If we consider the female classes of table 11, we get information as to the cross-over value of eosin and sable, namely, 42 units. The male classes will be considered in connection with the cross that follows.

The next experiment involves the same three gens which now enter in different relations. A double recessive, eosin vermilion (gray) female was mated to (red red) sable males and gave 202 wild-type^[5] females and 184 eosin vermilion males. Two F₁ pairs gave the results shown in table 12 (the four classes of females not being separated).

Table 12.—P₁ eosin vermilion F₁ wild-type ♀ × F₁ eosin vermilion ♂ ♂.

If we combine the data for males given in table 12 with those of table 11, we get the following cross-over values. Eosin vermilion, 32; vermilion sable, 12; eosin sable, 41.

LINKAGE OF MINIATURE AND SABLE.

The miniature wing has been described (Morgan, Science, 1911) and the wing figured (Morgan, Jour. Exp. Zool., 1911). The gen for miniature lies about 3 units to the right of vermilion, so that it is still closer to sable than is vermilion. The double recessive, miniature sable, was made up, and males of this stock were bred to wild females (long gray). The wild-type daughters were back-crossed to double recessive males and gave the results (mass cultures) shown in table 13.

Table 13.—P₁ wild ♀ ♀ × miniature sable ♂ ♂. B. C. F₁ wild-type ♀ ♀ × miniature sable ♂ ♂.

Since the results for the male and the female classes are expected to be the same, the sexes were not separated. The combined data give 7 per cent of crossing-over between miniature and sable.

LINKAGE OF VERMILION, SABLE, AND BAR.

Bar eye has been described by Mrs. S. C. Tice (1914). It is a dominant sex-linked character, whose locus, lying beyond vermilion and sable, is near the right end of the chromosome series, that is, at the end opposite yellow.

In the first cross of a balanced series of experiments for the gens vermilion, sable, and bar, vermilion (gray not-bar) entered from one side (♀) and (red) sable bar from the other (♂). The daughters were bar and the sons vermilion. The daughters were back-crossed singly to the triple recessive males vermilion sable (not-bar), and gave the data included in table 14.

In the second cross, vermilion sable (not-bar) went in from one side (♀) and (red, gray) bar from the other. The daughters were bar and the sons were vermilion sable. Since these sons have the three recessive factors, inbreeding of F₁ is equivalent to a triple back-cross. The results are given by pairs in table 15.

Table 14.—P₁ vermilion ♀ ♀ × sable bar ♂ ♂. B. C. F₁ bar ♀ × vermilion sable ♂ ♂.

Table 15.—P₁ vermilion sable ♀ ♀ × bar ♂ ♂. B. C. F₁ bar ♀ × vermilion sable ♂ ♂.

In the third cross, vermilion (gray) bar entered from one side (♀) and (red) sable (not-bar) from the other (♂). The daughters are bar and the sons vermilion bar. The daughters were back-crossed singly to vermilion sable males and gave the data in table 16.

Table 16.—P₁ vermilion bar ♀ ♀ × sable ♂ ♂. B. C. F₁ bar ♀ × vermilion sable ♂ ♂.

In the fourth cross, vermilion sable bar entered from one side, and (red gray not-bar) wild type from the other. The daughters were bar and the sons vermilion sable bar. The daughters were back-crossed singly to vermilion sable males, with the results shown in table 17.

Table 17.—P₁ vermilion sable bar ♀ ♀ × wild ♂ ♂. B. C. F₁ bar ♀ × vermilion sable ♂ ♂.

In tables 14 to 17 the calculations for the three cross-over values for vermilion, sable, and bar are given for the separate cultures and for the totals. The latter are here repeated.

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.

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

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 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.

In the F₂, from a cross of a double recessive (white vermilion) female by a triple recessive (eosin vermilion pink) male, there appeared, July 21, 1912, three white-eyed females which had two small, symmetrically placed, black, granular masses upon the thorax. These "dots" appeared to be dried exudations from pores. It did not seem possible that such an effect could be inherited, but as this condition had never been observed before, it seemed worth while to mate the three females to their brothers. In the next generation about 1 per cent of the males were dotted. From these females and males a stock was made up which in subsequent generations showed from 10 to 50 per cent of dot. Selection seemed to have no effect upon the percentage of dot. Although the stock never showed more than 50 per cent of dot, yet it was found that the normal individuals from the stock threw about the same per cent as did those that were dotted, so that the stock was probably genetically pure. The number of males which showed the character was always much smaller than the number of dotted females; in the hatches which produced nearly 50 per cent of dot, nearly all the females but very few of the males were dotted. Quite often the character showed on only one side of the thorax.

Since this character arose in an experiment involving several eye-colors an effort was made by crossing to wild and extracting to transfer the dot to flies normal in all other respects. This effort succeeded only partly, for a stock was obtained which differed from the wild type only in that it bore dot (about 30 per cent) and in that the eyes were vermilion. Several attempts to get the dot separated from vermilion failed. Since this was only part of the preliminary routine work necessary to get a mutant stock in shape for exact experimentation, no extensive records were kept.

LINKAGE OF VERMILION AND DOT.

When a dot male with vermilion eyes was bred to a wild female the offspring were wild-type males and females. These inbred gave the data shown in table 19.

Table 19.—P₁ vermilion dot ♂ × wild ♀ ♀. F₁ wild-type ♀ ♀ × F₁ wild-type ♂ ♂.

Only three dot individuals appeared in F₂, but since these were males the result indicates that the dot character is due to a sex-linked gen. These three males had also vermilion eyes, indicating linkage of dot and vermilion. The males show no deficiency in numbers, therefore the non-appearance of the dot can not be due to its being semi-lethal. It appears, therefore, that the expression of the character must depend on the presence of an intensifying factor in one of the autosomes, or more probably, like club, it appears only in a small percentage of flies that are genetically pure for the character.

The reciprocal cross (dot female with vermilion eyes by wild male) was made (table 20). The daughters were wild type and the sons vermilion. Not one of the 272 sons showed dot. If the gen is sex-linked the non-appearance of dot in the F₁ males can be explained on the ground that males that are genetically dot show dot very rarely, or that its appearance is dependent upon the intensification by an autosomal factor of the effect produced by the sex-linked factor for dot.

Table 20.—P₁ vermilion dot ♀ × wild ♂.

The F₂ generation is given in table 20. The dot reappeared in F₂ both in females and in males, but instead of appearing in 50 per cent of both sexes, as expected if it is simply sex-linked, it appeared in 4.0 per cent in the females and in only 0.4 per cent in the males. The failure of the character to be fully realized is again apparent, but here, where it is possible for it to be realized equally in males and females, we find that there are 50 females with dot to only 4 dot males. This would indicate that the character is partially "sex-limited" (Morgan, 1914d) in its realization. The dot appeared only in flies with vermilion eyes, indicating extremely strong linkage between vermilion and dot.

The evidence from the history of the stock, together with these experiments, shows that the character resembles club (wing) in that it is not expressed somatically in all the flies which are homozygous for it. In the case of club we were fortunate enough to find a constant feature which we could use as an index, but, so far as we have been able to see, there is no such constant accessory character in the case of the dot. Unlike club, dot is markedly sex-limited in its effect; that is, there is a difference of expression of the gen in the male and female. This difference recalls the sexual dimorphism of the eosin eye.

BOW.

In an F₂ generation from rudimentary males by wild females there appeared, August 15, 1912, a single male whose wings instead of being flat were turned down over the abdomen (fig. c). The curvature was uniform throughout the length of the wing. A previous mutation, arc, of this same type had been found to be a recessive character in the second group. The new mutation, bow, is less extreme than arc and is more variable in the amount of curvature. When the bow male was mated to wild females the offspring had straight wings.