The factors are located preferably by short distances (i.e., by those cases in which the amount of crossing-over is small), because when the amount of crossing-over is large a correction must be made for double crossing-over, and the correction can be best found through breaking up the long distances into short ones, by using intermediate points.
Conversely, when a long distance is indicated on the chromosome diagram, the actual cross-over value found by experiment (i.e., the percentage of cross-overs) will be less than the diagram indicates, because the diagram has been corrected for double crossing-over.
Diagram I.Diagram I.
Diagram I.
Diagram I has been constructed upon the basis of all the data summarized in table 65 (p.84) for the first or X chromosome. It shows the relative positions of the gens of the sex-linked characters ofDrosophila. One unit of distance corresponds to 1 per cent of crossing-over. Since all distances are corrected for double crossing-over and for coincidence, the values represent thetotalof crossing-over between the loci. The uncorrected value obtained in any experiment with two loci widely separated will be smaller than the value given in the map.
It may be asked what will happen when two factors whose loci are more than 50 units apart in the same chromosome are used in the same experiment? One might expect to get more than 50 per cent of cross-overs with such an experiment, but double crossing-over becomes disproportionately greater the longer the distance involved, so that in experiments the observed percentage of crossing-over does not rise above 50 per cent. For example, if eosin is tested against bar, somewhat under 50 per cent of cross-overs are obtained, but if the distance of bar from eosin is found by summation of the component distances the interval for eosin bar is 56 units.
In calculating the loci of the first chromosome, a system of weighting was used which allowed each case to influence the positions of the loci in proportion to the amount of the data. In this way advantage was taken of the entire mass of data.
The factors (lethal 1, white, facet, abnormal, notch, and bifid) which lie close to yellow were the first to be calculated and plotted. The next step was to determine very accurately the position of vermilion with respect to yellow. There are many separate experiments which influence this calculation and all were proportionately weighted. Then, using vermilion as the fixed point the factors (dot, reduplicated, miniature, and sable) which lie close to vermilion were plotted. The same process was repeated in locating bar with respect to vermilion and the factors about bar with reference to bar. The last step was to interpolate the factors (club, lethal 2, lemon, depressed, and shifted), which form a group about midway between yellow and vermilion. Of these, club is the only one whose location is accurate. The apparent closeness of the grouping of these loci is not to be taken as significant, for they have been placed only with reference to the distant points yellow and vermilion and not with respect to each other; furthermore, the data available in the cases of lemon and depressed are very meager.
The factors which are most important and are most accurately located are yellow, white (eosin), bifid, club, vermilion, miniature, sable, forked, and bar. Of these again, white (eosin), vermilion, and bar are of prime importance and will probably continue to claim first rank. Of the three allelomorphs, white, eosin, and cherry, eosin is the most useful.
NOMENCLATURE.
The system of symbols used in the diagrams and table headings is as follows: The factor or gen for a recessive mutant character is represented by a lower-case letter, as v for vermilion and m for miniature. The symbols for the dominant mutant characters bar, abnormal, and notch are B′, A′, and N′. There are now so many characters that it is impossible to represent all of them by a single letter. We therefore add a subletter in such cases, as bifid (bi), fused (fu), and lethal 2 (l2). In the case of multiple allelomorphs we usually use as the base of the symbol the symbol of that member of the system which was first found and add a letter as an exponent to indicate the particular member, as ysfor spot, wefor eosin, and wcfor cherry. The normal allelomorphs of the mutant gens are indicated by the converse letter, as V for not-vermilion, Bifor not-bifid, and b′ for not-bar. In the table headings the normal allelomorphs are indicated by position alone without the use of a symbol.
Thus the symbolsymbolindicates that the female in question carried eosin, not-vermilion, and bar in one chromosome and not-eosin, vermilion, and not-bar in the other. The symbolsymbolwhen used in the heading of a column in a table indicates that the flies classified under this heading are the result of single crossing-over between eosin and vermilion in a mother which was the compositionsymbol; the symbol tells at the same time that the flies that result from a single cross-over between eosin and vermilion in the mother are of the two contrary classes,eosin vermilion and bar. When a fly shows two or more non-allelomorphic characters the names are written from left to right in the order of their positions from the zero end of the map.
WHITE.
(Plate II, figure 11.)
The recessive character white eye-color, which appeared in May 1910, was the first sex-linked mutation inDrosophila(Morgan, 1910a, 1910b). Soon afterwards (June 1910) rudimentary appeared, and the two types were crossed (Morgan, 1910c). Under the conditions of culture the viability of rudimentary was extremely poor, but the data demonstrated the occurrence of recombination of the factors in the ovogenesis so that white and rudimentary, though both sex-linked, were brought together into the same individual. The results were not fully recognized as linkage, because white and rudimentary are so far apart in the chromosome that they seemed to assort freely from each other.
Owing to the excellent viability and the perfect sharpness of separation, white was extensively used in linkage experiments, especially with miniature and yellow (Morgan, 1911a; Morgan and Cattell, 1912 and 1913). White has been more extensively used than any other character inDrosophila, though it is now being used very little because of the fact that the double recessives of white with other sex-linked eye-colors, such as vermilion, are white, and consequently a separation into the true genetic classes is impossible. The place of white has been taken by eosin, which is an allelomorph of white and which can be readily used with any other eye-color.
The locus of white and its allelomorphs is only 1.1 units from that of yellow, which is the zero of the chromosome. Yellow and white are very closely linked, therefore giving only about one cross-over per 100 flies.
All the published data upon the linkage of white with other sex-linked characters have been collected into table 65.
RUDIMENTARY.
Rudimentary, which appeared in June 1910, was the second sex-linked character inDrosophila(Morgan, 1910c). Its viability has always been very poor; in this respect it is one of the very poorest of the sex-linked characters. The early linkage data (Morgan, 1911a) derived from mass cultures have all been discarded. By breeding from a single F1female in each large culture bottle it has been possible to obtain results which are fairly trustworthy (Morgan, 1912g; Morgan and Tice, 1914). These data appear in table 65, which summarizes all the published data.
The locus of rudimentary is at 55.1, for a long time the extreme right end of the known chromosome, though recently several mutants have been found to lie somewhat beyond it.
Fig. A. Rudimentary and normal fliesFig. A.a.rudimentary wing;b.the wild fly for comparison.
The rudimentary males are perfectly fertile, but the rudimentary females rarely produce any offspring at all, and then only a very few. The reason for this is that most of the germ-cells cease their development in the early growth stage of the eggs (Morgan, 1915a).
MINIATURE.
(Plate II. figures 7 and 8.)
The recessive sex-linked mutant miniature wings appeared in August 1910 (Morgan, 1911b and 1912a). The viability of miniature is fair, and this stock has been used in linkage experiments more than anyother, with the single exception of white. While the wings of miniature usually extend backwards, they are sometimes held out at right angles to the body, and especially in acid bottles the miniature flies easily become stuck to the food or the wings become stringy, so that other wing characters are not easy to distinguish in those flies which are also miniature. At present vermilion, whose locus is at 33, in being used more frequently in linkage work. The locus of miniature at 36.1 is slightly beyond the middle of the chromosome.
VERMILION.
(Plate II. figure 10.)
The recessive sex-linked mutant vermilion eye-color (Morgan, 1911cand 1912a) appeared in November 1910, and has appeared at least twice since then (Morgan and Plough, 1915). This is one of the best of the sex-linked characters, on account of its excellent viability, its sharp distinction from normal with very little variability, its value as a double recessive in combination with other sex-linked eye-colors, and because of its location at 33.0, very near to the middle of the known chromosome.
YELLOW.
(Plate I. figure 5.)
The recessive sex-linked mutant yellow body and wing-color appeared in January 1911 (Morgan, 1911cand 1912a). Its first appearance was in black stock; hence the fly was a double recessive, then called brown. Later the same mutation has appeared independently from gray stock. Yellow was found to be at the end of the X chromosome, and this end was arbitrarily chosen as the zero or the "left end," while the other gens are spoken of as lying at various distances to the right of yellow. Recently a lethal gen has been located less than one-tenth of a unit (-0.04) to the left of yellow, but yellow is still retained as the zero-point.
The viability of yellow is fairly good and the character can be separated from gray with great facility, and in consequence yellow has been used extensively, although at present it is being used less than formerly, since eosin lies only 1.1 units distant from yellow and is generally preferred.
ABNORMAL ABDOMEN.
(Plate I. figure 4.)
The dominant sex-linked character abnormal abdomen appeared in July 1911 (Morgan, 1911d). It was soon found that the realization of the abnormal condition depended greatly upon the nature of the environment (Morgan, 1912). Recently a very extensive study of this character has been published (Morgan, 1915). As this case has been reviewed in the introduction, there is little further to be said here.Because of the change that takes place as the culture grows older (the abnormal changing to normal), this character is not of much value in linkage work. The location of the factor in the X chromosome at 2.4 has been made out from the data given by Morgan (1915b). These data, which in general include only the abnormal classes, are summarized in table 1.
Table 1.—Linkage data, from Morgan, 1915b.
EOSIN.
(Plate II, figures 7 and 8.)
The recessive sex-linked mutation eosin eye-color appeared in August 1911 in a culture of white-eyed flies (Morgan 1912a). The eye-color is different in the male and female, the male being a light pinkish yellow, while the female is a rather dark yellowish pink. Eosin is allelomorphic to white and the white-eosin compound or heterozygote has the color of the eosin male. There is probably no special significance in this coincidence of color, since similar dilutions to various degrees have been demonstrated for all the other eye-colors tested (Morgan and Bridges, 1913). Since eosin is allelomorphic to white, its locus is also at 1.1. Eosin is the most useful character among all those in the left end of the chromosome.
BIFID.
The sex-linked wing mutant bifid, which appeared in November 1911, is characterized by the fusion of all the longitudinal veins into a heavy stalk at the base of the wing. The wing stands out from the body at a wide angle, so that the fusion is easily seen. At the tip of the wing the third longitudinal vein spreads out into a delta which reaches to the marginal vein. The fourth longitudinal vein reaches the margin only rarely. There is very often opposite this vein a great bay in the margin, or the whole wing is irregularly truncated.
The stock of bifid was at first extremely varied in the amount of this truncation. By selection a stock was secured which showed only very greatly reduced wings like those shown in figuresa,b. Another stock (figs.c,d) was secured by outcrossing and selection which showed wings of nearly normal size and shape, which always had the bifid stalk, generally the spread positions (not as extreme), and often the delta and the shortened fourth longitudinal vein. We believe that the extreme reduction in size seen in the one stock was due to an added modifier ofthe nature of beaded, since this could be eliminated by outcrossing and selection.
Fig. B. Bifid wingFig. B.—Bifid wing.canddshow the typical condition of bifid wings. All the longitudinal veins are fused into a heavy stalk at the base of the wing.ashows the typical position in which the bifid wings are held. The small size of the wings inaandbis due to the action of a modifier of the nature of "beaded" which has been eliminated inc,d.
Fig. B.—Bifid wing.canddshow the typical condition of bifid wings. All the longitudinal veins are fused into a heavy stalk at the base of the wing.ashows the typical position in which the bifid wings are held. The small size of the wings inaandbis due to the action of a modifier of the nature of "beaded" which has been eliminated inc,d.
LINKAGE OF BIFID WITH YELLOW, WITH WHITE, AND WITH VERMILION.
The stock of the normal (not-beaded) bifid was used by Dr. R. Chambers, Jr., for determining the chromosome locus of bifid by means of its linkage relations to vermilion, white, and yellow (Chambers, 1913). We have attempted to bring together in table 2 the complete data and to calculate the locus of bifid.
Table 2.—Linkage data, from Chambers, 1913.
In the crosses between white and bifid there were 1,127 cross-overs in a total of 20,800 available individuals, which gives a cross-over value of 5.3. In the crosses between yellow and bifid there were 182 cross-overs in a total of 3,175 available individuals, which gives a cross-over value of 5.8. In crosses between bifid and vermilion there were 806 cross-overs in a total of 2,509, which gives a cross-over value of 32.1. On the basis of all the data summarized in table 65, bifid is located at 6.3 to the right of yellow.
LINKAGE OF CHERRY, BIFID, AND VERMILION.
In a small experiment of our own, three factors were involved—cherry, bifid, and vermilion. A cherry vermilion female was crossed to a bifid male. Two daughters were back-crossed singly to white bifid males. The female offspring will then give data for the linkage of cherry white with bifid, while the sons will show the linkage of the three gens, cherry, bifid, and vermilion. The results are shown in table 3.
Table 3.—P1cherry vermilion ♀ ♀ × bifid ♂ ♂. B. C.[2]F1wild-type ♀ × white bifid ♂ ♂.
Both males and females give a cross-over value of 5 units for cherry bifid, which is the value determined by Chambers. The order of the factors, viz, cherry, bifid, vermilion, is established by taking advantage of the double cross-over classes in the males. The male classes give a cross-over value of 20 for bifid vermilion and 24 for cherry vermilion, which are low compared with values given by other experiments. The locus of bifid at 6.3 is convenient for many linkage problems, but this advantage is largely offset by the liability of the bifid flies to become stuck in the food and against the sides of the bottle. Bifid flies can be separated from the normal with certainty and with great ease.
REDUPLICATED LEGS.
In November 1912 Miss Mildred Hoge found that a certain stock was giving some males whose legs were reduplicated, either completely or only with respect to the terminal segments (described and figured, Hoge, 1915). Subsequent work by Miss Hoge showed that the condition was due to a sex-linked gen, but that at room temperature not all the flies that were genetically reduplicated showed reduplication. However, if the flies were raised through the pupa stage in the ice-box at a temperature of about 10° to 12° a majority of the flies which were expected to show reduplication did so. The most extremely reduplicated individual showed parts of 14 legs.
In studying the cross-over values of reduplicated, only those flies that have abnormal legs are to be used in calculation, as in the case of abnormal abdomen where the phenotypically normal individuals are partly genetically abnormal. Table 4 gives a summary of the data secured by Miss Hoge.
Table 4.—Summary of linkage data upon reduplicated legs, from Hoge, 1915.
The most accurate data, those upon the value for reduplicated and vermilion, give for reduplicated a distance of 1.7 from vermilion, either to the right or to the left. The distance from white is 29, which would place the locus for reduplication to the left of vermilion, which is at 33. The data for bar give a distance of 21, but since bar is itself 24 units from vermilion, this distance of 21 would seem to place the locus to the right of vermilion. The evidence is slightly in favor of this position to the right of vermilion at 34.7, where reduplicated may be located provisionally. In any case the locus is so near to that of vermilion that final decision must come from data involving double crossing-over,i. e., from a three-locus experiment.
LETHAL 1.
In February 1912 Miss E. Rawls found that certain females from a wild stock were giving only about half as many sons as daughters. Tests continuing through five generations showed that the sons that appeared were entirely normal, but that half of the daughters gave again 2 : 1 sex-ratios, while the other half gave normal 1 : 1 sex-ratios.
The explanation of this mode of transmission became clear when it was found that the cause of the death of half of the males was a particular factor that had as definite a locus in the X chromosome as have other sex-linked factors (Morgan, 1912e). Morgan mated females (from the stock sent to him by Miss Rawls) to white-eyed males. Half of the females, as expected, gave 2 : 1 sex-ratios, and daughters from these were again mated to white males. Here once more half of the daughters gave 2 : 1 sex-ratios, but in such cases the sons were nearly all white-eyed and only rarely a red-eyed son appeared, when under ordinary circumstances there should be just as many red sons as white sons. The total output for 11 such females was as follows (Morgan, 1914b): white ♀, 457; red ♀, 433; white ♂, 370; red ♂, 2. It is evident from these data that there must be present in the sex-chromosome a gen that causes the death of every male that receives this chromosome, and that this lethal factor lies very close to the factor for white eyes. The linkage of this lethal (now called lethal 1) to various other sex-linked gens was determined (Morgan 1914b), and is summarized in table 5. On the basis of these data it is found that the gen lethal 1 lies 0.4 unit to the left of white, or at 0.7.
Table 5.—Summary of linkage data upon lethal 1, from Morgan, 1914b, pp. 81-92.
LETHAL 1a.
In the second generation of the flies bred by Miss Rawls, one female gave (March 1912) only 3 sons, although she gave 312 daughters. It was not known for some time (see lethals 3 and 3a) what was the cause of this extreme rarity of sons. It is now apparent, however, that this mother carried lethal 1 in one X and in the other X a new lethal which had arisen by mutation. The new lethal was very close to lethal 1, as shown by the rarity of the surviving sons, which are cross-overs between lethal 1 and the new lethal that we may call lethal 1a. There is another class of cross-overs, namely, those which have lethal 1 and get lethal 1aby crossing-over. These doubly lethal males must also die, but since they are theoretically as numerous as the males (3) free from both lethals, we must double this number (3 × 2) to get the total number of cross-overs. There were 312 daughters, but as the sons are normally about 96 per cent of the number of the females,we may take 300 as the number of the males which died. There must have been, then, about 2 per cent of crossing-over, which makes lethal 1alie about 2 units from lethal 1. This location of lethal 1ais confirmed by a test that Miss Rawls made of the daughters of the high-ratio female. Out of 98 of these daughters none repeated the high sex-ratio and only 2 gave 1 ♀ : 1 ♂ ratios. The two daughters which gave 1 : 1 ratios are cross-overs. There should be an equal number of cross-overs which contain both lethals. These latter would not be distinguishable from the non-cross-over females, each of which carries one or the other lethal. In calculation, allowance can be made for them by doubling the number of observed cross-overs (2 × 2) and taking 98 - 2 as the number of non-cross-overs. The cross-over fraction {6 + 4}/{300 + 96} gives 2.6 as the distance between the two lethals. Lethal 1ais probably to the right of lethal 1 at 0.7 + 2.6 = 3.3.
SPOT.
(Plate II, figures 14 to 17.)
In April 1912 there was found in the stock of yellow flies a male that differed from yellow in that it had a conspicuous light spot on the upper surface of the abdomen (Morgan, 1914a). In yellow flies this region is dark brown in color. In crosses with wild flies the spot remained with the yellow, and although some 30,000 flies were raised, none of the gray offspring showed the spot, which should have occurred had crossing-over taken place. The most probable interpretation of spot is that it was due to another mutation in the yellow factor, the first mutation being from gray to yellow and the second from yellow to spot.
Spot behaves as an allelomorph to yellow in all crosses where the two are involved and is completely recessive to yellow,i. e., the yellow-spot hybrid is exactly like yellow. A yellow-spot female, back-crossed to a spot male, produces yellows and spots in equal numbers.
In a cross of spot to black it was found that the double recessive, spot black, flies that appear in F2have, in addition to the spot on the abdomen, another spot on the scutellum and a light streak on the thorax. These two latter characters ("dot and dash") are very sharply marked and conspicuous when the flies are young, but they are only juvenile characters and disappear as the flies become older. The spot flies never show the "dot and dash" clearly, and it only comes out when black acts as a developer. These characters furnish a good illustration of the fact that mutant gens ordinarily affect many parts of the body, though these secondary effects often pass unnoticed.
In the F2of the cross of spot by black one yellow black fly appeared, although none are expected, on the assumption that spot and yelloware allelomorphic. Unless due to crossing-over it must have been a mutation from spot back to yellow. Improbable as this may seem to those who look upon mutations as due to losses from the germ-plasm, yet we have records of several other cases where similar mutations "backwards" have taken place, notably in the case of eosin to white, under conditions where the alternative interpretation of crossing-over is excluded.
SABLE.
(Plate I, figure 2.)
In an experiment involving black body-color[3]a fly appeared (July 19, 1912) whose body-color differed slightly from ordinary black in that the trident mark on the thorax was sharper and the color itself was brighter and clearer. This fly, a male, was mated to black females and gave some black males and females, but also some gray (wild body-color) males and females, showing not only that he was heterozygous for ordinary recessive black, but at the same time that his dark color must be due to another kind of black. The gray F1flies when mated together gave a series of gray and dark flies in F2about as follows: In the females 3 grays to 1 dark; in the males 3 grays to 5 dark in color. The result indicated that the new black color, which we call sable, was due to a sex-linked factor. It was difficult to discover which of the heterogeneous F2males were the new blacks. Suspected males were bred (singly) to wild females, and the F2dark males, from those cultures that gave the closest approach to a 2 gray ♀ : 1 gray ♂ : 1 dark ♂, were bred to their sisters in pairs in order to obtain sable females and males. Thus stock homozygous for sable but still containing black as an impurity was obtained. It became necessary to free it from black by successive individual out-crossings to wild flies and extractions.
This account of how sable was purified shows how difficult it is to separate two recessive factors that give closely similar somatic effects. If a character like sable should be present in any other black stock, or if a character like black should be present in sable, very erratic results would be obtained if such stocks were used in experiments, before such a population had been separated into its component races.
Sable males of the purified stock were mated to wild females and gave wild-type (gray) males and females. These inbred gave the results shown in table 6.
No sable females appeared in F2, as seen in table 6. The reciprocal cross gave the results shown in table 7.
The F1males were sable like their mother. The evidence thus shows that sable is a sex-linked recessive character. Our next step was to determine the linkage relations of sable to certain other sex-linked gens, namely, yellow, eosin, cherry, vermilion, miniature, and bar.
Table 6.—P1wild ♀ ♀ × sable ♂. F1wild-type ♀ ♀ × F1wild-type ♂ ♂.
Table 7.—P1sable ♀ × wild ♂ ♂. F1wild-type ♀ × F1sable ♂.
LINKAGE OF YELLOW AND SABLE.
The factor for yellow body-color lies at one end of the known series of sex-linked gens. As already stated, we speak of this end as the left end of the diagram, and yellow as the zero in locating factors.
When yellow (not-sable) females were mated to (not-yellow) sable males they gave wild-type (gray) daughters and yellow sons. These inbred gave in F2two classes of females, namely, yellow and gray, and four classes of males, namely, yellow and sable (non-cross-overs), wild type and the double recessive yellow sable (cross-overs). From off-spring (F3) of the F2yellow sable males by F2yellow females, pure stock of the double recessive yellow sable was made up and used in the crosses to test linkage.
In color the yellow sable is quite similar to yellow black, that is, a rich brown with a very dark brown trident pattern on the thorax. Yellow sable is easier to distinguish from yellow than is yellow black, even when the flies have not yet acquired their adult body-color.
Yellow sable males were bred to wild females and F1consisted of wild-type males and females. These inbred gave the results shown in table 8.
Table 8.—P1wild ♀ ♀ × yellow sable ♂ ♂. F1wild-type ♀ ♀ × F1wild-type ♂ ♂.
Some of the F1females were back-crossed to yellow sable males and gave the data for table 9.
Table 9.—P1wild-type ♀ ♀ × yellow sable ♂ ♂. B. C. F1wild-type ♀ × yellow sable ♂ ♂.
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.—P1cherry ♀♀ × sable ♂♂. F1wild-type ♀ × F1cherry ♂ ♂.
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.—P1eosin sable ♀ × vermilion ♂♂. F1wild-type ♀♀ × F1eosin sable ♂♂.
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.