 are all of one class (wild type) and are useless for this calculation except as a check upon viability. The cross-over value of 43 per cent shows that crossing-over is very free. We interpret this to mean that sable is far from yellow in the chromosome. Since yellow is at one end of the known series, sable would then occupy a locus somewhere near the opposite end. This can be checked up by finding its linkage relations to the other sex-linked factors. {37} LINKAGE OF CHERRY AND SABLE.