THE BEIIAVIOR IN SUCCESSIVE NUCLEAR DIVISIONS OF A
        CHROMOSOME BROKEN A T MEIOSIS

              BY BARBARA MCCLLNTOCK   J


         DEPARTMENT OF BOTANY, UNIVERSITY OF MISSOURI

              Communicated July 7, 1939
If a chromatid in maize is broken at a meiotic anaphase and if it enters
a telophase nucleus, fusion between the two sister halves of this chromatid
will result at the position of breakage.'  Because of this fusion, the separa-
tion at a succeeding anaphase of the sister halves of this chromatid neces-
sarily produces a bridge configuration. The pull on the chromatin of this
bridge in the anaphase or telophase results in breakage and again a broken
chromosome enters each telophase nucleus. The following questions im-
mediately arise: Will this chromatid produce a bridge in the following
mitotic anaphase by a similar fusion of sister halves at the position of the
second break? Will this process continue indefinitely in each successive
division or will the broken end of a chromosome eventually "heal" and con-
tinue through mitotic cycles without further fusions, bridges and break-
ages?  It is stated in the previous publication that the evidence on this


406              GENETICS: B. McCLINTOCh-

PROC. N. A. S.

point was conflicting. It is the purpose of this note to indicate the nature
of this conflict.  In brief, the evidence shows that the breakage-fusion-
bridge-breakage cycle will continue, after an original meiotic break, in the
endosperm, whether the broken chromosome is contributed by the male
or by the female gamete.  In contrast to the endosperm, healing of the
broken end of the chromosome, i.e., discontinuance of the breakage-
fusion-bridge-breakage cycle, occurs in the embryo.

The method previously used to demonstrate, cytologically, the fusion
of broken ends of sister chromatids involved the use of an inversion.  In all
such cases the broken chromatid produced is deficient and would not be
expected to survive regularly through ovules or to be transmitted by pollen
grains.  Hence, a test of the behavior of such a broken chromosome could
not readily be obtained. Consequently, other methods have been used to
obtain a broken chromosome with at least a complete set of genes.

TheJirst method to be described involves the use of an x-ray induced re-
arrangement in chromosome 9. The morphology of a normal chromosome
9 is shown in a, figure 1. The short arm terminates in a large knob (stip-

                          FIGURE 1
a. Diagram of a normal chromosome 9. The stippled region represents the large
terminal knob, the dash line, the segment composed of small, loosely spaced chromo-
meres. The wide solid line represents the heavy, closely associated chromomeres adjacent
to the spindle fibre attachment region which is represented as a clear bulge.  The long
arm is represented by a thin, solid line.  The arrows point to the positions of the three
breaks, followed by refusions, which produced the chromosome diagrammed in b.
c.  The association of homologous regions, 1 to 5, of a normal chromosome 9 with a
very small knob and the rearranged chromosome 9 shown in b.  The cross indicates
a position of crossing-over to give rise to the dicentric chromosome shown in a, figures
3 and 4.

pled).  (The presence of a knob is not necessary for the functioning of
chromosome 9.  Strains with no knob or knobs of intermediate sizes are
known. The knob substance lengthens the chromosome at its end but does


VOL.~~, 1939        GENETICS: B. McCLINTOCK               407

not add necessary genie material.) The proximal one-third of the short arm
(5 and 6, figure 1) adjacent to the spindle fibre region, is characterized by
large, closely associated, deeply staining chromomeres, suggesting hetero-

.  chromatin in appearance. The distal two-thirds of the short arm (dash lines,
figure 1) is composed of small, widely spaced chromomeres.  The x-ray
induced rearrangement is shown in b, figure 1. The positions of breakage in
the normal chromosome 9 which gave rise to this rearrangementareindicated
by the arrows in a, figure 1.  The terminal large knob has been broken into
two unequal parts, the smaller part together with three-quarters of the
short arm being inserted into the long arm, the other section being inverted.
In plants possessing one normal chromosome 9 and one altered chromo-
some 9, homologous synaptic association of regions 1 to 5 (c, figure 1) fol-
lowed by a crossover within this region, results in a chromatid with two
spindle fibre attachment regions and a chromatid with no spindle fibre at-
tachment region (a, figures 3 and 4).  The composition of the chromatid
with two spindle fibre regions should be noted.  From right to left it is
composed of a chromosome 9 complete as far as and including the small
internal knob. This is followed by the conspicuous deeply staining region,
6, which joins the second spindle fibre attachment region. As the two
spindle fibre regions pass to opposite poles at anaphase I, a break must oc-
cur between these two regions.  Should it occur between the internal knob
and the second fibre region (at position of arrow in a, figures 3 and 4) one
of the two resulting chromatids will possess a full set of genes plus a short
duplication composed of a part of region 8 extending beyond the small
knob. This chromosome would be expected to be functional since no defi-
ciency is present. The main factor to be emphasized is the presence in this
chromosome of a broken end.  It is the purpose of this paper to test the
subsequent behavior of this broken end in the future nuclear cycles.

The genes Yg, C, Sh and Wx (Yg, normal green plant, yg, yellow-green
plant; C, colored aleurone, c, colorless aleurone; Sh, normal endosperm
development, sh, shrunken endosperm; Wx, normal starch in endosperm
and pollen, staining blue with iodine, wx, waxy endosperm whose starch
stains red with iodine) are located in the short arm of chromosome 9, the
order being : knob- Yg-C-Sh- Wx-spindle fibre region.'  The crossover
distance between the genes are 19, 3 and 21, respectively.3 All four genes
are located in the translocated segment 1 to L4  It should be emphasized
that Yg is a plant character whereas c, sh and wx are endosperm characters.
Plants possessing a normal chromosome 9 with the genes Yg-C (or c)-sh-wx
and the altered chromosome 9 containing the dominants only, when
crossed by yg-c-sh-wx or Yg-c-sh-wx, have given the results shown in
table 1.  The kernels have been divided into three main classes: I, full
colored, non-variegated kernels; II, c--Sh-wx kernels and III, kernels whose
aleurone color is variegated for C and c.  It will be noted that in the first


408             GENETICS: B. McCLINTOCK

PR0C.N. A.S

class, the non-variegated kernels, only 5 are detectable crossovers.  Of the
41 in the variegated class, 38 are recognizable crossovers.  The three that
contain Wx are variegated for Wx and wx as well as C and c. The type of
variegation is very specific. Colorless areas (6) of various sizes occur.

TABLE 1

DISTRIBUTION OF ENDOSPERM CHARACTERS RESULTING PROM THE CROSS

C(or c)-sh-zux-normal chromosome 9
' C-Sh-Wx-rearranged chromosome 9 X 3 c-sh-w-normal chromosome 9

I.   Non-variegated kernels :     c Sh wx      C Sh wx     Csh Wx
                         3185         4           1
II.   c sh wx kernels
     3128
III. Variegated kernels:   C-c (Sh-sh) + Wx-wx  C-c (Sh-sh) * wn  C-c sh wx
                          3              34          4

* Variegation for Sh and sh cannot be readily detected.

Within these areas there are no colored (C) cells. The kernels that are
likewise variegated for Wx and wx show a distinct relationship between
patterns of the C and c and of the Wx and wx variegations. All the C spots
are Wx. The c spots are of two types : (1) those that are totally wx and (2)
those showing smaller areas of wx within a c-Wx spot. Just such variega-
tion would be expected if the endosperms in these kernels had received
chromosomes 9 with broken ends, the broken chromosomes carrying the
dominant C or the dominants C and Wx, and if fusions of broken ends were
occurring in successive nuclear divisions, i.e., continuance of the breakage-
fusion-bridge-breakage cycle.  It is only necessary to assume that the
successive breaks are not always at the points of previous fusions.  Since
this is known to occur from direct cytological observations' and from ob-
servations of chromosomes which have undergone such successive breaks
(see figures 3 and 4), it is a justifiable assumption.  A chromosome 9 with a
broken end and carrying C and Wx could result from a crossover (c, figure 1)
between the translocation point to the right in the rearranged chromosome
9 and the gene Wx, followed by a break at anaphase I of the dicentric chro-
matid so produced (the chromatid recovered is that to the right of the
arrow, a, figures 3 and 4). This broken chromosome should give C-c and
Wx-wx variegation.  If the crossover occurred between Wx and C, the
chromosome with the broken end would contain the genes C and wx.  A
wx kernel with C-c variegation would resu t.  In the case being described,
variegation could result from fusions between the broken ends of two
chromosomes 9 (the two contributed by the female parent) or between ends
of sister halves of a chromosome.  That the latter can occur will be evident


VOL. 25, 1939        GENETICS: B. MCCLINTOCK                409

from the second case to be described.  Figure 2 has been constructed to
illustrate the method by which C-c and Wxezox variegations are produced
on the basis of fusion of broken ends of sister halves of a chromosome.  If
a broken chromosome with fused sister halves contains the genes C and Wx
(left, figure 2) and if the break in the following anaphase occurs between

C    c wx

                 C
                           c wx
c wx      TC t
c
  c wx
t           \ wx
                                   r

wx -
t

FIGURE 2

Diagram to illustrate the method by which variegation of the endosperm characters
C and Wx can be produced.  The figure to the left represents the two sister halves of
a broken chromosome 9 fused at the position of a previous break.  The chromosome
carries the genes C and Wx in the order given.  The spindle fibre attachment region
is represented as a clear bulging region.  Separation of the two halves in the following
anaphase produces a bridge configuration.  If breakage of the bridge occurs at the
position of the arrow, a chromosome lacking C will enter one nucleus (lower right) and
a chromosome duplicated for C will enter the sister nucleus (upper right).  If fusions
occur between the two split halves of each of these broken chromosomes, as represented
in the diagrams, a bridge will form in the following anaphase.  If breaks occur at the
positions of the arrows, Wx will be deleted from one of the resulting nuclei in each case.

Wx and C (arrow), the chromosome entering one nucleus will carry Wx
and a duplication with C (upper right, figure 2).  The chromosome enter-
ing the sister nucleus will have lost C but will possess Wx (lower right,
figure 2). Through fusions of broken ends of the two sister halves of each
of these chromosomes, in turn, an anaphase bridge will be produced in the
following division.  If the break occurs at the arrows in each case, Wx will
be removed from the chromosome in one of the two daughter nuclei, re-
spectively.  On this basis, two types of spots are expected in a variegated
kernel containing C and Wx:  (1) C-WX spots and (2) c-Wx spots within
which there are areas of wx.  Since the size of the recessive spots range
from large to very small, the breakage-fusion-bridge-breakage cycle must
continue in successive nuclear divisions in the endosperm tissue.

From genetic data on normal chromosome 9, it is known that within the
region 1 to 3, figure 1, single crossover chromatids are produced far more
frequently than double crossover chromatids.3  From the data in table 1.


410

GENETZCS: B. MCCLZNTOCK        PROC. N. A. S.

it is strikingly evident that the majority of detectable crossovers are as-
sociated with variegated endosperm.  Only 5 are not associated with
variegation. It was therefore suspected that the 41 variegated kernels
have obtained a chromosome with a broken end following a single crossover
as diagrannned in c, figure 1, whereas the 5 non-variegated kernels have ob-
tained a double crossover chromosome with a normal, non-broken end.
The latter, therefore, should not show variegation.  If the endosperm of a
kernel possesses either a single crossover chromosome (with a broken end)
or a double crossover chromosome (with a normal end), the embryo and
plant tissues arising from such a kernel must possess either a broken chromo-
some or a normal chromosome respectively. If the crossover kernels,
variegated and non-variegated, are sown, the plants arising from each
should clearly show the expected type of chromosome in the meiotic pro-
phases.  The following will describe these results.
If the variegated kernels from the cross Yg-C-&+x-normal chromosome

9/ Yg-C-S& Wz-altered chromosome 9 by yg-c--A-x-normal chromosome
9, possess a broken chromosome 9 introduced by the female parent, and if
the breakage-fusion-bridge-breakage cycle involving broken ends of sister
chromatids continues without healing of the broken ends, variegation for
the yg character should be evident in the plants coming from these kernels.
On the other hand, plants coming from the non-variegated kernels should
show no variegation for yg. Sixty non-variegated kernels of the C-Siz-Wx
class and 60 of the c-shw3c class were sown.  No evidence of yg appeared
in the resulting individuals. The 12 vmiegated kernels from this cross gave
rise to 6 Yg plants with no evidence of yg at any stage in the life of the
plants, 5 yg plants and 1 Yg plant which showed a single yg streak on the
first leaf but no other yg streaks throughout the life of the plant.  The yg
plants clearly indicated that a broken chromosome 9 was present, a break
having occurred to the right of the internal knob, a, figures 3 and 4, deleting
the ,Yg locus. Because the Yg plants did not show Yg-yg variegation it
was assumed that although they possessed a chromosome 9 with a broken
end, this broken end had healed in the embryo cells.  Since the Yg locus
is close to the right side of the small internal knob of the chromatid in a,
figures 3 and 4, the yg plants should show no knob on the chromosome 9
contributed by the female parent whereas the Yg plants could possess the
knob with or without an additional segment beyond the knob. Conse-
quently, all 12 plants coming from the variegated kernels were examined
at meiosis to obtain the constitution of the chromosome 9 contributed by
the female parent. All 5 yg plants showed a deficiency in one chromosome
9, the knob being lost in all cases, an extensive deficiency of the short arm
being present in two cases. Among the 7 Yg plants, the chromosome 9
contributed by the female parent was complete to and including the small
knob, with or without, in the several plants, respectively, an extension of


VOL. 25, 1939        GENETICS: B. McCLZNTOCK                411

chromatin beyond this small knob.  Regardless of its composition, the
broken chromosome 9 in all cells examined in any one plant was always
exactly the same in appearance.  In this particular cross, the plant tissues
have corroborated both genetically (yg plants) and cytologically (all plants)
the assumption of the presence of a broken chromosome 9, this assumption
having been based upon the variegation exhibited in the endosperm of the
kernels from which each plant arose.

Among the 26 examined plants coming from the variegated kernels of
both crosses (by Yg-c-sh-wx or yg-c-sh-wx), 10 have shown a chromosome
9 terminating in the small knob, i.e., a complete chromosome 9. Seven
have shown a deficient chromosome 9, either lacking the small knob or the
knob plus an adjacent section of the short arm.  In the remaining seven
cases, the chromosome 9 was complete to and including the small knob
but likewise possessed a section of chromatin extending beyond this knob.
The composition of the chromatin extension in each case introduces im-
portant evidence not only for the presence of a broken chromosome 9 but
also for the origin of this chromosome from a crossover diagrammed in
c, figure 1, followed by a break at anaphase I in the dicentric chromatid.
The extension always possessed a section of the easily identifiable chromatin
of region 6, figure 1, clearly indicating the origin of this chromosome from
one whose original composition was that of a, figures 3 and 4. A particu-
larly interesting example of a recovered broken chromosome is shown in c,
figure 3.  The simplest method of diagramming the origin of this chromo-

- .l..-ls.-.. w ~~~~~I~.~~-
                                      C
                          FIGURE 3
                See text for description.

some is given in a and b, figure 3.  In a, the dicentric chromatid is repre-
sented following a crossover as diagrammed in c, figure 1. The arrow
points to the position of breakage, the portion to the right of the break
being in the line of descent.  Fusion occurred between the two sister halves
of this chromosome as shown in b, figure 3. The arrow points to the posi-
tion of the second break, the upper chromatid being recovered as shown in
c, figure 3.  That the breakage-fusion-bridge-breakage cycle involving
sister chromatids must occur through several mitoses at least in the early


412

GENETICS: B. McCLINTOCK        PR0C.N. A.S.

divisions (embryo sac ?) following the original break is evident from the
composition of the recovered chromosome 9 in several of these cases.  One
example will be cited.  The constitution of the chromosome 9 contributed
by the female parent, as observed in the prophase of meiosis of this plant,
is shown in d, figure 4. At least three breaks, followed by fusions of broken
ends of sister halves of a chromosome are necessary to give rise to this
chromosome. These are illustrated in a, b and c, figure 4. The arrow

0

- . . . . . . . ..I
t                     a

      . . . . . . . . . . .
      . . . . . . . . . . .
  4               b
Is;; . . . . . . . . . .      J
     . . . . . . . ..I
t                      C


m . ..I. . . I...
                  d

    FIGURE 4
See test for description.

points to the position of the successive breaks, the upper chromatid in all
cases being in the line of descent.

The composition of the chromosome 9 contributed by the female parent
in these 26 cases has given abundant evidence of the presence in these
plants of a broken chromosome 9 and has verified the assumed origin of
such a broken chromosome.

If the variegated endosperm tissue is related to the presence of a broken
chromosome arising after a single crossover, each of the 5 crossover kernels
with no variegation (I, table 1) should possess an unbroken chromosome 9
which has arisen from a double crossover chromatid. Three of the 4 plants
coming from the C-Sh-wx kernels were examined. All three possessed the
rearranged chromosome (b, figure l), obviously the result of a double cross-
over. The plant coming from the C-.A--Wx kernel contained a normal chro-
mosome 9, likewise a double crossover.  Thus, the evidence from the cross-
over kernels, variegated and non-variegated, is in complete accord with the
prediction.

The second method involved the use of a chromosome 9 with a duplication
of nearly all of the short arm attached to the end of the normal short arm.
The order of the genes in the duplicated segment is inverted with respect
to those in the normal short arm as in an attached-X chromosome of
Drosophila. In plants containing a chromosome 9 with the duplication


VOL. 25, 1939        GENETZCS: B. McCLiNTOCK               413

and a normal chromosome 9, a number of types of 2-by-2 homologous
associations at meiotic prophase can occur. Dicentric chromatids are pro-
duced by a crossover following the association of the duplicated segment
with its homologous region in the normal chromosome 9, a, figure 5, result-
ing in a first division bridge or by a crossover as diagrammed in b, figure 5,
following the association of the duplicated segment with its homologous
region in the duplicated chromosome 9.  The latter will result in a second
division bridge. Counts of bridge configurations in AI and AU have given
13.8y0 (among 137 sporocytes) and 13.5% (among 74 sporocytes) respec-
tively.  It should be noted that the dicentric chromatid represents a com-
pletely duplicated chromosome attached at the end of the short arms, c,
figure 5. Breakage at anaphase of such a chromatid can produce two

  I
654321 123456,

   C
FIGURE .i

a.  Diagram of a meiotic prophase association of a normal chromosome 9 and a
chromosome 9 with a duplication of the short arm.  The duplicated segment is
homologously associated with the short arm of the normal chromosome 9.  A cross-
over as indicated produces a chromatid composed of two attached chromosomes 9 as
shown in C.  This will produce a bridge configuration at anaphase I.  b.  Association
of homologous regions within the duplicated chromosome 9.  A crossover as in-
dicated will give rise to the dicentric chromatid shown in c which will form a bridge
configuration in anaphase ZZ.

complete chromosomes 9 if the break occurs at the position of the arrow,
or one chromosome with a duplication and one chromosome with a de-
ficiency if the break occurs at any other position between the two spindle
tibre regions.  By this means, one can obtain a broken chromosome with
at least a complete set of genes.                            .
If such a broken chromosome carries dominant genes and if it is intro-

duced through the pollen to plants carrying the recessive alleles, a continued
breakage-fusion-bridge-breakage mechanism involving sister halves of a
chromosome, should produce variegation in the endosperm tissues.  When
successive genes are lost, those distal to the spindle fibre region should be
lost first followed by genes nearer to the spindle fibre region as illustrated
in figure 2. When such variegated kernels are sown, the plants coming
from them should show the presence of a broken chromosome 9 in the
meiotic prophase configurations.


414

GENETICS: B. MCCLINTOCK        PROC. N. A. S.

Plants carrying the duplicated chromosome 9 with the genes I and WX
in both the normal and the duplicated segment (I, inhibitor of color in the
aleurone layer, i, color in aleurone layer, located at the same position as
C in the crossover map) and a normal chromosome 9 with a large terminal
knob on the short arm and carrying the recessive alleles, when crossed to
plants with normal chromosomes 9 carrying i WC, have given the follow-
ing results: 62 I Wx, 0 I wx, 29 i Wx, 765 i wx and 30 I-i variegated
kernels.'  Among the 1-i variegated kernels 3 were totally WX. The re-
maining 27 were likewise variegated for Wx and wx, the pattern of variega-
tion being similar to that of the previous case described (substitute 1 for C
in figure 2). Cytological examination of plants coming from the I Wx, i wx
and variegated kernels have been made to determine the nature of the
chromosome 9 contributed by the pollen. All the plants coming from the

t tt t

FIGURE 6

Diagram to illustrate types of recovered broken chromosomes in 16 plants arising
from variegated kernels.  Since small duplicated segments, as illustrated in figure 4,
are not so readily detected in this material, the positions of breaks are referred to
the original dicentric chromatid, G, figure 5.  Only the middle section of the dicentric
chromatid is represented.  The chromatid recovered is the one to the right of the
arrow in each case.  In 8 cases, duplicated segments of various lengths, as shown
by the arrows, were present in the recovered broken chromosome.

I Wx, non-variegated class contained either the normal duplicated chromo-
some 9 or a normal chromosome 9 with the large terminal knob resulting
from a crossover which did not give rise to a dicentric chromatid (between
the normal chromosome 9 and the normal short arm of the duplicated
chromosome 9). All of the examined individuals derived from the i wx
kernels contained a normal chromosome 9 with a large terminal knob. All
the ndividuals coming from the variegated kernels have shown a broken
chromosome 9.  In each individual plant, the constitution of the broken
chromosome 9 was the same in all of the cells examined.  As has been shown
in the previously described case, the broken end of chromosome 9 must
heal in the very early embryo cells and remain healed throughout all later
nuclear cycles.  Examinations of anaphase configurations in root tips of
these plants have shown no indication of bridge configurations, which
agrees with similar observations made with the previously described case.
The types of broken chromosomes 9 as determined by meiotic prophase
examinations in plants arising from variegated kernels, are diagrammed in
figure 6.


Vo~.25, 1939        GENETICS: B. McCLINTOCK                415

The reciprocal cross (plants heterozygous for the duplication carrying
I Wx in the normal and the duplicated segment of the duplicated chromo-
some 9 and large knob, i wx in the normal chromosome 9 by plants carrying
a normal chromosome 9 with i wx) have given similar results. The types
of kernels obtained with their ratios are as follows:  438 I Wx, 0 I wx, 23
i wx, 445 i wx and 17 kernels variegated for 1-i and Wxzm. Chromosome
examinations in plants arising from the non-variegated and variegated
kernels have given the expected results:  non-broken and broken ends, re-
spectively.

It is obvious that when the proper genie characters are used, the presence
of individuals possessing a broken chromosome can be detected by the
variegation produced in the endosperm. This variegation can appear when
only one broken chromosome is present in the endosperm tissue. When
Wx as well as Tor C is present in the variegated kernels, the pattern of varie-
gation for each accords with the interpretation of a breakage-fusion-
bridge-breakage mechanism involving sister chromatids extending through
successive nuclear cycles in the developing endosperm.  It is at present
unexplained why this mechanism does not continue in the embryo tissue.
Here broken ends apparently heal and remain permanently healed.  This
is clearly demonstrated when the recovered broken chromosome has no
duplication or deficiency, arrow to right, figure 6.  These chromosomes be-
have in future generations as normal chromosomes 9, giving normal cross-
ing-over and transmissions.  No variegated kernels have appeared in
progeny tests of 8 such cases.  On the other hand, those broken chromo-
somes which have a newly derived duplicated segment, arrows to left of
first, figure 6, are the source of new broken chromosomes by the same
crossover mechanism as described for the original duplication.  Extensive
investigations with one derived duplication (the short duplication, second
arrow from right, figure 6) and preliminary tests with all the others have
repeated the original results.  Non-variegated and variegated kernels are
produced, the variegated kernels in all cases possessing newly broken chro-
mosomes.

In conclusion it can be stated that in maize a bridge configuration at a
meiotic anaphase results in the production of a broken chromosome.
Fusion occurs at the position of breakage between the two sister halves of
the broken chromosome.  Because of this fusion a bridge configuration oc-
curs in the following gametophyte division. This again results in a broken
chromosome entering each telophase nucleus. Fusion likewise can occur
between the broken ends of the sister halves of each of these chromosomes
thus continuing the cycle of bridge-breakage-fusion-bridge.  The variega-
tion exhibited in the endosperm tissues of kernels carrying broken chromo-
somes indicates that this cycle can continue, apparently uninterruptedly,
in this tissue.  In the embryo and plant tissues, however, the broken end


416     GENETICS: MAcDOWELL, POTTER AND TAYLOR  PROC. N. A. S.

becomes healed.  It remains permanently healed in future nuclear and
plant generations regardless of the type of tissue in which it is present.
Limitation of space does not allow a full discussion or even mention of
some interesting phases of these results. They will appear in a later publi-
cation.

The author wishes to express appreciation to Dr. L. J. Sadler for furnish-
ing the original material of the duplicated chromosome 9 and to Dr. Har-
riet B. Creighton for furnishing the original material of the x-ray induced
rearranged chromosome 9.

1 McClintock, B., Missouri Agr. Exp. Sta. Res. Bull., 290, (1938).
* McClintock, B., Proc. Nat. Ad. Sci. 17, 18,5-491 (1931); Creighton, H. B., Ibid.,
20, 111-115 (1934); Burnham, C. R., Am. Nut., 68, 81-82 (1934). Genetics, 19, -130-117
(1935).

3 Emerson, R. A., Beadle, G. W., and Fraser, A. C., Cornell Univ. Agr. Exp. Sk
Memoir, 160, (1935).

4 Due to limitation of space, evidence for this will be presented elsewhere.
6 The I Wx class is deficient in numbers since it comes from the infrequent crossovers
between the normal short arm of the duplicated chromosome and the short arm of the
normal chromosome or from the occasional transmission of the chromosome with the
duplication carrying I W'x. A pollen grain carrying the duplication functions less suc-
cessfully in competition with normal chromosome containing pollen grains.