The percentage of crossing-over between cherry and sable is 42. Since cherry is one point from yellow, this result agrees extremely well with the value 43 for yellow and sable. Since yellow and eosin lie at the left end of the first chromosome, the high values, namely, 43 and 42, agree in making it very probable that sable lies near the other end (i. e., to the right). Sable will lie farther to the right than vermilion, for vermilion has been shown elsewhere to give 33 per cent of crossing-over with eosin. The location of sable to the right of vermilion has in fact been substantiated by all later work.

LINKAGE OF EOSIN, VERMILION, AND SABLE.

Three loci are involved in the next experiment. Since eosin is an allelomorph of cherry, it should be expected to give with sable the same cross-over value as did cherry. When eosin (red) sable females were crossed to (red) vermilion (gray) males, the daughters were wild type and the males were eosin sable. Inbred these gave the classes shown in table 11.

Table 11.—P1 eosin sable ♀ × vermilion ♂♂. F1 wild-type ♀♀ × F1 eosin sable ♂♂.

Reference. F2 females. F2 males.

Eosin
sable.		 Wild-
type.		 Eosin. Sable. Eosin
sable.		 Ver-
milion.		 Eosin
ver-
milion.		 Sable. Eosin. Ver-
milion
sable.		 Eosin
ver-
milion
sable.		 Wild-
type.
26 I 132 171 113 109 127 163 75 76 37 14 2 5
26′I 96 146 86 78 74 128 76 59 18 21 4 3
Total. 228 317 199 187 201 291 151 135 55 35 6 8

If we consider the male classes of table 11, we find that the smallest classes are eosin vermilion sable and wild type, which are the expected double cross-over classes if sable lies to the right of vermilion, as indicated by the crosses with eosin and with yellow. The classes which represent single crossing-over between eosin and vermilion are eosin vermilion, and sable, and those which represent single crossing-over between vermilion and sable are eosin and vermilion sable. These relations are seen in diagram II.

Diagram II.—The upper line represents an X chromosome, the lower line its mate. The cross connecting lines indicate crossing-over between pairs of factors.

Non-cross-overs

Eosin sable.
Vermilion.
Single cross-overs

Eosin vermilion.
Sable.

Eosin.
Vermilion sable.
Double cross-overs

Eosin vermilion sable.
Wild-type.