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 classes 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 _assortment_ in the germ-cells of the
products of the segregation of two or more pairs of factors. If assortment
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 assortment 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 assortment 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 pass 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 assortment 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
sex-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 classes 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
sex-linked inheritance all sons show the characters of their mother. A male
transmits his sex-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 sex-linked. This term (sex-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 sex 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 sex factor
or sex factors in the X chromosome as we have for other factors. It is true
we do not know whether there is more than one sex-factor, because there is
no crossing-over in the male (the heterozygous sex), and crossing-over in
the female does not influence the distribution of sex, since like parts are
simply interchanged. It follows from this that we are unable as yet to
locate the sex 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 sex-linked.
CROSSING-OVER.
When two or more sex-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 sex-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 homologous piece of the other. The same conclusion
is reached if the cross is made in such a way that the same two sex-linked
characters enter, but, one from the mother and the other from the father.
The daughter gets one of her sex 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 classes._ Clearly, then, the interchange takes place irrespective
of the way in which the factors enter the cross. We call those classes that
arise through interchange between the chromosomes "cross-over classes" 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
sex 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 homologous 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 pass 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 sex
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
sex-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
assumption. 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
sex-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 pass 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 constitution XO. These males received their single X from
their father and therefore show the father's characters. While these XO
males are exceptions to sex-linked inheritance, the characters that they do
show are perfectly normal, that is, the miniature or the bar or other
sex-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 sex 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 sex-linked characters (a matroclinous exception), since she received
both her X chromosomes from her mother and none from her father. As far as
sex 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 sex or on the
visible characters (other than fertility) of the individual.
The evidence is equally positive that sex is quantitatively 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 assume 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
sex 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 sex is determined by the
combinations of the X chromosomes, and that the male and female
combinations are the causes of sex 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 classes 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
sex-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 prone 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 principal 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 sex-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 homologous 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 assortment 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
illustrated) 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 entitled "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}
SEX-LINKED LETHALS AND THE SEX 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 competition with wild forms, or, better said, were they left to
shift 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 classes are missing, and
the absence of these classes can be accounted for on the assumption 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 sex-ratio.
Whether a female bearing the same lethal twice (_i.e._, one homozygous 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 sex-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 assurance that the lethal will not
be lost. If instead of the single recessive used in this fashion, 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
homologous 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
sex-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 passes 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 SEX-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.
A demonstration is given in this instance of the interaction between a
given genotypic constitution and a special environment. The character
abnormal is a sex-linked dominant. Therefore, if an abnormal male is mated
to a wild female the daughters are heterozygous for abnormal, while the
sons, getting their X chromosome from their mother, are entirely normal. In
a wet environment all the daughters are abnormal and the sons normal. As
the culture dries out the daughters' color becomes normal in appearance.
But while the sons {17} will never transmit abnormality to any of their
descendants in any environment, the daughters will transmit (if bred to
normal males) in a suitable environment their peculiarity to half of their
daughters and to half of their sons. The experiment shows convincingly that
the abnormal abdomen appears in a special environment only in those flies
that have a given genotypic constitution.
As the cultures dry out the abnormal males are the first to change over to
normal, then the heterozygous females, and lastly the homozygous females.
It is doubtful if any far-reaching conclusion can be drawn from this
series, because the first and second classes differ from each other not
only in the presence of one or of two factors for abnormal, but also by the
absence in the first case (male) of an entire X chromosome with its
contained factors. The second and third classes differ from each other only
by the abnormal factor.
Similar results were found in the mutant type called reduplicated legs,
which is a sex-linked recessive character that appears best when the
cultures are kept at about 10deg C. As Miss M. A. Hoge has shown, this
character then becomes realized in nearly all of the flies that have the
proper constitution, but not in flies of normal constitution placed in the
same environment. Here the effect is produced by cold.
SEXUAL POLYMORPHISM.
Outside the primary and secondary sexual differences between the male and
the female, there is a considerable number of species of animals with more
than one kind of female or male. Darwin and his followers have tried to
explain such cases on the grounds that more than one kind of female (or
male) might arise through natural selection, in consequence of some
individuals mimicking a protected species. It is needless to point out here
how involved and intricate such a process would be, because the mutation
theory has cut the Gordian knot and given a simpler solution of the origin
of such diandromorphic and digynomorphic conditions.
In _Drosophila_ a mutant, eosin eye-color, appeared in which the female has
darker eyes than the male. If such stock is crossed with cherry (another
sex-linked recessive mutant, allelomorphic to eosin) the females in the F_2
generation are alike (for the pure eosin and the eosin-cherry compound are
not separable), but the cherry males and the eosin males are quite
different in appearance. Here we have a simulation, at least, of a
diandromorphic species. Such a group perpetuates itself, giving one type of
female (inasmuch as eosin and cherry females are very closely similar) and
two types of males, only one of which is like the females. A population of
this kind is very directly comparable to certain polymorphic types that
occur in nature. In _Colias philodice_ there is one type of male, yellow,
and two types of females, yellow and {18} white. In _Colias eurydice_ the
male is orange and the females are orange or white. In _Papilio turnus_ the
male is yellow and the females either yellow or black. Those cases are
directly comparable to an eosin-cherry population, except that in
Lepidoptera the female is heterozygous for the sex differential, in Diptera
the male.
Since in _Drosophila_ the results are explicable on a sex-linked basis, a
similar explanation may apply to polymorphism in butterflies. By suitable
combinations of eosin and cherry most of the cases of polymorphism in
butterflies may be simulated. To simulate the more complex cases, such as
that of _Papilio polytes_ and _memnon_, another allelomorph like eosin
would have to be introduced. A population of mixed cherry and white would
give three somatic types of females (cherry, cherry-white, and white) and
two of males (cherry and white).
FERTILITY AND STERILITY IN THE MUTANTS.
Aside from the decrease in fertility that occurs in certain stocks (a
question that need not be treated here), there are among the types
described in the text two cases that call for special comment. When the
mutant type called "rudimentary" was first discovered, it was found that
the females were sterile but the males were fully fertile. Later work has
revealed the nature of the sterility of the female. The ovaries are present
and in the young flies appear normal, but while in the normal flies the
eggs in the posterior portion enlarge rapidly during the first few days
after hatching, in the rudimentary females only a very few (about 15) eggs
enlarge. The other eggs in the ovary remain at a lower stage of their
development. Rarely the female lays a few eggs; when she does so some of
the eggs hatch, and if she has been mated to a rudimentary male, the
offspring are rudimentary females and males. The rudimentary females mate
in the normal time with rudimentary or with normal males, and their sexual
behavior is normal. Their sterility is therefore due to the failure of the
eggs to develop properly. Whether in addition to this there is some
incompatibility between the sperm and the eggs of this type (as supposed to
be the case at one time) is not conclusively disproved, but is not probable
from the evidence now available.
In the mutant called "fused" the females are sterile both with wild males
and with males from their own stock. An examination of the ovaries of these
females, made by Mr. C. McEwen, shows clearly that there are fewer than the
normal number of mature eggs, recalling the case of rudimentary.
It should be noticed that there is no apparent relation between the
sterility of these two types and the occurrence of the mutation in the X
chromosome, because other mutations in the X do not cause sterility, and
there is sterility in other mutant types that are due to factors in other
chromosomes. {19}
BALANCED INVIABILITY.
The determination of the cross-over values of the factors was at first
hindered because of the poor viability of some of the mutants. If the
viability of each mutant type could be determined in relation to the
viability of the normal, "coefficients of viability" could serve as
corrections in working with the various mutant characters. But it was found
(Bridges and Sturtevant, 1914) that viability was so erratic that
coefficients might mislead. At the same time it was becoming more apparent
that poor viability is no necessary attribute of a character, but depends
very largely on the condition of culture. Competition among larvae was
found to be the chief factor in viability. Mass cultures almost invariably
have extremely poor viability, even though an attempt is made to supply an
abundance of food. Special tests (Morgan and Tice, 1914) showed that even
those mutants which were considered the very poorest in viability were
produced in proportions fairly close to the theoretical when only one
female was used for each large culture bottle and the amount and quality of
food was carefully adjusted.
For the majority of mutants which did well even under heavy competition in
mass cultures the pair-breeding method reduced the disturbances due to
viability to a point where they were negligible.
Later a method was devised (Bridges, 1915) whereby mutations of poor
viability could be worked with in linkage experiments fairly accurately and
whereby the residual inviability of the ordinary characters could be
largely canceled. This method consists in balancing the data of a certain
class with poor viability by means of an equivalent amount of data in which
the same class occurs as the other member of the ratio. Thus in obtaining
data upon any linkage case it is best to have the total number of
individuals made up of approximately equal numbers derived from each of the
possible ways in which the experiment may be conducted. In the simplest
case, in which the results are of the form AB:Ab:aB:ab, let us suppose that
the class ab has a disproportionately low viability. If, then, ab occurs in
an experiment as a cross-over class, that class will be too small and a
false linkage value will be calculated. The remedy is to balance the
preceding data by an equal amount of data in which ab occurs as a
non-cross-over. In these latter the error will be the opposite of the
previous one, and by combining the two experiments the errors should be
balanced to give a better approximation to the true value. When equal
amounts of data, secured in these two ways, are combined, all four classes
will be balanced in the required manner by occurring both as
non-cross-overs and as cross-overs. The error, therefore, should be very
small. For three pairs of gens there are eight classes, and in order that
each of them may appear as a non-cross-over, as each single cross-over, and
as the double cross-over, four experiments must be made. {20}
HOW THE FACTORS ARE LOCATED IN THE CHROMOSOMES.
A character is in the first chromosome if it is transmitted by the
grandfather to half of his grandsons, while, in the reciprocal cross, the
mother transmits her character to all her sons (criss-cross inheritance)
and to half of her granddaughters and to half of her grandsons; in other
words, if the factor that differentiates the character has the same
distribution as the X chromosome. If, however, a new mutant type does not
show this sex-linked inheritance, its chromosome is determined by taking
advantage of the fact that in _Drosophila_ there is no crossing-over in the
male between factors in the same chromosome. For instance, if a new mutant
type is found not to be sex-linked, its group is determined by the
following tests: It is crossed to black, whose factor is known to be in the
second chromosome, and to pink, whose factor lies in the third chromosome.
If the factor of the new form should happen to be in the second chromosome,
then, in the cross with black, no double recessive can appear, so that the
F_{2} proportion is 2:1:1:0; but with pink, the mutant type should give the
proportion 9:3:3:1, typical of free assortment.
If, however, the factor of the new form is in the third chromosome, then,
when crossed to black, the double recessive and the 9:3:3:1 proportion
appear in F_{2}. But when crossed to pink no double recessive appears in
F_{2}, and the proportion 2:1:1:0 occurs.
If these tests show that the new mutant does not belong to either the
second or third chromosome, that is, if both with black and with pink the
9:3:3:1 ratio is obtained, then by exclusion the factor lies in the fourth
chromosome, in which as yet only two factors have been found.
We propose to give in a series of papers an account of the mutant races of
_Drosophila_ and the linkage shown in their inheritance. In this paper we
shall consider only the members of the first chromosome, describing a large
number of new mutants with their linkage relations and summarizing to date
all the linkage data relating to the first chromosome. In later papers we
propose to consider the members of the second, third, and fourth
chromosomes.
The list at the top of page 21 gives the names of the factors dealt with in
this paper. They stand in the order of their discovery, the mutant forms
reported here for the first time being starred.
In each experiment the percentage of crossing-over is found by dividing the
number of the cross-overs by the sum of the non-cross-overs and the
cross-overs, and multiplying this quotient by 100. The resulting
percentages, or cross-over values, are used as measures of the distances
between loci. Thus if the experiments give a cross-over value of 5 per cent
for white and bifid, we say that white and bifid lie 5 units apart in the X
chromosome. Other experiments show that yellow and white are about 1 unit
apart, and that yellow and bifid are about 6 units apart. We can therefore
construct a diagram with yellow as {21} the zero, with white at 1, and with
bifid at 6. If we know the cross-over values given by a new mutant with any
two mutants of the same chromosome whose positions are already determined,
then we can locate the new factor with accuracy, and be able to predict the
cross-over value which the new factor will give with any other factor whose
position is plotted.
_The sex-linked factors of Drosophila._
+------------+----------+-------+-------+--------+------------+---------+
| Gen. | Part |Figure.|Symbol.| Locus. | Date found.|Found by.|
| |affected. | | | | | |
+------------+----------+-------+-------+--------+------------+---------+
|White |Eye-color | 11 | w | 1.1 | May 1910 |Morgan. |
|Rudimentary |Wings | A | r | 55.1 | June 1910 |Morgan. |
|Miniature |Wings | 7-8 | m | 36.1 | Aug. 1910 |Morgan. |
|Vermilion |Eye-color | 10 | v | 33.0 | Nov. 1910 |Morgan. |
|Yellow |Body-color| 5 | y | 0.0 | Jan. 1911 |Wallace. |
|Abnormal |Abdomen | 4 | A' | 2.4 | July 1911 |Morgan. |
|Eosin |Eye-color | 7-8 | w^e | 1.1 | Aug. 1911 |Morgan. |
|Bifid |Wings | B | b_i | 6.3 | Nov. 1911 |Morgan. |
|Reduplicated|Legs | | | 34.7 | Nov. 1911 |Hoge. |
|Lethal 1 |Life | | l_1 | 0.7 | Feb. 1912 |Rawls. |
|Lethal 1_a_*|Life | | l_1a | 3.3 | Mar. 1912 |Rawls. |
|Spot* |Body-color| 14-17 | y^s | 0.0 | April 1912 |Cattell. |
|Sable* |Body-color| 2 | s | 43.0 | July 1912 |Bridges. |
|Dot* |Thorax | | | 33 +/- | July 1912 |Bridges.
|
|Bow* |Wings | C | | | Aug. 1912 |Bridges. |
|Lemon* |Body-color| 3 | l_m | 17.5 | Aug. 1912 |Wallace. |
|Lethal 2 |Life | | l_2 | 12.5+/- | Sept. 1912 |Morgan.
|
|Cherry |Eye-color | 9 | w^c | 1.1 | Oct. 1912 |Safir. |
|Fused* |Venation | D | f_u | 59.5 | Nov. 1912 |Bridges. |
|Forked* |Bristles | E | f | 56.5 | Nov. 1912 |Bridges. |
|Shifted* |Venation | F | s_h | 17.8 | Jan. 1913 |Bridges. |
|Lethal sa |Life | | l_sa | 23.7 | Jan. 1913 |Stark. |
|Bar |Eye-shape | 12-13 | B' | 57.0 | Feb. 1913 |Tice. |
|Notch |Wing | | N' | 2.6 | Mar. 1913 |Dexter. |
|Depressed* |Wing | G | d_p | 18.0 | April 1913 |Bridges. |
|Lethal sb |Life | | l_sb | 16.7 | April 1913 |Stark. |
|Club* |Wings | H | c_l | 14.6 | May 1913 |Morgan. |
|Green* |Body-color| | | | May 1913 |Bridges. |
|Chrome* |Body-color| | | | Sept. 1913 |Bridges. |
|Lethal 3 |Life | | l_3 | 26.5 | Dec. 1913 |Morgan. |
|Lethal 3_a_ |Life | | l_3a | 19.5 | Jan. 1914 |Morgan. |
|Lethal 1_b_*|Life | | l_1b | 1.1- | Feb. 1914 |Morgan. |
|Facet* |Eye | | f_a | 2.2 | Feb. 1914 |Bridges. |
|Lethal _sc_ |Life | | l_sc | 66.2 | April 1914 |Stark. |
|Lethal _sd_ |Life | | l_sd | | May 1914 |Stark. |
|Furrowed |Eye | | f_w | 38.0 | Nov. 1914 |Duncan. |
+------------+----------+-------+-------+--------+------------+---------+
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. {22}
0.0 | Yellow, spot
0.7 | Lethal I
1.1-| Lethal Ib
1.1 | White, eosin, cherry
2.2 | Facet
2.4 | Abnormal
2.6 | Notch
3.3 | Lethal Ia
|
6.3 | Bifid
|
|
|
|
|
|
12.5 | Lethal II
14.6 | Lethal sb
16.7 | Club
17.5 | Lemon
17.8 | Shifted
18.0 | Depressed
19.5 | Lethal IIIa
|
|
23.7 | Lethal sa
|
26.5 | Lethal III
|
|
|
33.0 | Vermilion
33.+/- | Dot
34.7 | Reduplicated
36.1 | Miniature
38.0 | Furrowed
|
|
43.0 | Sable
|
|
|
|
|
|
55.1 | Rudimentary
56.5 | Forked
57.0 | Bar
59.5 | Fused
|
|
|
66.2 | Lethal sc
DIAGRAM I.
{23}
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 of _Drosophila_. 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 the _total_ of 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. {24}
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 (b_i), fused (f_u), and lethal 2 (l_2). 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 y^s for spot, w^e for eosin, and w^c
for cherry. The normal allelomorphs of the mutant gens are indicated by the
converse letter, as V for not-vermilion, B_i for 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 symbol [draw] indicates 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 symbol [draw] when 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 composition [draw]; 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 bar and
vermilion. 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.
* * * * *
{25}