Sex-linked Inheritance in Drosophila

by Thomas Hunt Morgan; Calvin B. Bridges

Table 9.—P₁ wild-type ♀ ♀ × yellow sable ♂ ♂. B. C. F₁ wild-type ♀ × yellow sable ♂ ♂.

Reference.	Non-cross-overs.		Cross-overs.		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.

LINKAGE OF CHERRY AND SABLE.

The origin of cherry eye-color ([Plate II], fig. 9) has been given by Safir (Biol. Bull., 1913). From considerations which will be discussed later in this paper we regard cherry as allelomorphic to white in a quadruple allelomorph system composed of white, eosin, cherry, and their normal red allelomorph. Cherry will then occupy the same locus as white, which is one unit to the right of yellow, and will show the same linkage relations to other factors as does white. A slightly lower cross-over value should be given by cherry and sable than was given by yellow and sable.

When cherry (gray) females were crossed to (red) sable males the daughters were wild type and the sons cherry. Inbred these gave the results shown in table 10.

Table 10.—P₁ cherry ♀♀ × sable ♂♂. F₁ wild-type ♀ × F₁ cherry ♂ ♂.

Reference.	Wild- type ♀.	Cherry ♀.	Non-cross- over ♂.		Cross-over ♂.		Total males.	Cross- over value.
			Cherry.	Sable.	Cherry sable.	Wild- type.
24 I	94	105	51	42	20	43	156	40
55 I	101	131	63	52	38	48	201	43
55′ I	96	94	52	31	29	30	142	42
Total	291	330	166	125	87	121	499	42