 1996 Oxford University Press
Human Molecular Genetics, 1996, Vol. 5 Review
1495–1504
Recombination and nondisjunction in humans and
flies
Kara E. Koehler1, R. Scott Hawley1, Stephanie Sherman2,3 and Terry Hassold3,*
1Department
of Genetics, Section of Molecular and Cellular Biology, University of California at Davis, Davis,
CA 95616, USA, 2Department of Genetics and Molecular Medicine, Emory University School of Medicine,
Atlanta, GA 30322, USA and 3Department of Genetics and The Center for Human Genetics, Case Western
Reserve University and University Hospitals of Cleveland, Cleveland, OH 44106, USA
Received June 25, 1996
Recent studies of Drosophila and humans indicate that aberrant genetic recombination is an important
component of nondisjunction in both species. In both, a proportion of nondisjunction is associated with failure
to pair and/or recombine and in both, exchanges which are either too distal or too proximal increase the likelihood
of malsegregation. In this review we provide two perspectives on these observations: first, a review of exchange
and chromosome segregation in model organisms, focusing on Drosophila, and secondly an overview of
nondisjunction in humans. This format allows us to describe the paradigms developed from studies of model
organisms and to ask whether these paradigms apply to the human situation.
INTRODUCTION
Over the past decade, studies of model organisms—in particular,
studies of yeast and Drosophila—have yielded remarkable
advances in our understanding of how meiotic chromosome
pairing, synapsis and segregation occur in lower eukaryotes (1,2).
Further, these studies have provided compelling evidence that
abnormalities in genetic recombination perturb normal meiotic
chromosome segregation. Meiotic mutants which reduce the
level of recombination invariably increase the frequency of
nondisjunction and, in some situations, the location of exchanges
on the bivalent also affects the likelihood of nondisjunction.
Our understanding of the human meiotic process, and of factors
which affect meiotic chromosome segregation, is much less
advanced. Nevertheless, recent studies of human trisomies
suggest similarities between nondisjunction in our species and
nondisjunction in model organisms. Specifically, reductions in
recombination are a feature of all human trisomies thus far studied
and, in several trisomies, the location of the exchanges is different
than those observed in normal meioses.
In this review we provide a summary of nondisjunction from two
perspectives: first, a review of exchange and chromosome segregation in model organisms, focusing on Drosophila, and secondly an
overview of nondisjunction in humans. This format allows us to
describe the paradigms developed from studies of model organisms
and to ask whether these paradigms apply to the human situation.
EXCHANGE AND CHROMOSOME SEGREGATION IN
MODEL ORGANISMS
Genetic recombination, or crossing-over, is the primary means by
which the disjunction of homologous chromosomes at meiosis I
*To whom correspondence should be addressed
(MI) is ensured. Recombination events themselves do not commit
chromosomes to segregate; rather, disjunction is mediated by
specific structures, known as chiasmata, that are formed at the
sites of exchange. We and others have recently shown that, in
addition to ensuring the segregation of homologous chromosomes, chiasmata play a crucial part in the control of the meiotic
cell cycle (3–6).
How chiasmata work
Chiasmata serve to link homologous chromosomes together by
taking advantage of the strong sister chromatid cohesion maintained along both of the homologs. The resulting pair of
interlocked chromosomes, known as a bivalent, then attaches to
the spindle such that one kinetochore is oriented towards each
pole. The process by which that orientation is achieved—indeed,
the process that ensures disjunction—is explained by the simple
physical model described below.
In centriolar meiotic systems, and thus in the male meiotic
systems of most animals, during prometaphase each kinetochore
of the bivalent attaches to the spindle and begins to move toward
one of the two poles. Most bivalents achieve a bipolar orientation
immediately (7). The high frequency of initial proper orientation
is due both to the fact that chromosomal spindle fibers connect to
the pole to which a given centromere most nearly points and to the
observation that at the start of prometaphase, the two homologous
centromeres are usually oriented in opposite directions, such that
if one centromere is pointed at one pole the other centromere is
pointed at the opposite pole [Ostergren (8), as modified by
Nicklas (7)]. To quote Nicklas (9), ‘while the upper half-bivalent’s
kinetochores most nearly face the upper pole, those of its partner
most nearly face the lower pole, simply because bivalents are so
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constructed’. Once the two centromeres have oriented towards
opposite poles, the progression of the two centromeres towards
the poles is halted at the metaphase plate by the chiasmata. This
represents a stable position in which the bivalent will remain until
anaphase I.
Although the mechanism described above is sufficient to
explain bivalent attachment in many meiotic systems, it should be
remembered that in some oocytes the spindle appears to be
acentriolar and is organized by the chromosomes themselves. A
well-studied example of this phenomenon occurs in the oocytes
of the fruit fly Drosophila melanogaster. Prior to metaphase, the
chromosomes of Drosophila oocytes are condensed into a single
mass known as the karyosome. Meiotic spindle formation begins
with the establishment of an array of microtubules, lacking a
defined pole, that emanate from the major chromosomes. As
prometaphase continues, these bundles of microtubules are
sculpted together on each side of the metaphase plate to form a
bipolar spindle. It is often possible to observe that the early
spindle is comprised of four sets of microtubule bundles,
presumably corresponding to each of the four pairs of homologous chromosomes. The spindle then lengthens and tapers.
The ability of chromosomes to organize a functional spindle is
not limited to Drosophila oocytes. Chromosomes have been
shown to organize the spindle in the absence of centrosomes in
several meiotic systems. At least in Drosophila, and probably in
other similar meiotic systems as well, the co-orientation of
homologous chromosomes appears to reflect maintenance of
heterochromatin pairing in the vicinity of the centromeres
(10,11). Because these paired centromeres are both involved in
organizing the spindle, their first poleward movements along the
developing spindles will serve to lock them in opposite orientations.
Thus, the ability of bivalents to orient their centromeres
towards opposite poles is achieved by balancing the tension
between the bivalent and the two spindle poles. This balancing of
tension requires chiasmata to hold the two homologs together.
This rather simple mechanism of chiasma function requires that
each bivalent must be capable of orienting independently of other
bivalents in the cell.
The functional significance of chiasma position
The number and position of recombination events is also tightly
controlled. In metazoans, exchange only occurs in the euchromatin,
and the amount of exchange is not proportional to physical
distance (12). For each of the five major chromosome arms of
Drosophila melanogaster, the frequency of exchange is extremely
low near the base and tip of the euchromatin and reaches its
highest levels in an interval beginning approximately 30% of the
distance from the tip to the base of the euchromatin and ending
some 50–60% of that distance. This pattern is not unique to
Drosophila females but is a general feature of chiasma distribution in a large number of organisms (13).
The distribution of meiotic recombinational events results from
the combined action of three types of genetic control: (i)
trans-acting regulators of exchange position, which appear to act
at the level of entire chromosome arms; (ii) local cis-acting
regulators of exchange; and (iii) chromosomal elements such as
centromeres and telomeres that can suppress exchange in a polar
fashion over long chromosomal distances (2). All three of these
levels of regulation appear to act towards one simple purpose: to
keep exchanges, and hence chiasmata, a substantial distance
away from the centromeres and telomeres.
As virtually all exchanges are sufficient to ensure segregation,
it seems paradoxical that the organism goes to such enormous
efforts to position precisely its exchanges. The positioning of
exchanges reflects a compromise between the problems inherent
in resolving exchanges at anaphase I, which will preclude more
proximal exchanges, and the fact that very distal exchanges
occasionally do not ensure segregation. Recall that the resolution
of chiasmata occurs not by a terminalization process, but rather
by a precise release of sister chromatid cohesion between the
chiasma and the telomere (13). Moreover, sister chromatid
cohesion must be maintained proximal to that chiasma and most
essentially in the centromeric regions to ensure reductional
separation at MI. Resolution of extremely proximal exchanges
would require the release of sister chromatid cohesion in regions
too close to the centromere to allow normal centromere function
at MI. By limiting most exchanges to regions in the more distal
euchromatin, the cell also limits the extent of sister chromatid
release required for proper separation and chiasma resolution at
the onset of anaphase, thus keeping its centromeres out of
jeopardy.
Evidence in both flies and yeast argues that distal exchanges are
not always by themselves sufficient to ensure segregation
(reviewed in 2). In an elegant series of studies of mini-chromosome recombination in yeast, Dawson and his collaborators (14)
have shown that the ‘ability of an exchange to enhance
segregation fidelity appears to be proportional to its distance from
the telomere’. Similarly, when the two back-up achiasmate
segregational mechanisms in Drosophila melanogaster are
mutationally ablated, bivalents with very distal chiasmata often
nondisjoin at very high frequencies (15,16), suggesting that the
ability of a very distal chiasma to guarantee segregation may often
depend on additional functions, such as achiasmate back-up
systems. We are fond of the idea that very distal exchanges are
simply locked in by far too little chromosomal material and that
the resolution of chiasmata at the very start of sister chromatid
release may well result in a precocious separation of homologous
centromeres. Indeed, as noted below, the ord and mei-S332
mutations, which cause precocious sister chromatid separation at
anaphase I, also cause high levels of nondisjunction.
In summary, the precise genetic control of exchange position
reflects a delicate balance between sites of exchange that allow
chiasma function and those sites which can permit the sister
chromatid release required for chiasma resolution. Exchanges
must be proximal enough to provide sufficient sister chromatid
adhesion distal to the cross-over in order to ensure that the
bivalents stay locked together. This requirement for more
proximal exchanges is balanced against the need to prevent sister
chromatid separation anywhere even remotely in the vicinity of
the centromere.
Cases where even properly positioned chiasmata do not
ensure segregation
There are two important categories into which the nondisjunction
of exchange bivalents fall: so-called ‘spontaneous’ nondisjunction and nondisjunction due to meiotic mutations (a third
category, radiation-induced nondisjunction, will not be considered here). Although each type of event is discussed in detail
below, it may be said in summary that in each case it is a failure
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of some other component of the meiotic process, such as the
non-repair of DNA damage or the construction of a faulty spindle,
that induces the nondisjunction. Chiasmata cannot be blamed for
these segregational failures any more than automobiles can be
blamed for the effects of adverse road conditions.
Spontaneous nondisjunction in Drosophila. In humans, considerable
information is available on the the incidence and origin of
spontaneous nondisjunction, as described later in this article. In
contrast, in yeast and many other model organisms intolerance to
aneuploidy has prevented extensive studies of spontaneous nondisjunction; thus, relatively little is known about its occurrence in most
model systems. It is best characterized in Drosophila melanogaster.
Instead of trisomic conceptuses or disomic gametes, nondisjunctional events in Drosophila can be recovered as ‘exceptional’
females who have received both of their X chromosomes from their
mothers and no sex chromosomes from their fathers. The single
published study of spontaneous X chromosome nondisjunction in
Drosophila (17) was completed over 30 years ago and raised more
questions than it answered; however, it has provided us with the only
previous insights into the mechanisms responsible for the spontaneous failure of chromosomes to segregate from each other during
meiosis.
In many ways the data of Merriam and Frost (17) parallel the
basic observations in humans. They suggested, albeit without the
use of a centromere marker for verification, that the majority of
spontaneous X chromosome nondisjunction in flies occurred at
MI; as demonstrated in Table 1 and discussed later in this paper,
this is also the case for most human trisomies. Additionally, they
noted that exchange precedes spontaneous nondisjunction in as
many as 75% of cases, although there was also an excess of
achiasmate bivalents among nondisjunctional X chromosomes.
As discussed later, the majority of cases of human nondisjunction
are preceded by exchange but—especially for the sex chromsomes—a proportion are associated with achiasmate bivalents.
Thus, it is reasonable to expect that the origin of nondisjunction
in flies will be useful and relevant in elucidating similar
mechanisms in humans.
This classic study in Drosophila (17) also reported a number of
unusual phenomena that remained unaddressed until recently
(18). One such observation was the possible occurrence of
pericentromeric exchange, a phenomenon recently reported for
human X chromosome nondisjunction (19). Another unusual
result was the appearance of new lethal mutations at a rate several
orders of magnitude greater than background levels on nondisjunctional X chromosomes. Although this idea has not been
addressed experimentally in humans, at least one case of trisomy
21 has been noted to carry a new cytogenetic heteromorphism not
attributable to either parent (20).
These findings in flies and humans led Hawley et al. (18) to
propose that at least some spontaneous nondisjunction—in both
species—is the result of the repair of double-stranded breaks
(DSBs) generated by the movement of transposable elements
during meiotic prophase (Fig. 1). Such repair events will form
Holliday junctions and thus strongly resemble normal meiotic
recombination intermediates. Hawley et al. (18) further suggested that if such repair events occur at the wrong time or wrong
place during meiosis, either outside of the narrow temporal
window allowed for normal meiotic exchange or in a location in
which recombination is normally prohibited, such as the centromeric
heterochromatin, the resulting physical connection between the
Figure 1. A possible transposon-based model for spontaneous nondisjunction
(A) Hawley et al. (18) have proposed a model in which at least some
spontaneous nondisjunction is the result of the repair of DSBs generated by the
movement of transposable elements during meiotic prophase. DSBs in the
DNA resulting from transposon excision (1) are repaired off of the homolog of
the damaged chromosome (2), and the resulting repair events are thought to
form recombination intermediates (3) similar or identical to those produced by
normal meiotic recombination. (B) This model further suggests that such a
physical connection between the two homologous chromosomes could, if it
occurred at the wrong time or place, actually impair the proper segregation of
homologs during anaphase I. For example, an exchange of meiotic origin in the
central euchromatin (4) ensures regular disjunction, while a proximal
DSB-repair event in the heterochromatin (5) may cause nondisjunction, either
by interlocking the homologs at anaphase I or by compromising the integrity
of the centromere. The latter event could result in precocious sister chromatid
separation at MI.
homologs could interfere with rather than ensure their proper
segregation at anaphase I.
An experimental reexamination of these ideas, using Drosophila,
is currently underway and has uncovered two lines of evidence
supporting the idea that spontaneous nondisjunction is correlated
with the movement of transposable elements. First, a reversion
rate of at least 1% due to the excision of a transposable element
has been observed (Koehler and Hawley, unpublished observations) at a locus whose normal reversion rate is 1.5×10–5 (21).
Second, we have recovered new lethal mutations at a rate of
roughly 2% from a population of spontaneously nondisjoining
Drosophila X chromosomes (Koehler and Hawley, unpublished
observations). The transposon-based model for spontaneous
nondisjunction postulated by Hawley et al. (18) predicts that the
molecular lesions associated with the elevated mutation rate
originally observed by Merriam and Frost (17) will be defects due
to the insertion of a transposable element into a vital gene.
Regardless of whether this is verified by molecular studies, the
origin of these mutations will provide crucial insights into the
mechanism of spontaneous nondisjunction.
Meiotic mutants that allow the nondisjunction of exchange
bivalents. There are six Drosophila meiotic mutations that cause
chiasmate bivalents (exchange bivalents) to nondisjoin at high
frequency, two of which also impair sister chromatid cohesion.
These mutations are ald, mei-38, ord, mei-S332, nodDTW (when
homozygous), and ncd. The functions of the first two genes, ald
and mei-38, remain unclear. Other than noting that these two
mutations apparently define some function necessary for chiasma
function, these mutants will not be considered further. Both
nod DTW and ncd define functions required for spindle assembly
and/or maintenance at prometaphase, while ord and mei-S332
define functions required for the control of sister chromatid
cohesion. Each of these functions is considered in detail below.
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Chiasma failure under conditions where spindle structure and/or
assembly is perturbed. The nod gene in Drosophila melanogaster
encodes a kinesin-like protein required for normal meiotic
segregation (15,22,23). Although recessive loss-of-function nod
alleles affect only the segregation of achiasmate bivalents, a
dominant allele (nod DTW) impairs the segregation of both
chiasmate and achiasmate bivalents at MI (16,23). Interestingly,
the number and frequency of exchanges in nondisjoining
bivalents of nod DTW-bearing oocytes parallel those observed for
nondisjoining chromosomes 21 in humans (18). Both are
enriched not only for achiasmate chromosomes but for bivalents
with distally located exchange events; indeed, as discussed later,
in trisomy 21-generating meioses, recombination in distal 21q is
actually increased over that observed in normal female meioses
(Fig. 2).
However, it is important to note that the defect in nod DTW is not
in recombination per se; e.g. the mutation does not affect the
frequency of recombination. Instead, the chromosomes in mutant
oocytes are defective in their ability to maintain contact with both
the homolog comprising the other half of the bivalent and with the
forming meiotic spindle, resulting in precocious disassociation of
one or more bivalents or univalent chromosomes from the
defective spindle. Although proximal exchanges are still sufficient to ensure proper segregation in this situation, bivalents with
very distal exchanges or without any chiasmata at all are much
more likely to nondisjoin (16,24). Such chromosomes, ejected
from the developing spindle, might frequently either be lost or
reform a connection with one of the two poles at random. We
understand these observations and ideas in very simple mechanical terms: on a normal spindle there are several mechanisms,
including tight centromere apposition as a result of a proximal
chiasma and/or proper NOD function, to hold homologs together
and to hold bivalents on the spindle.
We have previously proposed that age impairs the ability of
human females to form a normal meiotic spindle (18), perhaps
through the time-dependent decay of an important component on
which spindle function is dependent. Observations similar to
those made for nod DTW have been made for mutations at the ncd
locus in Drosophila melanogaster (Paul Szauter, personal
communication). It is possible that there are multiple elements
whose functional deterioration could potentially have such an
impact in human females as well. Like the nod gene, ncd encodes
a kinesin that is required for proper meiotic spindle assembly
(25). It has been recently demonstrated that the NCD protein is
critical to the stable assembly of a bipolar spindle in living
Drosophila oocytes (26).
with random segregation at both divisions in both sexes. Like the
original allele, these new alleles also decrease the frequency of
exchange in females by 10–20-fold and ablate the ability of those
exchanges that do occur to ensure segregation. Thus, the
pleiotropic effects of the original allele on sister chromatid
adhesion, exchange and chiasma function must be due to a single
biochemical defect. Presumably, that defect is in sister chromatid
adhesion, and the effects on exchange and chiasma function are
consequences of that defect. The ord gene has been recently
cloned and found to encode a novel protein; mutational analysis
of a number of ord alleles suggests that the C-terminal portion of
this protein is required for proper sister chromatid adhesion and
plays a part in moderating protein binding (29).
The mei-S332 mutation elevates equational nondisjunction in
both sexes. Like ord, it has been shown cytologically to exhibit
defects in sister chromatid cohesion in spermatocytes, although
these do not appear until anaphase I, later than the first
manifestation of the ord defect. However, unlike ord, mei-S332
does not alter recombination location or frequency. Because of
this and the other phenotypic differences between these two
mutations, mei-S332 has been postulated to define a function
important for maintaining sister chromatid cohesion in the
centromeric region (27,30). Consistent with this idea, MEI-S332
has recently been localized specifically to meiotic centromeres,
where it remains until dissociation or degradation at anaphase II,
concomitant with sister centromere separation (30). The cytology
of mei-S332 mutants suggests that the majority of nondisjunction
occurs through the following pathway: sister chromatids separate
precociously by anaphase I, but complete the reductional division
in this separated state. However at meiosis II (MII) the two sister
chromatids segregate randomly.
The observations on ord and mei-S332 are intriguing, in light
of recent cytogenetic studies of nondisjunction in human oocytes.
In analyses of unfertilized MII oocytes obtained from patients
attending fertility clinics, Angell et al. (31) found a high
proportion of oocytes contain either missing or extra single
chromatids—not missing or extra intact chromosomes. While
these observations are yet to be confirmed, they suggest that
errors in segregation at MI may result in the premature
segregation of sister chromatids prior to MII. As the precocious
equational segregation of chromosomes that nondisjoin at MI has
been documented in a number of experimental systems (for
review see 13,32), this may represent an important potential
mechanism for nondisjunction in many organisms.
The ord and mei-S332 mutations and sister-chromatid cohesion.
In both males and females ord causes a dramatic increase in both
reductional and equational nondisjunction as detected genetically. In cytological studies of male meiosis, Goldstein (27)
observed abnormal sister chromatid association during prophase
as well as precocious sister chromatid separation during anaphase
I. In fact, the majority of chromosome misbehavior in ord mutants
appears to be precocious sister chromatid separation at the first
rather than the second meiotic division (27).
Recently, five additional alleles of ord have been characterized
(28); a decrease in the frequency of sister chromatid cohesion is
observed cytologically in the male germline for all five of these
alleles (female meiosis has not been examined). Indeed, for the
strongest allele, both genetic and cytological studies are consistent
Control of the metaphase-to-anaphase transition is a central
component of cell cycle regulation, and arrest at metaphase I is
a common component of oogenesis in both flies and mammals.
In many male meiotic systems, the failure to recombine will result
in the production of two univalent chromosomes, neither of
which can be balanced at the metaphase plate. Several lines of
evidence indicate that the absence of kinetochore tension in these
organisms results in the production of a chemical signal that
triggers a terminal metaphase arrest. For example, Li and Nicklas
(33) recently reported that in praying mantid spermatocytes, the
absence of tension on even a single kinetochore is sufficient to
signal a meiotic error and trigger a prolongation of metaphase
which results in cell death. Restoration of tension on that
kinetochore by micromanipulation relieves that error signal and
Chiasma release and the cell cycle
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allows the cell to proceed into anaphase I. The triggering of a
terminal metaphase I arrest by unaligned chromosomes acts to
prevent these spermatocytes from producing mature, but potentially aneuploid, sperm (33). Thus, in these males the function of
an arrest at metaphase is to abort an aberrant meiotic process.
We have recently shown that metaphase I arrest in Drosophila
is triggered by the presence of chiasmata (34). Although the
presence of even a single chiasmate bivalent is sufficient to trigger
metaphase arrest, in the absence of chiasmata metaphase arrest
does not occur. While it is possible that the arrest reflects nothing
more than the physical inability of homologs to separate until all
of the chiasmata are resolved, we favor a model in which the
presence of chiasmata induces one of a number of cell cycle
regulators to cause arrest. According to either model, the release
of this metaphase arrest is concomitant with chiasma resolution.
In both the mantid spermatocyte and the Drosophila oocyte,
kinetochore tension is being used to halt the cell cycle at
metaphase, albeit towards rather different ends. This dramatic
sexual asymmetry reflects the fact that evolution has allowed
tension to be used in two very different ways to regulate the
meiotic process in males and females.
The absence of a chromosome misalignment checkpoint during
Drosophila female meiosis (and many others, reviewed in 35) is
striking. Meiosis proceeds unarrested with any number of
univalent chromosomes or spindle disorganization, demonstrating
that there is no checkpoint for mono-oriented or misaligned
chromosomes in Drosophila female meiosis. A similar case can
be made for oocytes from XO female mice. In this case, oocytes
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enter anaphase with a single unpaired X chromosome and without
signaling a metaphase I checkpoint. Furthermore, the univalent
segregates either equationally or reductionally at the first
division, showing that the nature of the spindle attachment
(bipolar or monopolar) does not effect the metaphase-to-anaphase
transition (35). It is conceivable that most or all oocyte systems
lack a method to detect mono-oriented chromosomes on the
meiotic spindle. This may be related to the fact that, unlike
spermatocytes and yeast cells, most oocytes naturally arrest at
some point during MI or II. The lack of a metaphase checkpoint
in human oocytes may cause the very high frequency of meiotic
errors in human oocytes as compared with sperm (35).
MOLECULAR STUDIES OF HUMAN NONDISJUNCTION
As is obvious from the preceding discussion, analysis of meiotic
mutants has guided our thinking about the relationship of
exchange and chromosome segregation in lower organisms. In
contrast, meiotic mutants which affect human chromosome
segregation have not yet been identified; thus, this approach has
not been available. Instead, investigations of human meiotic
nondisjunction have typically involved studying the incidence or
origin of the additional chromosome in trisomic conceptuses. The
occurrence of such abnormalities is a major concern to human
reproductive health. Trisomy is the most common class of human
chromosome abnormality, occurring in at least 0.3% of all
newborns and in approximately 25% of spontaneous abortions.
Considered as a class, it is the most common known cause of
mental retardation and the leading cause of pregnancy wastage.
Figure 2. Genetic maps of chromosomes 16 and 21, generated from normal female meioses (from CEPH family data) and trisomies of maternal origin. (A) For trisomy
16, there is an overall reduction in map length, attributable to a reduction of recombination in proximal 16p and 16q. (B) For trisomy 21, both MI and MII maps are
associated with aberrant recombination; in the former case, the map is shorter, especially in proximal 21q and in the latter, the map is longer, especially in proximal 21q.
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Table 1. Molecular studies of parental and meiotic stage of origin in autosomal and sex chromosome trisomies (adapted from
reference 54 and Hassold and Sherman, unpublished results)
Trisomy
No informative
Paternal
cases
I
2–12
16
13–15
54
16
62
18
101
21*
776
22
Maternal
II
133
XXX
46
I
Percentage
II
I or II
3
1
4
2
9
17
27
19
XXY
I or II
2
58
2
maternal
13
81
12
8
27
87
51
1
10
100
16
35
41
90
556
176
6
94
11
89
40
13
22
56
24
10
10
94
*For trisomy 21, we have presented only those cases having information on both parent and meiotic stage of origin of trisomy.
Despite the high frequency of human trisomy and its obvious
clinical importance, until recently little was known about the
origin or causes of human nondisjunction. However, beginning in
the late 1980s, several laboratories initiated DNA polymorphism
studies of trisomic fetuses or liveborns and their parents;
subsequently, a considerable amount of information has accrued
on the parent and meiotic stage of origin of human trisomy and,
more recently, on the association of aberrant recombination and
nondisjunction.
Most human trisomies originate in maternal meiosis
Molecular studies of the origin of trisomy are now available on
over 1000 trisomic conceptuses (Table 1). The most extensively
studied condition has been trisomy 21, with over 750 informative
cases reported (36–38). Maternal errors predominate, accounting
for over 90% of cases. It has not been possible to determine
unequivocally the meiotic stage of origin due to the lack of a
chromosome 21-specific centromeric polymorphism. Nevertheless,
two groups (36,38) have used proximal 21q DNA markers to
infer the meiotic stage of origin of trisomy 21 and have reported
similar results. For maternally derived cases, over 75% are
compatible with MI nondisjunction, while for paternal trisomy
MII cases predominate (Table 1). A small proportion, approximately 5%, of cases are apparently due to mitotic nondisjunction
(36).
The other trisomies that have been studied display remarkable
variability in origin (Table 1). For example, paternal MI
nondisjunction is responsible for nearly 50% of cases of 47,XXY,
while in trisomy 18 maternal MII errors predominate and for
trisomy 16, all cases may derive from maternal MI nondisjunction. For the other trisomies, relatively few cases have yet been
studied. Nevertheless, the available evidence suggests that
maternal errors predominate for most trisomies, but that individual chromosomes may have specific liabilities to paternal and
maternal nondisjunctional mechanisms.
Aberrant recombination is a feature of human trisomy
Several groups have used centromere mapping techniques (39)
either (i) to compare the frequency and location of genetic
exchanges occurring between nondisjoining chromosomes in
trisomy-generating meioses with those occurring between normally disjoining chromosomes in normal meioses or (ii) to
compare genetic linkage maps based on trisomy-generating
meioses with those constructed from normal meioses (e.g. from
conventional linkage studies of CEPH families). Using these
approaches, data on recombination have been obtained for six
nondisjunctional conditions: paternally derived cases of 47,XXY,
maternally derived cases of sex chromosome trisomy and
autosomal trisomies 16, 18 and 21, and maternal uniparental
disomy 15 (Table 2). With the exception of trisomy 21, all
mapping studies of individual trisomies have been based on a
relatively small number of cases, with the reported effects of
recombination appearing to vary among the different conditions.
Thus, interpretations of the data must be made with caution.
Nevertheless, all studies are in agreement in suggesting an
association of abnormal recombination and human trisomy, and
several general features are now beginning to emerge.
A proportion of human trisomy is associated with failure of
recombination. Similar to the observations of Merriam and Frost
(17) for Drosophila, a significant excess of zero-exchange events
has been reported for several of the human trisomies. This is
especially true for paternally derived 47,XXYs, as indicated by
studies of crossing-over in the XpYp pseudoautosomal region in
39 paternally derived 47,XXYs (40). Assuming that normally, a
single chiasma within this region joins the MI XY bivalent,
approximately one-half of the 47,XXYs would be expected to
show evidence of recombination (in the other half, both
recombinant or both non-recombinant meiotic products would be
recovered, and the exchange would go undetected). Instead,
Hassold et al. (40) found evidence for recombination in only six
of the 39 cases, a significant reduction from expectation.
Subsequently, they used the data to generate a genetic linkage
map of the pseudoautosomal region and found it to be significantly
shorter than the normal male map of the region (13 cM vs.
52 cM).
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Table 2. Summary of studies of aberrant recombination and the genesis of human trisomy
Parental origin
Trisomy (ref)
of trisomy
Stage of
Observed effect
Associated with parental age?
Origin
Paternal
47, XXY (40)
MI
No observable recombination in pseudoautosomal
region in 33 of 39 cases of paternal 47,XXY; map
length of 13 cM vs 52 cM in normal male meioses
No
Maternal
47, XXY,
47, XXX (19,43)
MI
Overall decrease in recombination, due to absence
of recombination in some cases; however, other
cases associated with increase in recombination in
the pericentromeric region; overall map length =
98 cM vs 181 cM in normal female meioses
Yes; cases with no detectable recombination have
increased maternal age; cases with recombination
near centromere not associated with increased age
MII
Overall decrease in recombination, but increase in
pericentromeric region; map length = 140 cM vs
181 cM in normals
No
Uniparental
disomy 15
(45,47)
MI
Reduced and possibly altered distribution of
recombinational events; map length = 105 cM vs
174 cM in normals (47)
Presumably, since UPD 15 is age-dependent
Trisomy 16 (44)
MI
Reduced recombination restricted to proximal
regions of chromosome; map length = 129 cM vs
172 cM in normals
Presumably, since trisomy 16 is thought to be
entirely age-related
Trisomy 18 (46)
MI
Overall reduction in recombination; map length =
104 cM vs 163 cM in normals
Probably, although only a small number (16) of
MI cases have yet been studied
MII
Overall increase (although not significant) in
recombination; map length = 191 cM vs 163 cM
in normals
Yes; MII cases elevated in maternal age by
comparison with controls
MI
Reduced recombination, especially in proximal
21q; map length = 33 cM vs 72 cM for normals
Yes; similar reductions in recombination seen in
younger and older mothers
MII
Increased recombination, especially in proximal
21q; map length = 107 cM vs 72 cM for normals
Yes; similar increases in recombination seen in
younger and older women
Trisomy 21 (38)
Thus, the overwhelming majority of paternal 47,XXYs are
apparently associated with failure of pairing and/or recombination. This observation is somewhat surprising, as evidence from
the mouse suggests that abnormalities in XY pairing lead to germ
cell death rather than to increased nondisjunction (e.g. 41).
However, Ashley et al. (42) have recently suggested that, rarely,
unrecombined sex chromosomes may complete meiosis to form
functional (although frequently aneuploid) sperm. They studied
male mice with a rearranged Y chromosome and a high frequency
of sex chromosome univalency in which—using FISH—it was
possible to distinguish between sperm or spermatids carrying
recombinant and those carrying non-recombinant sex chromosomes. Most sperm were chromosomally normal and had normal
levels of XY recombination, indicating that most cells with
unrecombined sex chromosomes were eliminated at MI. However, a small proportion (4%) of gametes were aneuploid, either
XY or ‘O’, suggesting that some of the cells with unrecombined
sex chromosomes were able to proceed to MII and beyond. In
humans, the available data suggest that a similar process may well
be the most common cause of male sex chromosome nondisjunction.
Failure of recombination also appears to be a feature of
maternal nondisjunction, especially cases involving the X
chromosome. In studies of sex chromosome trisomy of maternal
MI origin (19,43), approximately 40% (32 of 82) of all examined
cases showed no evidence of recombination between the
nondisjoined Xs. As at least three to four exchanges are predicted
for the X chromosome bivalent, this represents a significant
departure from expectation.
However, evidence for a similar effect involving autosomes is
not nearly as compelling. In trisomies 16 and 21, an excess of
achiasmate bivalents has been reported (44,38). However, in
trisomy 16, exchanges were identified in approximately 80% (44
of 56) of cases, with the most significant difference from normal
female meioses being an increased frequency of one-exchange
events (44); and in trisomy 21 as well, studies of 173 cases of
maternal MI origin indicated a significant increase in the
frequency of one-exchange events (38). Similarly, there has been
relatively little evidence for a significant increase in the frequency
of zero-exchange events in either maternal uniparental disomy
15, based on studies of 27 cases (45) or maternal MI trisomy 18,
based on studies of 16 cases (46). These observations are
compatible with a role of reduced—but not absent—recombination
in the genesis of most cases of autosomal trisomy. Thus, in
humans the association between absence of recombination and
nondisjunction may be largely restricted to the sex chromosomes.
Reduced recombination is a feature of most, if not all, human
trisomies of MI origin. Reductions in recombination have been
observed in all trisomies of MI origin, with most trisomy-based
genetic linkage maps being approximately one-half to two-thirds
the length of the comparable maps from normal meioses (Table
2). However, there is mounting evidence that the reductions are
not randomly located on the chromosome arms; instead, there
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Figure 3. The possible association of chiasma location and human nondisjunction. (A) Bivalents joined only by distally placed chiasmata may be susceptible to
premature separation at MI. When followed by a normal MII, the resulting disomic gamete would contain nondisjoined chromosomes having genetically different
centromeres; thus nondisjunction would be scored as occurring at MI. We suggest that any of several abnormalities—including impairment of meiotic motors or sister
chromatid cohesion proteins—may increase the likelihood of nondisjunction of these bivalents. (B) Bivalents joined by proximally-placed chiasmata may be unable
to separate at MI. At MII, the bivalent might undergo a reductional division, resulting in a disomic gamete in which the nondisjoined chromosomes have identical
centromeres; thus, nondisjunction would be scored as arising at MII, even though the precipitating event was at MI. We suggest that some such nondisjunction is
attributable to transposon mobilization (Fig. 1), and that this process may contribute to the maternal age effect on human trisomy.
appear to be specific ‘cold spots’ in the trisomy-generating
meioses. This is especially true for trisomies 16 and 21, the two
autosomal conditions for which there are the most data (Fig. 2).
In trisomy 16, there is little evidence for a reduction in
recombination in either distal 16p or 16q. However, in the
proximal regions of the chromosome Hassold et al. (44) reported
a 20-fold decrease in recombination, with a map length of 77 cM
in normals being reduced to only 4 cM in the trisomy 16 cases.
Similarly, in trisomy 21 the reductions in recombination are
restricted to proximal 21q; indeed in distal 21q the map is actually
expanded by comparison with normals (38). Less information is
available for other autosomal trisomies, but preliminary data from
UPD 15 suggest that proximal exchanges may be diminished in
this condition as well (45,47).
Thus it may be that, at least for autosomal trisomies of maternal
MI origin, the absence of a proximally located chiasma imparts
an increased risk for nondisjunction. This mimics observations on
the nod DTW mutation in Drosophila (see preceding discussion of
model organisms), raising the possibility that—in humans as well
as flies—there is a back-up system to normal chiasmate
segregation. If components of this system (such as a human
homolog of NOD) degrade, achiasmate bivalents or bivalents
with distal-only exchanges may be preferentially affected; e.g. the
centromeres may be less able to align when the exchange is
physically distant, resulting in a bipolar attachment for one or
both members of the bivalent. Alternatively, distal-only exchanges may increase the likelihood of premature separation of
the bivalent—resulting in functional univalency—because of a
decreased ability to maintain sister chromatid cohesion (Fig. 3).
Impairment in the functioning of sister chromatid cohesion
proteins (such as a human homolog of ORD) might well
preferentially affect this type of bivalent.
Regardless of the correctness of these or other models, the
available evidence from humans suggests that it is altered
placement of recombinational events, and not simply reduced
recombination per se, which is the important determinant of
maternal MI nondisjunction.
‘MII’ nondisjunction is associated with aberrant recombination.
As genetic recombination occurs at MI, there was no obvious
reason to suspect that nondisjunction at MII would be associated
with aberrant recombination and this was consistent with early
studies of recombination in maternal MII trisomy 21 (38).
However, with the acquisition of new data, it is clear that this
interpretation is incorrect. Studies of maternal MII trisomy 21
now indicate a significant effect of aberrant recombination
although, in contrast to the situation with MI trisomy, the
association is with increased recombination. Sherman (unpublished observations) has estimated the overall length of the MII
map to be 107 cM, significantly elevated from the 72 cM for
normal female meioses (Fig. 2). The distribution of exchanges is
also different from normal, with an increased frequency of
proximal exchanges in the MII population.
There is also suggestive evidence that aberrant recombination
is important in MII nondisjunction involving other chromosomes.
In maternal MII sex chromosome trisomy, significantly increased
pericentromeric recombination has been reported, although the
overall length of the X chromosome map is shorter than the
normal map (19). In maternal MII trisomy 18, there is an
increase—although non-significant—in the overall length of the
map (46).
How can an event which occurs at MI affect segregation at MII?
One explanation may be that the precipitating event occurs at MI,
with aberrant segregation occurring at both divisions (Fig. 3). For
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example, proximal chiasmata may predispose to ‘chromosome
entanglement’ (48) at MI, with the bivalent being unable to
separate and passing intact to the MII plate; at MII the bivalent
divides reductionally, resulting in a disomic gamete. In this
situation, the additional chromosomes would have identical
centromeres, and thus be scored as an MII trisomy, even though
the origin of the abnormality was at MI.
One implication of this model is that ‘true’ MII errors may be
relatively rare in humans. That is, in studies of trisomies of maternal
origin, MII errors are typically reported to account for approximately
20–30% of all cases (Table 1). If these errors instead reflect
abnormal processing of specific MI chiasmate configurations, it may
be that virtually all human nondisjunction is attributable to errors at
maternal MI, with errors at maternal MII and paternal MI and MII
occurring at low, ‘background’ levels. This is intuitively an attractive
idea, as the first meiotic division in females is unique in that it takes
at least 10 and possibly as many as 40–50 years to complete.
However, before seriously considering this posssibility, it will be
important to verify the apparent association between aberrant
recombination and MII nondisjunction.
Maternal age-dependent trisomy is associated with aberrant
recombination. An obvious implication of the above studies is an
association between aberrant recombination and age-dependent
nondisjunction. For example, in trisomy 16, thought to be
completely maternal age-dependent (49), the genetic map is
significantly shorter than the normal map (44); in trisomy 21,
significant reductions in recombination have been identified in
both older and younger women (38); and in maternal sex
chromosome trisomy, the mean maternal age is significantly
elevated for the zero-exchange cases (19). Thus, it is clear that
aberrant recombination plays an important part in the genesis of
the maternal age effect on trisomy.
However, there is less certainty regarding the underlying basis
of the association. An early, and still much debated, model was
proposed by Henderson and Edwards over 25 years ago (50).
Based on observations of declining chiasma frequencies and an
increased frequency of univalents in aging mouse oocytes, they
suggested that reductions in crossing-over with resultant univalent
formation might explain age-dependent trisomy. However, as
chiasma formation occurs prenatally in the female, they were
forced to postulate the existence of a gradient, or ‘production
line’, in the fetal ovary that causes the first formed oocytes to have
a higher chiasma frequency than those formed later. Assuming
that oocytes are ovulated in the same order that they enter meiosis,
those that are ovulated last would be the most susceptible to
nondisjunction due to a reduction in chiasmata.
Recent studies of the mouse suggest that the last-formed
oocytes are, indeed, the last to be ovulated (51), fulfilling one
prediction of the ‘production line’ model. Studies of human
trisomies fulfill a second prediction, namely, that age-dependent
trisomy is associated with reductions in recombination. However,
other predictions of the model are less easily reconciled with the
mapping data from normal and trisomy-generating meioses. For
example, conventional linkage studies of chromosome 21
(Audrey Lynn, personal communication) and chromosome 22
(52) provide no evidence that recombination declines with
maternal age in normal meioses. Further, in studies of trisomies
16 and 21 of maternal MI origin, the magnitude of the reduction
in recombination is similar in younger and older women (Hassold
and Merrill, unpublished observations; Sherman, unpublished
1503
observations). Finally, cases of trisomy 21 of maternal MII origin
are associated with increased recombination, despite the fact that
this condition is clearly maternal age-dependent.
Thus, the mapping data provide little evidence that the
association of recombination and age-related trisomy is established prenatally. Instead, it seems more likely that the effect
involves two ‘hits’: first, the establishment in fetal meiosis of a
‘susceptible’ meiotic configuration and second, age-related
degradation of a meiotic process which increases the risk of
nondisjunction of bivalents with susceptible configurations. For
example, for the chromosome 21 bivalent the presence of a single,
distally placed exchange may constitute a susceptible configuration (Fig. 3). In the ‘young’ ovary, the presence of a single
chiasma—regardless of its location—may be sufficient to ensure
normal segregation. However, with advancing age normal
segregation may become increasingly dependent on the presence
of a proximal, ‘anchoring’ chiasma. In its absence, the homologs
may separate prematurely; for example, due to an age-related
defect in a motor protein (such as NOD in Drosophila) which
normally helps to hold the homologs in close register (2), or to
age-related degregation of a protein involved in sister chromatid
cohesion (such as ORD or MEI-S332 in Drosophila). As a
consequence in the older ovary, bivalents with distal-only
exchanges may function independently of one another at MI,
increasing the likelihood that they will migrate to the same pole.
However, what is the explanation for the increase in exchanges—
particularly in the proximal regions of the chromosome—
observed in some maternal age-dependent conditions
(e.g. trisomy 21 of maternal MII origin)? One possibility is that
such configurations also confer an increased susceptibility, but
for a reason opposite that for distally placed exchanges; i.e. in
these instances the configurations interfere with the ability of the
bivalent to become disentangled at MI (Fig. 3). This might be a
consequence of an age-related shortening in the meiotic cell cycle
(53), which limits the amount of time to resolve chiasmata, or to
age-related degradation of human homologs of proteins, such as
NCD, which normally promote poleward movement of the
chromosomes. As a result, the bivalent fails to separate and the
homologs move together to the pole. At MII reduction occurs,
leading to disomy and an apparent MII error.
In the above two models of age-dependent trisomy it is
assumed that aberrant recombination is correlated with, but is not
the proximal cause of, age-related trisomy. Rather, it is assumed
that with increasing maternal age, bivalents joined together by too
few chiasmata (or by distally placed chiasmata) or by too many
chiasmata (or proximally placed chiasmata), are more susceptible
to nondisjunction.
However, in trisomies associated with an excess number of
exchanges, it may be that aberrant recombination is, indeed, the
proximal cause of nondisjunction. As discussed earlier, Hawley
et al. (18) have suggested that some nondisjunction in Drosophila
results from the repair of double strand breaks generated by
transposon mobilization and we suggest that some age-related
human trisomy arises in a similar fashion. In this instance, the
‘recombinational’ events occur in the arrested (dictyotene) stage
of oogenesis. They do not lead to functional chiasmata, but rather
may cause the chromosomes to interlock and prevent their
separation at MI (Fig. 1). If the bivalent passes intact to the MII
plate, the nondisjunctional product may again appear to result
from an MII event, as in Figure 3. One implication of this model
is that there may be an environmental component to human
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age-related nondisjunction. That is, in addition to repair of
tranposition events, other DNA damaging agents may lead to
chromosome interlocking and eventual nondisjunction. With age,
the likelihood of such an event presumably increases, generating
an age effect on trisomy; and, if pericentromeric ‘exchanges’
promote interlocking, recombination in trisomy-generating
meioses will appear to be increased in the proximal regions.
However, such recombination should not display interference,
providing a possible means for testing this model.
SUMMARY
Studies of Drosophila and humans indicate several similarities in
nondisjunctional mechanisms in the two species. In both, a
proportion of nondisjunction is associated with achiasmate
bivalents and in both, exchanges which are either too distal or too
proximal increase the likelihood of mal-segregation.
Analyses of meiotic mutants in Drosophila have identified
several likely candidates for these recombination-associated
events: abnormalities in meiotic motors and sister chromatid
cohesion proteins have been shown to increase the likelihood of
nondisjunction in distal-only exchange bivalents, and transposon
mobilization in the pericentromeric region may also be a source
of nondisjunction in flies. The relevance of these mutational
processes to human nondisjunction is not yet clear, and the
technical and ethical difficulties associated with human meiotic
analyses will complicate attempts to examine them. Nevertheless,
recent advances in molecular cytogenetic technology now make
it possible to study chromosome segregation in human gametes
(35), and the utilization of knockout mice makes it possible to
study the effect of specific mutations on meiotic chromosome
behavior. By combining these with other emerging technologies,
we may finally be able characterize the mechanisms underlying
human nondisjunction; given the extraordinary incidence and
clinical significance of trisomy to human well-being, it is
important that such attempts begin.
ACKNOWLEDGMENTS
We gratefully acknowledge Drs Patricia Hunt and Helen Salz for
their critical reading of the manuscript. Some of the research
described in this manuscript was supported by NIH grants
R01-HD21341 (to T.H.), P01-HD32111 (to T.H. and S.S.) and
R01-GM51444 (to R.S.H.), and by American Cancer Society
grant DB-39E (to R.S.H.).
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