Sex-linked Inheritance In Drosophila Part 1

s.e.x-linked Inheritance in Drosophila.

by Thomas Hunt Morgan and Calvin B. Bridges.

PART I. INTRODUCTORY.

MENDEL'S LAW OF SEGREGATION.

Although the ratio of 3 to 1 in which contrasted characters reappear in the second or F_2 generation is sometimes referred to as Mendel's Law of Heredity, the really significant discovery of Mendel was not the 3 to 1 ratio, but the segregation of the characters (or rather, of the germinal representatives of the characters) which is the underlying cause of the appearance of the ratio. Mendel saw that the characters with which he worked must be represented in the germ-cells by specific producers (which we may call factors), and that in the fertilization of an individual showing one member of a pair of contrasting characters by an individual showing the other member, the factors for the two characters meet in the hybrid, and that _when the hybrid forms germ-cells the factors segregate from each other without having been contaminated one by the other._ In consequence, half the germ-cells contain one member of the pair and the other half the other member. When two such hybrid individuals are bred together the combinations of the pure germ-cells give three cla.s.ses of offspring, namely, two hybrids to one of each of the pure forms. Since the hybrids usually can not be distinguished from one of the pure forms, the observed ratio is 3 of one kind (the dominant) to 1 of the other kind (the recessive).

There is another discovery that is generally included as a part of Mendel's Law. We may refer to this as the _a.s.sortment_ in the germ-cells of the products of the segregation of two or more pairs of factors. If a.s.sortment takes place according to chance, then definite F_2 ratios result, such as 9:3:3:1 (for two pairs) and 27:9:9:9:3:3:3:1 (for three pairs), etc. Mendel obtained such ratios in peas, and until quite recently it has been generally supposed that free a.s.sortment is the rule when several pairs of characters are involved. But, as we shall try to show, the emphasis that has been laid on these ratios has obscured the really important part of Mendel's discovery, namely, _segregation_; for with the discovery in 1906 of the fact of linkage the ratios based on free a.s.sortment were seen to hold only for combinations of certain pairs of characters, not for other combinations. But the principle of segregation still holds for each pair of characters. Hence segregation remains the cardinal point of Mendelism.

Segregation is to-day Mendel's Law.

LINKAGE AND CHROMOSOMES.

It has been found that when _certain_ characters enter a cross together (_i. e._, from the same parent) their factors tend to pa.s.s into the same gamete of the hybrid, with the result that other ratios than the chance ratios described by Mendel are found in the F_2 generation. {6} Such cases of linkage have been described in several forms, but nowhere on so extensive a scale as in the pomace fly, _Drosophila ampelophila_. Here, over a hundred characters that have been investigated as to their linkage relations are found to fall into four groups, the members of each group being linked, in the sense that they tend to be transmitted to the gametes in the same combinations in which they entered from the parents. The members of each group give free a.s.sortment with the members of any of the other three groups. A most significant fact in regard to the linkage shown by the _Drosophila_ mutants is that _the number of linked groups corresponds to the number of pairs of the chromosomes._ If the gens for the Mendelian characters are carried by the chromosomes we should expect to find demonstrated in _Drosophila_ that there are as many groups of characters that are inherited together as there are pairs of chromosomes, provided the chromosomes retain their individuality. The evidence that the chromosomes are structural elements of the cell that perpetuate themselves at every division has continually grown stronger. That factors have the same distribution as the chromosomes is clearly seen in the case of s.e.x-linked characters, where it can be shown that any character of this type appears in those individuals which from the known distribution of the X chromosomes must also contain the chromosome in question. For example, in _Drosophila_, as in many other insects, there are two X chromosomes in the cells of the female and one X chromosome in the cells of the male. There is in the male, in addition to the X, also a Y chromosome, which acts as its mate in synapsis and reduction. After reduction each egg carries an X chromosome. In the male there are two cla.s.ses of sperm, one carrying the X chromosome and the other carrying the Y chromosome. Any egg fertilized by an X sperm produces a female; any egg fertilized by a Y sperm produces a male. The scheme of inheritance is as follows.

+------------------------------+ Eggs X--X Sperm X--Y +------------------------------+ Daughter XX Son XY +------------------------------+

The sons get their single X chromosome from their mother, and should therefore show any character whose gen is carried by such a chromosome. In s.e.x-linked inheritance all sons show the characters of their mother. A male transmits his s.e.x-linked character to his daughters, who show it if dominant and conceal it if recessive. But any daughter will transmit such a character, whether dominant or recessive, to half of her sons. The path of transmission of the gen is the same as the path followed by the X chromosome, received here {7} from the male. Many other combinations show the same relations. In the case of non-disjunction, to be given later, there is direct experimental evidence of such a nature that there can no longer be any doubt that the X chromosomes are the carriers of certain gens that we speak of as s.e.x-linked. This term (s.e.x-linked) is intended to mean that such characters are carried by the X chromosome. It has been objected that this use of the term implies a knowledge of a factor for s.e.x in the X chromosome to which the other factors in that chromosome are linked; but in fact we have as much knowledge in regard to the occurrence of a s.e.x factor or s.e.x factors in the X chromosome as we have for other factors. It is true we do not know whether there is more than one s.e.x-factor, because there is no crossing-over in the male (the heterozygous s.e.x), and crossing-over in the female does not influence the distribution of s.e.x, since like parts are simply interchanged. It follows from this that we are unable as yet to locate the s.e.x factor or factors in the X chromosome. The fact that we can not detect crossing-over under this condition is not an argument against the occurrence of linkage. We are justified, therefore, in speaking of the factors carried by the X chromosome as s.e.x-linked.

CROSSING-OVER.

When two or more s.e.x-linked factors are present in a male they are always transmitted together to his daughters, as must necessarily be the case if they are carried by the unpaired X chromosome. If such a male carrying, let us say, two s.e.x-linked factors, is mated to a wild female, his daughters will have one X chromosome containing the factors for both characters, derived from the father, and another X chromosome that contains the factors that are normal for these two factors (the normal allelomorphs). The sons of such a female will get one or the other of these two kinds of chromosomes, and should be expected to be like the one or the other grandparent. In fact, most of the sons are of these two kinds. But, in addition, there are sons that show one only of the two original mutant characters. Clearly an interchange has taken place between the two X chromosomes in the female in such a way that a piece of one chromosome has been exchanged for the h.o.m.ologous piece of the other. The same conclusion is reached if the cross is made in such a way that the same two s.e.x-linked characters enter, but, one from the mother and the other from the father.

The daughter gets one of her s.e.x chromosomes from her mother and the other from her father. She should produce, then, two kinds of sons, one like her mother and one like her father. In fact, the majority of her sons are of these two kinds, but, in addition, there are two other kinds of sons, one kind showing both mutant characters, the other kind showing normal characters. Here again the results must be due to interchange between the two X's in the hybrid female. _The number of_ {8} _the sons due to exchange in the two foregoing crosses is always the same, although they are of contrary cla.s.ses._ Clearly, then, the interchange takes place irrespective of the way in which the factors enter the cross. We call those cla.s.ses that arise through interchange between the chromosomes "cross-over cla.s.ses" or merely "cross-overs." The phenomenon of holding together we speak of as linkage.

By taking a number of factors into consideration at the same time it has been shown that _crossing-over involves large pieces of the chromosomes_.

The X chromosomes undergo crossing-over in about 60 per cent of the cases, and the crossing-over may occur at any point along the chromosome. When it occurs once, whole ends (or halves even) go over together and the exchange is always equivalent. If crossing-over occurs twice at the same time a middle piece of one chromosome is intercalated between the ends of the other chromosome. This process is called double crossing-over. It occurs not oftener than in about 10 per cent of cases for the total length of the X chromosome. Triple crossing-over in the X chromosome is extremely rare and has been observed only about a half dozen times.

While the genetic evidence forces one to accept crossing-over between the s.e.x chromosomes in the female, that evidence gives no clue as to how such a process is brought about. There are, however, certain facts familiar to the cytologist that furnish a clue as to how such an interchange might take place. When the h.o.m.ologous chromosomes come together at synapsis it has been demonstrated, in some forms at least, that they twist about each other so that one chromosome comes to lie now on the one side now on the other of its partner. If at some points the chromosomes break and the pieces on the same side unite and pa.s.s to the same pole of the karyokinetic spindle, the necessary condition for crossing-over will have been fulfilled.

THE Y CHROMOSOME AND NON-DISJUNCTION.

Following Wilson's nomenclature, we speak of both X and Y as s.e.x chromosomes. Both the cytological and the genetic evidence shows that when two X chromosomes are present a female is produced, when one, a male. This conclusion leaves the Y chromosome without any observed relation to s.e.x-determination, despite the fact that the Y is normally present in every male and is confined to the male line. The question may be asked, and in fact has been asked, why may not the presence of the Y chromosome determine that a male develop and its absence that a female appear? The only answer that has yet been given, outside of the work on _Drosophila_, is that since in some insects there is no Y chromosome, there is no need to make such an a.s.sumption. But in _Drosophila_ direct proof that Y has no such function is furnished by the evidence discovered by Bridges in the case of non-disjunction. (Bridges, 1913, 1914, 1916, and unpublished results.) {9}

Ordinarily all the sons and none of the daughters show the recessive s.e.x-linked characters of the mother when the father carries the dominant allelomorph. The peculiarity of non-disjunction is that sometimes a female produces a daughter like herself or a son like the father, although the rest of the offspring are perfectly regular. For example, a vermilion female mated to a wild male produces vermilion sons and wild-type daughters, but rarely also a vermilion daughter or a wild-type son. The production of these exceptions (primary exceptions) by a normal XX female must be due to an aberrant reduction division at which the two X chromosomes fail to disjoin from each other. In consequence both remain in the egg or both pa.s.s into the polar body. In the latter case an egg without an X chromosome is produced. Such an egg fertilized by an X sperm produces a male with the const.i.tution XO. These males received their single X from their father and therefore show the father's characters. While these XO males are exceptions to s.e.x-linked inheritance, the characters that they do show are perfectly normal, that is, the miniature or the bar or other s.e.x-linked characters that the XO male has are like those of an XY male, showing that the Y normally has no effect upon the development of these characters. But that the Y does play some positive role is proved by the fact that all the XO males have been found to be absolutely sterile.

While the presence of the Y is necessary for the fertility of the male, it has no effect upon s.e.x itself. This is shown even more strikingly by the phenomenon known as secondary non-disjunction. If the two X chromosomes that fail to disjoin remain in the egg, and this egg is fertilized by a Y sperm, an XXY individual results. This is a female which is like her mother in all s.e.x-linked characters (a matroclinous exception), since she received both her X chromosomes from her mother and none from her father. As far as s.e.x is concerned this is a perfectly normal female. The extra Y has no effect upon the appearance of the characters, even in the case of eosin, where the female is much darker than the male. The only effect which the extra Y has is as an extra wheel in the machinery of synapsis and reduction; for, on account of the presence of the Y, both X's of the XXY female are sometimes left within the ripe egg, a process called secondary non-disjunction. In consequence, an XXY female regularly produces exceptions (to the extent of about 4 per cent). A small percentage of reductions are of this XX-Y type; the majority are X-XY. The XY eggs, produced by the X-XY reductions, when fertilized by Y sperm, give XYY males, which show no influence of the extra Y except at synapsis and reduction. By mating an XXY female to an XYY male, XXYY females have been produced and these are perfectly normal in appearance. We may conclude from the fact that visibly indistinguishable males have been produced with the formulas XO, XY, and XYY, and {10} likewise females with the formulas XX, XXY, and XXYY, that the Y is without effect either on the s.e.x or on the visible characters (other than fertility) of the individual.

The evidence is equally positive that s.e.x is quant.i.tatively determined by the X chromosome--that two X's determine a female and one a male. For in the case of non-disjunction, a zero or a Y egg fertilized by an X sperm produces a male, while conversely an XX egg fertilized by a Y sperm produces a female. It is thus impossible to a.s.sume that the X sperms are normally female-producing because of something else than the X or that the Y sperm produce males for any other reason than that they normally fertilize X eggs. Both the X and the Y sperm have been shown to produce the s.e.x opposite to that which they normally produce when they fertilize eggs that are normal in every respect, except that of their X chromosome content. These facts establish experimentally that s.e.x is determined by the combinations of the X chromosomes, and that the male and female combinations are the causes of s.e.x differentiation and are not simply the results of maleness and femaleness already determined by some other agent.

Cytological examination has demonstrated the existence of one XXYY female, and has checked up the occurrence in the proper cla.s.ses and proportions of the XXY females. Numerous and extensive breeding-tests have been made upon the other points discussed. The evidence leaves no escape from the conclusion that the genetic exceptions are produced as a consequence of the exceptional distribution of the X chromosomes and that the gens for the s.e.x-linked characters are carried by those chromosomes.

MUTATION IN DROSOPHILA AMPELOPHILA.

The first mutants were found in the spring of 1910. Since then an ever-increasing series of new types has been appearing. An immense number of flies have come under the scrutiny of those who are working in the Zoological Laboratory of Columbia University, and the discovery of so many mutant types is undoubtedly due to this fact. But that mutation is more frequent in _Drosophila ampelophila_ than in some of the other species of _Drosophila_ seems not improbable from an extensive examination of other types. It is true a few mutants have been found in other _Drosophilas_, but relatively few as compared with the number in _D. ampelophila_. Whether _ampelophila_ is more p.r.o.ne to mutate, or whether the conditions under which it is kept are such as to favor this process, we have no knowledge.

Several attempts that we have made to produce mutations have led to no conclusive results.

The mutants of _Drosophila_ have been referred to by Baur as "mutations through loss," but inasmuch as they differ in no respect that we can discover from other mutants in domesticated animals and plants, there is no particular reason for putting them into this category unless {11} to imply that new characters have not appeared, or that those that have appeared must be due to loss in the sense of absence of something from the germ-plasm.

In regard to the first point, several of the mutants are characterized by what seem to be additions. For example, the eye-color sepia is darker than the ordinary red. At least three new markings have been added to the thorax. A speck has appeared at the base of the wing, etc. These are recessive characters, it is true, but the character "streak," which consists of a dark band added to the thorax, is a dominant. If dominance is supposed to be a criterion as to "presence," then it should be pointed out that among the mutants of _Drosophila_ a number of dominant types occur.

But clearly we are not justified by these criteria in inferring anything whatever in regard to the nature of the change that takes place in the germ-plasm. Probably the only data which give a basis for attempting to decide the nature of the change in the germ-plasm are from cases where multiple allelomorphs are found. Several such cases are known to us, and two of these are found in the X chromosome group, namely, a quadruple system (white, eosin, cherry, red), and a triple system (yellow, spot, gray). In such cases each member acts as the allelomorph of any other member, and only two can occur in any one female, and only one in any male.

If the normal allelomorph is thought of as the positive character, which one of the mutants is due to its loss or to its absence? If each is produced by a loss it must be a different loss that acts as an allelomorph to the other loss. This is obviously absurd unless a different idea from the one usually promulgated in regard to "absence" is held.

MULTIPLE ALLELOMORPHS.

It appears that Cuenot was the first to find a case (in mice) in which the results could be explained on the basis that more than two factors may stand in the relation of allelomorphs to each other. In other words, a given factor may become the partner of more than one other factor, although, in any one individual, no more than two factors stand in this relation. While it appears that his evidence as published was not demonstrative, and that, at the time he wrote, the possibility of such results being due to very close linkage could not have been appreciated as an alternative explanation, nevertheless it remains that Cuenot was right in his interpretation of his results and that the factors for yellow, gray, gray white-belly, and black in mice form a system of quadruple allelomorphs.

There are at least two such systems among the factors in the first chromosome in _Drosophila_. The first of these includes the factor for white eyes, that for eosin eyes, and that for cherry eyes, and of course that allelomorph of these factors present in the wild fly and which when present gives the red color. In this instance the normal {12} allelomorph dominates all the other three, but in mice the mutant factor for yellow dominates the wild or "normal" allelomorph.

The other system of multiple allelomorphs in the first chromosome is a triple system made up of yellow (body-color), spot (on abdomen), and their normal allelomorph--the factor in the normal fly that stands for "gray."

In general it may be said that there are two princ.i.p.al ways in which it is possible to show that certain factors (more than two) are the allelomorphs of each other. First, if they are allelomorphs only two can exist in the same individual; and, in the case of s.e.x-linked characters, while two may exist in the same female, only one can exist in the male, for he contains but one X chromosome. Second, all the allelomorphs should give the same percentages of crossing-over with each other factor in the same chromosome.

It is a question of considerable theoretical importance whether these cases of multiple allelomorphs are only extreme cases of linkage or whether they form a system quite apart from linkage and in relation to normal allelomorphism. It may be worth while, therefore, to discuss this question more at length, especially because _Drosophila_ is one of the best cases known for such a discussion.

The factors in the first chromosome are linked to each other in various degrees. When they are as closely linked as yellow body-color and white eyes crossing-over takes place only once in a hundred times. If two factors were still nearer together it is thinkable that crossing-over might be such a rare occurrence that it would require an enormous number of individuals to demonstrate its occurrence. In such a case the factors might be said to be completely linked, yet each would be supposed to have its normal allelomorph in the h.o.m.ologous chromosome of the wild type. Imagine, then, a situation in which one of these two mutant factors (a) enters from one parent and the other mutant factor (b) from the other parent. The normal allelomorph of a may be called A. It enters the combination with b, while the normal allelomorph B of b enters the combination with a. Since b is completely linked to A and a to B, the result will be the same as though a and b were the allelomorphs of each other, for in the germ-cells of the hybrid aBAb the a.s.sortment will be into aB and Ab, which is the same as though a and b acted as segregating allelomorphs.

There is no way from Mendelian data by which this difference between a true case of multiple allelomorphs and one of complete linkage (as just ill.u.s.trated) can be determined. There is, however, a different line of attack which, in a case like that of _Drosophila_, will give an answer to this question. The answer is found in the way in which the mutant factors arise. This argument has been fully developed in the book ent.i.tled "The Mechanism of Mendelian Inheritance," and will therefore not be repeated here. It must suffice to say that if two mutant {13} types that behave as allelomorphs of each other arise separately from the wild form, one of them must have arisen as a double mutation of two factors so close to each other as to be completely linked--a highly improbable occurrence when the infrequency of mutations is taken into consideration.[1] The evidence opposed to such an interpretation is now so strong that there can be little doubt that multiple allelomorphs have actually appeared.

On _a priori_ grounds there is no reason why several mutative changes might not take place in the same locus of a chromosome. If we think of a chromosome as made up of a chain of chemical particles, there may be a number of possible recombinations or rearrangements within each particle.

Any change might make a difference in the end-product of the activity of the cell, and give rise to a new mutant type. It is only when one arbitrarily supposes that the only possible change in a factor is its loss that any serious difficulty arises in the interpretation of multiple allelomorphs.

One of the most striking facts connected with the subject of multiple allelomorphs is that the same kind of change is effected in the same organ.

Thus, in the quadruple system mentioned above, the color of the eye is affected. In the yellow-spot system the color of the body is involved. In mice it is the coat-color that is different in each member of the series.

While this is undoubtedly a striking relation and one which seems to fit well with the idea that such effects are due to mutative changes in the same fundamental element that affects the character in question, yet on the other hand it would be dangerous to lay too much emphasis on this point, because any given organ may be affected by other factors in a similar manner, and also because a factor frequently produces more than a single effect. For instance, the factor that when present gives a white eye affects also the general yellowish pigment of the body. If red-eyed and white-eyed flies are put for several hours into alcohol, the yellowish body-color of the white-eyed flies is freely extracted, but not that of the red-eyed flies. In the living condition the difference between the body-colors of the red- and of the white-eyed flies is too slight to be visible, but after extraction in alcohol the difference is striking. There are other effects also that follow in the wake of the white factor. Now, it is quite conceivable that in some specific case one of the effects might be more striking than the one produced in that organ more markedly affected by the other factor of the allelomorphic series. In such a case the relation mentioned above might seemingly disappear. For this reason it is well not to insist too strongly on the idea that multiple allelomorphs affect the same part in the same way, even although at present that appears to be the rule for all known cases.

{14}

s.e.x-LINKED LETHALS AND THE s.e.x RATIO.

Most of the mutant types of _Drosophila_ show characteristics that may be regarded as superficial in so far as they do not prevent the animal from living in the protected life that our cultures afford. Were they thrown into open compet.i.tion with wild forms, or, better said, were they left to s.h.i.+ft for themselves under natural conditions, many or most of the types would no doubt soon die out. So far as we can see, there is no reason to suppose that the mutations which can be described as superficial are disproportionally more likely to occur than others. Of course, superficial mutations are more likely to survive and hence to be seen; while if mutations took place in important organs some of them would be expected to affect injuriously parts essential to the life of the individual and in consequence such an individual perishes. The "lethal factors" of _Drosophila_ may be supposed to be mutations of some such nature; but as yet we have not studied this side of the question sufficiently, and this supposed method of action of the lethals is purely speculative. Whatever the nature of the lethals' action, it can be shown that from among the offspring obtained from certain stocks expected cla.s.ses are missing, and the absence of these cla.s.ses can be accounted for on the a.s.sumption that there are present mutant factors that follow the Mendelian rule of segregation and which show normal linkage to other factors, but whose only recognizable difference from the normal is the death of those individuals which receive them. The numerical results can be handled in precisely the same way as are other linkage results.

There are some general relations that concern the lethals that may be mentioned here, while the details are left for the special part or are found in the special papers dealing with these lethals. A factor of this kind carried by the X chromosome would be transmitted in the female line because the female, having two X chromosomes, would have one of them with the normal allelomorph (dominant) of the lethal factor carried by the other X chromosome. Half of her sons would get one of her X's, the other half the other. Those sons that get the lethal X will die, since the male having only one X lacks the power of containing both the lethal and its normal allelomorph. The other half of the sons will survive, but will not transmit the lethal factor. In all lethal stocks there are only half as many sons as daughters. The heterozygous lethal-bearing female, fertilized by a normal male, will give rise to two kinds of daughters; one normal in both X's, the other with a normal X and a lethal-bearing X chromosome. The former are always normal in behavior, and the latter repeat in their descendants the 2:1 s.e.x-ratio.

Whether a female bearing the same lethal twice (_i.e._, one h.o.m.ozygous for a given lethal) would die, can not be stated, for no such females are obtainable, because the lethal males, which alone could bring about {15} such a condition, do not exist. The presumption is that a female of this kind would also die if the lethal acts injuriously on some vital function or structure.

Since only half of the daughters of the lethal-bearing females carry the lethal, the stock can be maintained by breeding daughters separately in each generation to insure obtaining one which repeats the 2:1 ratio. There is, however, a much more advantageous way of carrying on the stock--one that also confirms the sufficiency of the theory.

In carrying on a stock of a lethal, advantage can be taken of linkage. A lethal factor has a definite locus in the chromosome; if, then, a lethal-bearing female is crossed to a male of another stock with a recessive character whose factor lies in the X chromosome very close to the lethal factor, half the daughters will have lethal in one X and the recessive in the other. The lethal-bearing females can be picked out from their sisters by the fact that they give a 2:1 s.e.x-ratio, and by the fact that nearly all the sons that do survive show the recessive character. If such females are tested by breeding to the recessive males, then the daughters which do not show the recessive carry the lethal, except in the few cases of crossing-over. Thus in each generation the normal females are crossed to the recessive males with the a.s.surance that the lethal will not be lost. If instead of the single recessive used in this fas.h.i.+on, a double recessive of such a sort that one recessive lies on each side of the lethal is used, then in each generation the females which show neither recessive will almost invariably contain the lethal, since a double cross-over is required to remove the lethal.

It is true that females carrying two _different_ lethals might arise and not die, because the injurious effect of each lethal would be dominated by its allelomorph in the other X chromosome. Such females can not be obtained by combining two existing lethals, since lethal males do not survive. They can occur only through a new lethal arising through mutation in the h.o.m.ologous chromosome of a female that already carries one lethal. Rare as such an event must be, it has occurred in our cultures thrice. The presence of a female of this kind will be at once noticed by the fact that she produces no sons, or very rarely one, giving in consequence extraordinary s.e.x-ratios. The rare appearance of a son from such a female can be accounted for in the following way: If crossing-over occurs between her X chromosomes the result will be that one X will sometimes contain two lethals, the other none. The latter, if it pa.s.ses into a male, will lead to the development of a normal individual. The number of such males depends on the distance apart of the two lethals in the chromosome. There is a crucial test of this hypothesis of two lethals in females giving extraordinary ratios. This test has been applied to the cases in which such females were found, by Rawls (1913), by Morgan (1914_c_), and again by Stark (1915), and it has been found to confirm the explanation. The daughters of {16} such a female should all (excepting a rare one due to crossing-over) give 2:1 ratios, because each daughter must get one or the other X chromosome of her mother, that is, one or the other lethal. Although the mother was fertilized by a normal male, every daughter is heterozygous for one or the other of the lethal factors. The daughters of the two-lethal females differ from the daughters of the one-lethal female in that the former mother, as just stated, gives all lethal-bearing daughters; the latter transmits her lethal to only half of her daughters.

INFLUENCE OF THE ENVIRONMENT ON THE REALIZATION OF TWO s.e.x-LINKED CHARACTERS.

The need of a special environment in order that certain mutant characters may express themselves has been shown for abnormal abdomen (Morgan, 1912_d_, 1915_b_) and for reduplication of the legs (Hoge, 1915). In a third type, club, described here (page 69), the failure of the unfolding of the wing which occurs in about 20 per cent of the flies is also without much doubt an environmental effect, but as yet the particular influence that causes the change is unknown.

A very extensive series of observations has been made on the character called abnormal abdomen. In pure cultures kept moist with abundance of fresh food all the flies that hatch for the first few days have the black bands of the abdomen obliterated or made faint and irregular. As the bottles get dry and the food becomes scarce the flies become more and more normal, until at last they are indistinguishable from the normal flies.

Nevertheless these normal-looking flies will give rise in a suitable environment to the same kind of flies as the very abnormal flies first hatched. By breeding from the last flies of each culture, and in dry cultures, flies can be bred from normal ancestors for several generations, and then by making the conditions favorable for the appearance of the abnormal condition, the flies will be as abnormal as though their ancestors had always been abnormal. Here, then, is a character that is susceptible to the variations in the environment, yet whatever the realized condition of the soma may be, that condition has no effect whatever on the nature of the germ-plasm. A more striking disproof of the theory of the inheritance of acquired characters would be hard to find.