Separation anxiety: the etiology of nondisjunction in flies and people

Risk of Down syndrome birth: Consanguineous marriage is associated with maternal meiosis-II nondisjunction at younger age and without any detectable recombination error

Consanguineous marriage was examined as a risk factor for Down syndrome birth. We genotyped Down syndrome family trios using short tandem repeat markers on 21q-to interpret the parental and meiotic stage of origin of errors as well as to record recombination profile along long arm of chromosome 21. We then compared nonconsanguineous (N = 811) group with-the consanguineous (N =157) marriages. We report for the first time that consanguineous marriage is associated with an increased risk for nondisjunction of chromosome 21 in oocytes-during the second meiotic division. We observed the absence of recombination more frequently in younger mothers in nonconsanguineous meiosis I cases. This was in contrast to an equal distribution of nonrecombinant cases across the age categories in the meiosis I consanguineous group. Moreover, the non-consanguineous group exhibited preferential telomeric recombination in meiosis I error among younger women and centromeric recombination in meiosis II errors in older women. In contrast, the consanguineous group exhibited medially placed recombination events in both meiosis I and meiosis II nondisjunction errors. Additionally, we recorded reduced maternal age at conception in the-consanguineous group. These findings suggest novel risk factors associated that increase the risk of chromosome 21 nondisjunction in the families with consanguinity.

Meiotic Consequences of Genetic Divergence Across the Murine Pseudoautosomal Region

The production of haploid gametes during meiosis is dependent on the homology-driven processes of pairing, synapsis, and recombination. On the mammalian heterogametic sex chromosomes, these key meiotic activities are confined to the pseudoautosomal region (PAR), a short region of near-perfect sequence homology between the X and Y chromosomes. Despite its established importance for meiosis, the PAR is rapidly evolving, raising the question of how proper X/Y segregation is buffered against the accumulation of homology-disrupting mutations. Here, I investigate the interplay of PAR evolution and function in two interfertile house mouse subspecies characterized by structurally divergent PARs, Mus musculus domesticus and M. m. castaneus. Using cytogenetic methods to visualize the sex chromosomes at meiosis, I show that intersubspecific F₁ hybrids harbor an increased frequency of pachytene spermatocytes with unsynapsed sex chromosomes. This high rate of asynapsis is due, in part, to the premature release of synaptic associations prior to completion of prophase I. Further, I show that when sex chromosomes do synapse in intersubspecific hybrids, recombination is reduced across the paired region. Together, these meiotic defects afflict ∼50% of spermatocytes from F₁ hybrids and lead to increased apoptosis in meiotically dividing cells. Despite flagrant disruption of the meiotic program, a subset of spermatocytes complete meiosis and intersubspecific F₁ males remain fertile. These findings cast light on the meiotic constraints that shape sex chromosome evolution and offer initial clues to resolve the paradox raised by the rapid evolution of this functionally significant locus.

Recurrent Trisomies: Chance or Inherited Predisposition?

Two patients experiencing recurring trisomic pregnancies involving a different chromosome each time are presented. Mechanisms to explain recurrent trisomies include a gene or genes predisposing to nondisjunction in general or to nondisjunction of the acrocentric chromosomes, maternal age effects, and germ-line mosaicism. Genetic counseling is complicated by the lack of a clear explanation for the recurrences, difficulty in quoting a specific recurrence risk, concern regarding the risk for uniparental disomy, and the frustration, grief and guilt reactions of the patients.

Genetic considerations in recurrent pregnancy loss

Human reproduction is remarkably inefficient; nearly 70% of human conceptions do not survive to live birth. Spontaneous fetal aneuploidy is the most common cause for spontaneous loss, particularly in the first trimester of pregnancy. Although losses owing to de novo fetal aneuploidy occur at similar frequencies among women with sporadic and recurrent losses, some couples with recurrent pregnancy loss have additional associated genetic factors and some have nongenetic etiologies. Genetic testing of the products of conception from couples experiencing two or more losses may aid in defining the underlying etiology and in counseling patients about prognosis in a subsequent pregnancy. Parental karyotyping of couples who have experienced recurrent pregnancy loss (RPL) will detect some couples with an increased likelihood of recurrent fetal aneuploidy; this may direct interventions. The utility of preimplantation genetic analysis in couples with RPL is unproven, but new approaches to this testing show great promise.

Altered patterns of multiple recombinant events are associated with nondisjunction of chromosome 21

We have previously examined characteristics of maternal chromosomes 21 that exhibited a single recombination on 21q and proposed that certain recombination configurations are risk factors for either meiosis I (MI) or meiosis II (MII) nondisjunction. The primary goal of this analysis was to examine characteristics of maternal chromosomes 21 that exhibited multiple recombinant events on 21q to determine whether additional risk factors or mechanisms are suggested. In order to identify the origin (maternal or paternal) and stage (MI or MII) of the meiotic errors, as well as placement of recombination, we genotyped over 1,500 SNPs on 21q. Our analyses included 785 maternal MI errors, 87 of which exhibited two recombinations on 21q, and 283 maternal MII errors, 81 of which exhibited two recombinations on 21q. Among MI cases, the average location of the distal recombination was proximal to that of normally segregating chromosomes 21 (35.28 vs. 38.86 Mb), a different pattern than that seen for single events and one that suggests an association with genomic features. For MII errors, the most proximal recombination was closer to the centromere than that on normally segregating chromosomes 21 and this proximity was associated with increasing maternal age. This pattern is same as that seen among MII errors that exhibit only one recombination. These findings are important as they help us better understand mechanisms that may underlie both age-related and nonage-related meiotic chromosome mal-segregation.

Molecular basis of maternal age-related increase in oocyte aneuploidy

Aneuploidy is one of the most common and serious pregnancy complications in humans. Most conceptuses with autosomal aneuploidy die in utero, resulting in early pregnancy loss. However, some fetuses with aneuploidy survive to term but suffer from disorders associated with congenital anomalies and mental retardation, such as Down syndrome with trisomy 21. Three general characteristics of this condition are well acknowledged: (i) in most cases the extra chromosome is of maternal origin; (ii) most cases are derived from a malsegregation event in meiosis I; and (iii) the frequency of these errors increases with maternal age. The basis for the age-dependent increase in meiosis I errors has been a long-standing enigma. Many investigators have addressed the nature of this biological phenomenon through genomic analyses of extra chromosome 21 using polymorphic markers to determine the frequency or location of crossovers that should ensure faithful chromosome segregation. Cytogenetic analyses of in vitro unfertilized oocytes have also been performed. However, no definitive conclusions regarding meiosis I errors have yet been reached from such studies. Recent findings in conditional knock-out mice for meiosis-specific cohesin have shed further light on this issue. The present review focuses on the current understanding of age-related aneuploidy and provides an overview of the mechanisms involved. We refer to recent data to illustrate some of the new paradigms that have arisen in this field.

Genetic analysis of genome-scale recombination rate evolution in house mice

The rate of meiotic recombination varies markedly between species and among individuals. Classical genetic experiments demonstrated a heritable component to population variation in recombination rate, and specific sequence variants that contribute to recombination rate differences between individuals have recently been identified. Despite these advances, the genetic basis of species divergence in recombination rate remains unexplored. Using a cytological assay that allows direct in situ imaging of recombination events in spermatocytes, we report a large (∼30%) difference in global recombination rate between males of two closely related house mouse subspecies (Mus musculus musculus and M. m. castaneus). To characterize the genetic basis of this recombination rate divergence, we generated an F2 panel of inter-subspecific hybrid males (n = 276) from an intercross between wild-derived inbred strains CAST/EiJ (M. m. castaneus) and PWD/PhJ (M. m. musculus). We uncover considerable heritable variation for recombination rate among males from this mapping population. Much of the F2 variance for recombination rate and a substantial portion of the difference in recombination rate between the parental strains is explained by eight moderate- to large-effect quantitative trait loci, including two transgressive loci on the X chromosome. In contrast to the rapid evolution observed in males, female CAST/EiJ and PWD/PhJ animals show minimal divergence in recombination rate (∼5%). The existence of loci on the X chromosome suggests a genetic mechanism to explain this male-biased evolution. Our results provide an initial map of the genetic changes underlying subspecies differences in genome-scale recombination rate and underscore the power of the house mouse system for understanding the evolution of this trait.

Homologous recombination is an indispensable feature of the mammalian meiotic program and an important mechanism for creating genetic diversity. Despite its central significance, recombination rates vary markedly between species and among individuals. Although recent studies have begun to unravel the genetic basis of recombination rate variation within populations, the genetic mechanisms of species divergence in recombination rate remain poorly characterized. In this study, we show that two closely related house mouse subspecies differ in their genomic recombination rates by ∼30%, providing an excellent model system for studying evolutionary divergence in this trait. Using quantitative genetic methods, we identify eight genomic regions that contribute to divergence in global recombination rate between these subspecies, including large effect loci and multiple loci on the X-chromosome. Our study uncovers novel genomic loci contributing to species divergence in global recombination rate and offers simple genetic explanations for rapid phenotypic divergence in this trait.

Telomere length is associated with types of chromosome 21 nondisjunction: a new insight into the maternal age effect on Down syndrome birth

Advanced maternal age is a well-documented risk factor of chromosome 21 nondisjunction in humans, but understanding of this association at the genetic level is still limited. In particular, the state of maternal genetic age is unclear. In the present study, we estimated maternal genetic age by measuring telomere length of peripheral blood lymphocytes among age-matched mothers of children with Down syndrome (cases: N = 75) and mothers of euploid children (controls: N = 75) in an age range of 18-42 years. All blood samples were taken within 1 week of the birth of the child in both cases and controls. The telomere length estimation was performed by restriction digestion--Southern blot hybridization method. We stratified the cases on the basis of centromeric STR genotyping into maternal meiosis I (N = 48) and maternal meiosis II (N = 27) nondisjunction groups and used linear regression to compare telomere length as a function of age in the euploid, meiosis I and meiosis II groups. Our results show that all three groups have similar telomere length on average for younger mothers. As age increases, all groups show telomere loss, but that loss is largest in the meiosis II mother group and smallest in the euploid mother group with the meiosis I mother group in the middle. The regression lines for all three were statistically significantly different from each other (p < 0.001). Our results do not support the theory that younger women who have babies with Down syndrome do so because are 'genetically older' than their chronological age, but we provide the first evidence that older mothers who have babies with Down syndrome are 'genetically older' than controls, who have euploid babies at the same age. We also show for the first time that telomere length attrition may be associated in some way with meiosis I and meiosis II nondisjunction of chromosome 21 and subsequent Down syndrome births at advanced maternal age.

Chromosome 21 non-disjunction and Down syndrome birth in an Indian cohort: analysis of incidence and aetiology from family linkage data

We analysed the family linkage data obtained from short tandem repeat (STR) genotyping of 212 unrelated Indian families having a single Down syndrome (DS) baby each, in order to explore the incidence and aetiology of this human aneuploidy in our cohort. The estimated values of maternal meiotic I and meiotic II non-disjunction (NDJ) errors of chromosome 21 (Ch 21) were ~78 and ~22%, respectively. Within the paternal outcome group, about 47 and 53% were accounted for NDJ at meiosis I and meiosis II, respectively. We estimated only ~2% post-zygotic mitotic errors. The comparison of average age of conception between controls and DS-bearing mothers revealed a significant difference (P<0·001) with DS-bearing women were on an average older than controls and meiotic II non-disjoined mothers were oldest among meiotic outcome groups. Our linkage analysis suggested an overall reduction in recombination by more than 50% on meiotic I non-disjoined maternal Ch 21 with error prone to susceptible chiasma formation within the ~5·1 kbp segment near the telomeric end. We stratified meiotic I non-disjoined women in three age groups, viz. young (⩽28 years), middle (29–34 years) and old (⩾35 years) and found linear decrease in the frequency of achiasmate meiosis from the young to the old group. In contrary, a linear increase in the multiple chiasma frequency from the young to the old group was observed. Considering these results together, we propose that the risk factors for Ch 21 NDJ are of two types, one being ‘maternal age-independent’ and the other being ‘maternal age-dependent’. Moreover, a comparison of our present Indian dataset with that of other published data of ethnically different populations suggested that the genetics that underlies the NDJ of Ch 21 is probably universal irrespective of racial difference across human populations. The present study is the first population-based report on any DS cohort from the Indian subcontinent and our work will help future workers in understanding better the aetiology of this birth defect.

On the origin of the maternal age effect in trisomy 21 Down syndrome: the Oocyte Mosaicism Selection model

We have recently documented that trisomy 21 mosaicism is common in human foetal ovaries. On the basis of this observation we propose that the maternal age effect in Down syndrome (DS) is caused by the differential behaviour of trisomy 21 in relation to disomy 21 oocytes during development from foetal life until ovulation in adulthood. In particular, we suggest that trisomy 21 oocytes, lagging behind those that are disomic, may escape the timed pruning of the seven million in foetal life to the 300–400 finally selected for ovulation. The net effect of this preferential elimination will be an accumulation of trisomy 21 oocytes in the ovarian reserve of older women. We here highlight the implications of this Oocyte Mosaicism Selection (OMS) model with respect to the prevalent view that the maternal age effect is complex, dependent on many different biological and environmental factors. We examine conclusions drawn from recent large-scale studies in families, tracing DNA markers along the length of chromosome 21q between parents and DS children, in comparison to the OMS model. We conclude that these family linkage data are equally compatible with the maternal age effect originating from the accumulation of trisomy 21 oocytes with advancing maternal age. One relatively straightforward way to get to grips with what is actually going on in this regard would be to compare incidence of trisomy 21 oocytes (and their pairing configurations) in foetal ovaries with that in oocytes at the meiosis I stage from adult women.

Etiology of Down syndrome: Evidence for consistent association among altered meiotic recombination, nondisjunction, and maternal age across populations

Down syndrome caused by meiotic nondisjunction of chromosome 21 in humans, is well known to be associated with advanced maternal age, but success in identifying and understanding other risk factors has been limited. Recently published work in a U.S. population suggested intriguing interactions between the maternal age effect and altered recombination patterns during meiosis, but some of the results were counter-intuitive. We have tested these hypotheses in a population sample from India, and found that essentially all of the results of the U.S. study are replicated even in our ethnically very different population. We examined meiotic recombination patterns in a total of 138 families from the eastern part of India, each with a single free trisomy 21 child. We genotyped each family with a set of STR markers using PCR and characterized the stage of origin of nondisjunction and the recombination pattern of maternal chromosome 21 during oogenesis. Our sample contains 107 maternal meiosis I errors and 31 maternal meiosis II errors and we subsequently stratified them with respect to maternal age and the number of detectable crossover events. We observed an association between meiosis I nondisjunction and recombination in the telomeric 5.1 Mb of chromosome 21. By contrast, in meiosis II cases we observed preferential pericentromeric exchanges covering the proximal 5.7 Mb region, with interaction between maternal age and the location of the crossover. Overall reduction of recombination irrespective of maternal age is also evident in meiosis I cases. Our findings are very consistent with previously reported data in a U.S. population and our results are the first independent confirmation of those previous reports. This not only provides much needed confirmation of previous results, but it suggests that the genetic etiology underlying the occurrence of trisomy 21 may be similar across human populations.

New insights into human nondisjunction of chromosome 21 in oocytes

Nondisjunction of chromosome 21 is the leading cause of Down syndrome. Two risk factors for maternal nondisjunction of chromosome 21 are increased maternal age and altered recombination. In order to provide further insight on mechanisms underlying nondisjunction, we examined the association between these two well established risk factors for chromosome 21 nondisjunction. In our approach, short tandem repeat markers along chromosome 21 were genotyped in DNA collected from individuals with free trisomy 21 and their parents. This information was used to determine the origin of the nondisjunction error and the maternal recombination profile. We analyzed 615 maternal meiosis I and 253 maternal meiosis II cases stratified by maternal age. The examination of meiosis II errors, the first of its type, suggests that the presence of a single exchange within the pericentromeric region of 21q interacts with maternal age-related risk factors. This observation could be explained in two general ways: 1) a pericentromeric exchange initiates or exacerbates the susceptibility to maternal age risk factors or 2) a pericentromeric exchange protects the bivalent against age-related risk factors allowing proper segregation of homologues at meiosis I, but not segregation of sisters at meiosis II. In contrast, analysis of maternal meiosis I errors indicates that a single telomeric exchange imposes the same risk for nondisjunction, irrespective of the age of the oocyte. Our results emphasize the fact that human nondisjunction is a multifactorial trait that must be dissected into its component parts to identify specific associated risk factors.

Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age

Mammalian oocytes begin meiosis in the fetal ovary, but only complete it when fertilized in the adult reproductive tract. This review examines the cell biology of this protracted process: from entry of primordial germ cells into meiosis to conception. The defining feature of meiosis is two consecutive cell divisions (meiosis I and II) and two cell cycle arrests: at the germinal vesicle (GV), dictyate stage of prophase I and at metaphase II. These arrests are spanned by three key events, the focus of this review: (i) passage from mitosis to GV arrest during fetal life, regulated by retinoic acid; (ii) passage through meiosis I and (iii) completion of meiosis II following fertilization, both meiotic divisions being regulated by cyclin-dependent kinase (CDK1) activity. Meiosis I in human oocytes is associated with an age-related high rate of chromosomal mis-segregation, such as trisomy 21 (Down's syndrome), resulting in aneuploid conceptuses. Although aneuploidy is likely to be multifactorial, oocytes from older women may be predisposed to be becoming aneuploid as a consequence of an age-long decline in the cohesive ties holding chromosomes together. Such loss goes undetected by the oocyte during meiosis I either because its ability to respond and block division also deteriorates with age, or as a consequence of being inherently unable to respond to the types of segregation defects induced by cohesion loss.

Chromosome malorientations after meiosis II arrest cause nondisjunction

This study investigated the basis of meiosis II nondisjunction. Cold arrest induced a fraction of meiosis II crane fly spermatocytes to form (n + 1) and (n - 1) daughters during recovery. Live-cell liquid crystal polarized light microscope imaging showed nondisjunction was caused by chromosome malorientation. Whereas amphitely (sister kinetochore fibers to opposite poles) is normal, cold recovery induced anaphase syntely (sister fibers to the same pole) and merotely (fibers to both poles from 1 kinetochore). Maloriented chromosomes had stable metaphase positions near the equator or between the equator and a pole. Syntelics were at the spindle periphery at metaphase; their sisters disconnected at anaphase and moved all the way to a centrosome, as their strongly birefringent kinetochore fibers shortened. The kinetochore fibers of merotelics shortened little if any during anaphase, making anaphase lag common. If one fiber of a merotelic was more birefringent than the other, the less birefringent fiber lengthened with anaphase spindle elongation, often permitting inclusion of merotelics in a daughter nucleus. Meroamphitely (near amphitely but with some merotely) caused sisters to move in opposite directions. In contrast, syntely and merosyntely (near syntely but with some merotely) resulted in nondisjunction. Anaphase malorientations were more frequent after longer arrests, with particularly long arrests required to induce syntely and merosyntely.

A linkage map for brown trout (Salmo trutta): chromosome homeologies and comparative genome organization with other salmonid fish

We report on the construction of a linkage map for brown trout (Salmo trutta) and its comparison with those of other tetraploid-derivative fish in the family Salmonidae, including Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), and Arctic char (Salvelinus alpinus). Overall, we identified 37 linkage groups (2n = 80) from the analysis of 288 microsatellite polymorphisms, 13 allozyme markers, and phenotypic sex in four backcross families. Additionally, we used gene-centromere analysis to approximate the position of the centromere for 20 linkage groups and thus relate linkage arrangements to the physical morphology of chromosomes. Sex-specific maps derived from multiple parents were estimated to cover 346.4 and 912.5 cM of the male and female genomes, respectively. As previously observed in other salmonids, recombination rates showed large sex differences (average female-to-male ratio was 6.4), with male crossovers generally localized toward the distal end of linkage groups. Putative homeologous regions inherited from the salmonid tetraploid ancestor were identified for 10 pairs of linkage groups, including five chromosomes showing evidence of residual tetrasomy (pseudolinkage). Map alignments with orthologous regions in Atlantic salmon, rainbow trout, and Arctic char also revealed extensive conservation of syntenic blocks across species, which was generally consistent with chromosome divergence through Robertsonian translocations.

Evidence of a high proportion of premature unbalanced separation of sister chromatids in the first polar bodies of women of advanced age

BACKGROUND

Maternal ageing is the only aetiological factor unequivocally linked to aneuploidy. Two mechanisms seem to explain these abnormalities in oocytes: non-disjunction and premature unbalanced separation of sister chromatids (PSSC). Previous studies of unfertilized oocytes argue for a major role of PSSC in the aetiology of aneuploidy for women of advanced age, but in vitro ageing of the oocytes could influence the results.

METHODS

Owing to the high prevalence of aneuploidy in women of advanced age, chromosomal screening of the first polar body just before ICSI was offered to women (from 38 years of age) included in an assisted reproduction programme.

RESULTS

Among 141 oocytes from 29 women (mean age 40 years and 2 months), 43 (30.5%) were abnormal. Sixty-five abnormalities were found and PSSC was involved in 80% of cases.

CONCLUSION

These results are in accordance with previous studies and confirm, in 'fresh' oocytes, the major role of PSSC in the aetiology of aneuploidy in women of advanced age.

Effect of maternal age on the frequency of cytogenetic abnormalities in human oocytes

The cytogenetic investigation of human oocytes was initiated in the Sixties, and for the last four decades, this field of research has never stopped progressing as new technologies appear. Numerous karyotyping studies and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes, but also displaying a great variability in results, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. Essentially, the most relevant analyses have led to the estimate that 15-20% of human oocytes display chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies has also been investigated in human oocytes. Most previous studies have failed to confirm any relationship between maternal age and aneuploidy frequency in human oocytes, whereas the more recent reports based on large samples of oocytes or polar bodies have provided evidence for a direct correlation between increased aneuploidy frequency and advanced maternal age, and have clarified the contribution of the various types of malsegregation in the maternal age-dependent aneuploidies.

Maternal meiosis II nondisjunction in trisomy 21 is associated with maternal low socioeconomic status

PURPOSE

We evaluated whether the association of socioeconomic risk factors for trisomy 21 differed by type of maternal meiotic error.

METHODS

We determined meiotic errors by DNA analysis for 150 trisomy 21 cases, and maternal lifetime exposures to low socioeconomic factors by questionnaire.

RESULTS

Mothers of meiosis II cases were significantly more likely to be exposed to four low socioeconomic factors than mothers of meiosis I cases (odds ratio = 9.50; 95% confidence interval = 1.8-49.8).

CONCLUSION

Maternal lifetime exposure to poor socioeconomic environment is a risk factor for a trisomy 21, particularly if nondisjunction leads to a maternal meiosis II.

The chromosomal analysis of human oocytes. An overview of established procedures

The cytogenetic survey of mature human oocytes has been and remains a subject of great interest because of the prevalence of aneuploidy of maternal origin in abnormal human conceptuses, and the lack of understanding about the non-disjunction processes in human meiosis. The first attempts to analyse the chromosomal content of human female gametes were made in the early 1970s, and led to limited data because of the paucity of materials and the inadequacy of the procedure used. The years to follow brought a resurgence of interest in this field, because of the development of human IVF techniques which made oocytes unfertilized in vitro available for cytogenetic analysis. Numerous studies have since been performed. However, the difficulties in obtaining good chromosome preparations and of performing accurate chromosome identification have reduced the viability of these studies, resulting in large variations in the reported incidences of chromosomal abnormalities. The further introduction of new procedures for oocyte fixation and the screening of large oocyte samples have allowed more reliable data to be obtained and to identify premature chromatid separation as a major mechanism in aneuploidy occurrence. The last decade has been privileged to witness the adaptation of molecular cytogenetic techniques to human oocytes, and thus various powerful procedures have been tried not only on female gametes, but also on polar bodies, involving sequential and multicolour fluorescent in situ hybridization (FISH) labelling, comparative genomic hybridization (CGH), spectral karyotyping and alternative methods such as primed in situ labelling (PRINS) and peptide nucleic acid (PNA) techniques. A large body of data has been obtained, but these studies also display a great variability in the frequency of abnormalities, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. However, molecular cytogenetic approaches have also evidenced the co-existence of both whole chromosome non-disjunction and chromatid separation in maternal aneuploidy. In addition, the extension of these techniques to oocyte polar body materials has provided additional data on the mechanism of meiotic malsegregation. Improvements of some of these techniques have already been reported. The further development of new approaches for the in situ analysis of human meiosis will increase the impact of cytogenetic investigation of human oocytes in the understanding of aneuploidy processes in humans.

Variation in human meiotic recombination

As recently as 20 years ago, there was relatively little information about the number and distribution of recombinational events in human meiosis, and we knew virtually nothing about factors affecting patterns of recombination. However, the generation of a variety of linkage-based genetic mapping tools and, more recently, cytological approaches that enable us to directly visualize the recombinational process in meiocytes, have led to an increased understanding of human meiosis. In this review, we discuss the different approaches used to study meiotic recombination in humans, our understanding of factors that affect the number and location of recombinational events, and clinical consequences of variation in the recombinational process.

Âge maternel et anomalies chromosomiques dans les ovocytes humains

Maternal ageing is the only etiological factor unequivocally associated with the occurrence of aneuploid conceptuses. Molecular studies of trisomies have demonstrated that the pattern of recombinaison was an important predisposing factor to meiotic nondisjunction. To complete this data, a large chromosomal study has been undertaken on 1,397 unfertilised human oocytes recovered from women participating in in vitro fertilization programmes. Conventional whole chromosome nondisjunction and premature chromatid separation were the major types of numerical abnormalities observed. A positive relationship was found between maternal age and these two types of nondisjunction, but the most significant correlation was observed with chromatid separation resulting in the presence of free chromatid in metaphase II oocyte. These data revealed that chromatid separation was an essential factor in the age-dependent occurrence of aneuploidy. This finding provided new insights into the mechanism of nondisjunction in female meiosis since disturbance in molecular chromatid cohesion by cohesins might be a causal mechanism predisposing to nondisjunction and involved in the maternal age effect.

Effects of male age on the frequencies of germinal and heritable chromosomal abnormalities in humans and rodents

OBJECTIVE

To review evidence regarding the effects of male age on germinal and heritable chromosomal abnormalities using available human and rodent studies and to evaluate possible underlying mechanisms.

DESIGN

Review of English language-published research using MEDLINE database, excluding case reports and anecdotal data.

RESULT(S)

There was little evidence from offspring or germ cell studies for a generalized male age effect on autosomal aneuploidy, except in rodents. Sex chromosomal nondisjunction increased with age in both human and rodent male germ cells. Both human and rodent data showed age-related increases in the number of sperm with chromosomal breaks and fragments and suggest that postmeiotic cells are particularly vulnerable to the effects of aging. Translocation frequencies increased with age in murine spermatocytes, at rates comparable to mouse and human somatic cells. Age-related mechanisms of induction may include accumulation of environmental damage, reduced efficiency of DNA repair, increased genomic instability, genetic factors, hormonal influences, suppressed apoptosis, or decreased effectiveness of antioxidants and micronutrients.

CONCLUSION(S)

The weight of evidence suggests that the increasing trend toward fathering at older ages may have significant effects on the viability and genetic health of human pregnancies and offspring, primarily as a result of structural chromosomal aberrations in sperm.

To err (meiotically) is human: the genesis of human aneuploidy

Aneuploidy (trisomy or monosomy) is the most commonly identified chromosome abnormality in humans, occurring in at least 5% of all clinically recognized pregnancies. Most aneuploid conceptuses perish in utero, which makes this the leading genetic cause of pregnancy loss. However, some aneuploid fetuses survive to term and, as a class, aneuploidy is the most common known cause of mental retardation. Despite the devastating clinical consequences of aneuploidy, relatively little is known of how trisomy and monosomy originate in humans. However, recent molecular and cytogenetic approaches are now beginning to shed light on the non-disjunctional processes that lead to aneuploidy.

Reading between the LINEs: human genomic variation induced by LINE-1 retrotransposition

The insertion of mobile elements into the genome represents a new class of genetic markers for the study of human evolution. Long interspersed elements (LINEs) have amplified to a copy number of about 100,000 over the last 100 million years of mammalian evolution and comprise approximately 15% of the human genome. The majority of LINE-1 (L1) elements within the human genome are 5' truncated copies of a few active L1 elements that are capable of retrotransposition. Some of the young L1 elements have inserted into the human genome so recently that populations are polymorphic for the presence of an L1 element at a particular chromosomal location. L1 insertion polymorphisms offer several advantages over other types of polymorphisms for human evolution studies. First, they are typed by rapid, simple, polymerase chain reaction (PCR)-based assays. Second, they are stable polymorphisms that rarely undergo deletion. Third, the presence of an L1 element represents identity by descent, because the probability is negligible that two different young L1 repeats would integrate independently between the exact same two nucleotides. Fourth, the ancestral state of L1 insertion polymorphisms is known to be the absence of the L1 element, which can be used to root plots/trees of population relationships. Here we report the development of a PCR-based display for the direct identification of dimorphic L1 elements from the human genome. We have also developed PCR-based assays for the characterization of six polymorphic L1 elements within the human genome. PCR analysis of human/rodent hybrid cell line DNA samples showed that the polymorphic L1 elements were located on several different chromosomes. Phylogenetic analysis of nonhuman primate DNA samples showed that all of the recently integrated "young" L1 elements were restricted to the human genome and absent from the genomes of nonhuman primates. Analysis of a diverse array of human populations showed that the allele frequencies and level of heterozygosity for each of the L1 elements was variable. Polymorphic L1 elements represent a new source of identical-by-descent variation for the study of human evolution. [The sequence data described in this paper have been submitted to the GenBank data library under accession nos. AF242435-AF242451.]

Requirement of the spindle checkpoint for proper chromosome segregation in budding yeast meiosis

The spindle checkpoint was characterized in meiosis of budding yeast. In the absence of the checkpoint, the frequency of meiosis I missegregation increased with increasing chromosome length, reaching 19% for the longest chromosome. Meiosis I nondisjunction in spindle checkpoint mutants could be prevented by delaying the onset of anaphase. In a recombination-defective mutant (spo11Delta), the checkpoint delays the biochemical events of anaphase I, suggesting that chromosomes that are attached to microtubules but are not under tension can activate the spindle checkpoint. Spindle checkpoint mutants reduce the accuracy of chromosome segregation in meiosis I much more than that in meiosis II, suggesting that checkpoint defects may contribute to Down syndrome.

Recombination within the human MHC

Recombination (crossing over) in the human MHC is thought to have played a role in generation of novel alleles at various HLA loci. It is also responsible for the diversity observed at the haplotype level, although the functional consequences of this activity are not clear. Historic and family studies of recombination have provided estimations of recombination fractions across the MHC and identified potential hotspots for recombination in the class II region. Other characteristics of recombination in the human MHC such as haplotype specificity in recombination frequency and localized sequence motifs involved in recombination have been considered, but have been difficult to address given the constraints of human population studies. Single-sperm typing holds promise in overcoming some of the limitations inherent in the study of recombination in human populations. Both family-based and sperm typing analyses of recombination, along with our knowledge of linkage disequilibrium patterns in the MHC, may provide novel information regarding the evolution of HLA haplotypes that will be difficult to obtain by other means.

Genetics of oocyte ageing

OBJECTIVES

Correlations between parental age, aneuploidy in germ cells and recent findings on aetiological factors in mammalian trisomy formation are reviewed.

METHODS

Data from observations in human oocytes, molecular studies on the origin of extra chromosomes in trisomies, experiments in a mouse model system, and transgenic approaches are shown.

RESULTS

Errors in chromosome segregation are most frequent in meiosis I of oogenesis in mammals and predominantly predispose specific chromosomes and susceptible chiasmate configurations to maternal age-related nondisjunction. Studies on spindle structure, cell cycle and chromosome behaviour in oocytes of the CBA/Ca mouse used as a model for the maternal age-effect suggest that hormonal homeostasis and size of the follicle pool influence the quality, maturation competence and spindle size of the mammalian oocyte. Predisposition to errors in chromosome segregation are critically dependent on altered cell cycles. Compromised protein synthesis and mitochondrial function affect maturation kinetics and spindle formation, and cause untimely segregation of chromosomes (predivision), mimicking an aged phenotype.

CONCLUSIONS

Altered cell cycles and untimely resolution of chiasmata but also nondisjunction of late segregating homologues caused by asynchrony in cytoplasmic and nuclear maturation appear to be causal to errors in chromosome segregation with advanced maternal age. Oocytes appear to lack checkpoints guarding against untimely chromosome segregation. Genes and exposures affecting pool size, hormonal homeostasis and interactions between oocytes and their somatic compartment and thus quality of follicles and oocytes have the potential to critically influence chromosome distribution in female meiosis and affect fertility in humans and other mammals.

Chromosomal drive and the evolution of meiotic nondisjunction and trisomy in humans

Trisomy is a genetic abnormality of considerable medical importance. The most familiar example is trisomy 21, which causes Down Syndrome [Cummings, M. R. (1988) Human Heredity: Principles and Issues (West Publishing Company, New York)]. In a classic paper, Axelrod and Hamilton [Axelrod, R. & Hamilton, W. D. (1981) Science 211, 1390-1396] offered a chromosomal drive (CD) hypothesis based on the game iterated prisoner's dilemma (IPD) to explain the evolution of an increased frequency of trisomic pregnancies with maternal age. In this paper we explore this hypothesis and its predictions in detail. On closer examination we find that IPD does not provide an adequate model for the CD hypothesis. Therefore, we develop a more suitable model and explore the conditions necessary for it to explain maternal age-dependent trisomy. Our results demonstrate that a relationship between the decay of a female's reproductive potential and chromosomal drive must exist for the CD hypothesis to work. With appropriate parameter values, a comparison of model predictions with empirical estimates for the age-dependence of trisomy reveals a striking correspondence. We point out a close correspondence between other predictions made by the CD hypothesis and empirical observations, as well.

Chromosome 21 detection in human oocyte fluorescence in situ hybridization: possible effect of maternal age

PURPOSE

The purpose of this study was to evaluate, among 100 uncleaved oocytes, the incidence of numerical and structural chromosome 21 and X abnormalities and to analyze the influence of various factors, such as in vitro (IVF) indications, follicle stimulation protocols, and women's age.

METHODS

We investigated 150 uncleaved oocytes from 128 patients after an IVF attempt. After cytogenetic analysis (Giemsa) 100 oocytes (66%) were selected for fluorescence in situ hybridization (FISH). Fluorescent probes for human chromosomes X and 21 were used simultaneously according to standard procedures for their hybridization and detection.

RESULTS AND CONCLUSIONS

We analyzed by the FISH protocol 100 metaphase II oocytes with 22 to 25 chromosomes. Our results demonstrate a high rate of disomy for chromosome 21 in human oocytes. Among them, eight were disomic (8%) and three were nullosomic (3%) for chromosome 21. Only one disomy of chromosome X was noted. The various indications of IVF and the different folliculogenesis stimulating protocols did not seem to influence the results but suggested a correlation between the maternal age and the aneuploidy rate of chromosome 21.

The mechanisms involved in formation of deletions and duplications of 15q11-q13

Haplotype analysis was undertaken in 20 cases of 15q11-q13 deletion associated with Prader-Willi syndrome (PWS) or Angelman syndrome (AS) to determine if these deletions arose through unequal meiotic crossing over between homologous chromosomes. Of these, six cases of PWS and three of AS were informative for markers on both sides of the deletion. For four of six cases of paternal 15q11-q13 deletion (PWS), markers on both sides of the deletion breakpoints were inferred to be of the same grandparental origin, implying an intrachromosomal origin of the deletion. Although the remaining two PWS cases showed evidence of crossing over between markers flanking the deletion, this was not more frequent than expected by chance given the genetic distance between proximal and distal markers. It is therefore possible that all PWS deletions were intrachromosomal in origin with the deletion event occurring after normal meiosis I recombination. Alternatively, both sister chromatid and homologous chromosome unequal exchange during meiosis may contribute to these deletions. In contrast, all three cases of maternal 15q11-q13 deletion (AS) were associated with crossing over between flanking markers, which suggests significantly more recombination than expected by chance (p = 0.002). Therefore, there appears to be more than one mechanism which may lead to PWS/AS deletions or the resolution of recombination intermediates may differ depending on the parental origin of the deletion. Furthermore, 13 of 15 cases of 15q11-q13 duplication, triplication, or inversion duplication had a distal duplication breakpoint which differed from the common distal deletion breakpoint. The presence of at least four distal breakpoint sites in duplications indicates that the mechanisms of rearrangement may be complex and multiple repeat sequences may be involved.

Chiasmata, crossovers, and meiotic chromosome segregation

Meiotic recombination events are probably critical for the completion of several meiotic processes. In addition, recombination is likely to be involved in the events that lead up to synapsis of homologues in meiotic prophase. Recombination events that ultimately become resolved as exchanges are needed for the formation of chiasmata. Chiasmata maintain the association of paired homologues following loss of the synaptonemal complex and participate in the mechanism that signals that the bivalent has attached to the spindle in a bipolar orientation that will result in meiosis I disjunction.

Spontaneous X chromosome MI and MII nondisjunction events in Drosophila melanogaster oocytes have different recombinational histories

Recent studies of human oocytes have demonstrated an enrichment for distal exchanges among meiosis I (MI) nondisjunction events and for proximal exchanges among meiosis II (MII) events. Our characterization of 103 cases of spontaneous X chromosome nondisjunction in Drosophila oocytes strongly parallels these observations. The recombinational histories of MI (97/103) and MII (6/103) nondisjunctional ova were strikingly different. MI nondisjunction occurred primarily in oocytes with non-exchange X chromosomes; of the new nondisjoining exchange bivalents, most carried distal crossovers. Thus, spontaneous MI nondisjunction reflects the failure of the achiasmate segregation systems. MII nondisjunction occurred only in oocytes with proximal exchanges. We propose several models to explain how very proximal exchanges might impair proper segregation.

Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II

The cause of non-disjunction of chromosome 21 remains largely unknown. Advanced maternal age is associated with both maternal meiosis I (MI) and meiosis II (MII) non-disjunction events. While reduced genetic recombination has been demonstrated in maternal MI errors, the basis for MII errors remains uncertain. We studied 133 trisomy 21 cases with maternal MII errors to test the hypothesis that segregation at MII may also be influenced by genetic recombination. Our data support a highly significant association: MII non-disjunction involves increased recombination that is largely restricted to proximal 21q. Thus, while absence of a proximal recombination appears to predispose to non-disjunction in MI, the presence of a proximal exchange predisposes to non-disjunction in MII. These findings profoundly affect our understanding of trisomy 21 as they suggest that virtually all maternal non-disjunction results from events occurring in meiosis I.

Kinesin-related proteins in the mammalian testes: candidate motors for meiosis and morphogenesis

The kinesin superfamily of molecular motors comprises proteins that participate in a wide variety of motile events within the cell. Members of this family share a highly homologous head domain responsible for force generation attached to a divergent tail domain thought to couple the motor domain to its target cargo. Many kinesin-related proteins (KRPs) participate in spindle morphogenesis and chromosome movement in cell division. Genetic analysis of mitotic KRPs in yeast and Drosophila, as well as biochemical experiments in other species, have suggested models for the function of KRPs in cell division, including both mitosis and meiosis. Although many mitotic KRPs have been identified, the relationship between mitotic motors and meiotic function is not clearly understood. We have used sequence similarity between mitotic KRPs to identify candidates for meiotic and/or mitotic motors in a vertebrate. We have identified a group of kinesin-related proteins from rat testes (termed here testes KRP1 through KRP6) that includes new members of the bimC and KIF2 subfamilies as well as proteins that may define new kinesin subfamilies. Five of the six testes KRPs identified are expressed primarily in testes. Three of these are expressed in a region of the seminiferous epithelia (SE) rich in meiotically active cells. Further characterization of one of these KRPs, KRP2, showed it to be a promising candidate for a motor in meiosis: it is localized to a meiotically active region of the SE and is homologous to motor proteins associated with the mitotic apparatus. Testes-specific genes provide the necessary probes to investigate whether the motor proteins that function in mammalian meiosis overlap with those of mitosis and whether motor proteins exist with functions unique to meiosis. Our search for meiotic motors in a vertebrate testes has successfully identified proteins with properties consistent with those of meiotic motors in addition to uncovering proteins that may function in other unique motile events of the SE.

Cytogenetics of uncleaved oocytes and arrested zygotes in IVF programs

PURPOSE

Cytogenetic studies of arrested oocytes and zygotes were used to understand in vitro fertilization (IVF) failures.

METHODS

We investigated the cytogenetics (Giemsa banding and FISH) of 710 uncleaved oocytes and 94 arrested zygotes from 208 patients undergoing IVF procedures.

RESULTS AND CONCLUSIONS

Of uncleaved oocytes without a polar body, 39% were judged cytogenetically abnormal (17% unbalanced predivision and 21.5% diploid). Of 575 oocytes with a polar body, 124 (21.5%) showed numerical or structural chromosome aberrations. In arrested zygotes, approximately equal cases were found with separate condensed haploid complements (no syngamy), nuclear asynchrony and pulverized DNA, and apparently cytogenetically normal zygotes arrested at mitosis. These data on chromosome abnormalities were also analyzed with respect to two ovarian stimulation protocols and to maternal age. Both ovarian stimulation protocols and to maternal age. Both ovarian stimulation protocols showed the same levels of chromosome abnormalities. Overall chromosome abnormalities and premature chromosome condensation were also unchanged with maternal age. These data illustrate the significance of chromosome aberrations in IVF failures.

Meiotic recombination and germ cell aneuploidy

Data on human trisomic conceptuses suggest that the extra chromosome commonly has a maternal origin, and the amount and position of crossing-over on nondisjoined chromosomes is commonly altered. These observations may provide important clues to the etiology of human germ cell aneuploidy, especially in regard to evaluating whether environmental factors play a role. There is concordance of effects of environmental agents on fungi, plants, and animals, which suggests that the overall process of meiosis is well conserved and that chemical and physical agents can affect meiotic recombination, leading to aneuploidy. It seems likely that meiosis in humans will fit the general pattern of meiosis in terms of sensitivity to radiation and chemicals. Thus studies on other organisms provide some insight into the procedures necessary for obtaining useful human data. For example, frequencies of spontaneous meiotic recombination are not uniform per physical length in Drosophila, and different regions of a chromosome respond differently to treatment. Treatments that relieve constraints on the distribution of meiotic exchange, without changing greatly the overall frequency of exchange, may increase the number of univalents and give the impression that there are chromosome-specific responses. Recombination studies that monitor one or a few relatively short genetic regions may also give a false impression of the effects of a treatment on recombination. In addition, meiotic mutants in Saccharomyces and Drosophila highlight a number of processes that are important for production of an exchange event and the utility of that event in the proper segregation of both homologues and sisters. They also suggest that tests for pairing at pachytene, chiasmata at diplotene, and genetic crossing-over may give different results.

Parental age-related aneuploidy in human germ cells and offspring: a story of past and present

Parental age is the most important aetiological factor in trisomy formation in humans. Cytogenetic studies on germ cells reviewed here imply that (i) 2-4% sperm are aneuploid and 8.6% oocytes from IVF are hyperploid (ii) a paternal age effect may exist, and (iii) oocytes of aged women contain precociously separated chromatids in metaphase II. Trisomy data suggest that most aneuploidy is generated during meiosis I of oogenesis and is maternal age-dependent. Trisomy 18 is unique, originating mostly from maternal meiosis II errors. The extra gonosome in 47, XXY derives mostly from a paternal meiosis I error. Trisomy of individual chromosomes may remain low, linearly rise, or exponentially increase with advanced maternal age. Maternal age related trisomies involve achiasmatic and normochiasmate chromosomes, and chromosomes with disturbed recombination and distally located chiasmata. Hypotheses on the origin of the maternal age effect are critically reviewed. One model is presented that relates to altered cell cycle and protein phosphorylation in oocytes of aged mammals and accounts for most of the observed data in humans and in experimental studies. Aneuploidy may thus involve a predetermined component but is possibly also influenced by extrinsic factors reducing oocyte quality or depleting the oocyte pool precociously. Areas of future research are proposed to elucidate (i) the significance of early disturbances in the prenatal ovary, (ii) parameters diminishing the quality of oocytes in dictyate stage, and (iii) mechanisms enabling oocytes to process all chromosomal configurations successfully during later stages of oogenesis. Studies with newly developed and existing animal models appear indispensable to identify exposures affecting chromosome disjunction during meiosis, especially in the aging female.

The incidence, origin, and etiology of aneuploidy

Aneuploidy, the presence of an extra or missing chromosome, is the most frequent cause of mental retardation and pregnancy loss in our species. Studies can be divided into those of incidence, origin, and etiology. Trisomy 21 is the most common aneuploidy among liveborns whereas monosomy X and trisomy 16 are the most frequent causes of pregnancy loss. Aneuploidy primarily arises by the process of nondisjunction in the first meiotic division of maternal meiosis; however, this varies among chromosomes in that some show a significant proportion of paternal and/or meiosis II errors. The most common etiological factor associated with aneuploidy is advancing maternal age and it is generally agreed that this is a result of the increasing likelihood of nondisjunction in the aging ovary. There has been intense debate as to the existence of of a paternal age effect and recent studies on human sperm suggest that there may be a small effect for the sex chromosomes. Furthermore, recent molecular studies on trisomic conceptuses have revealed a second etiological factor associated with nondisjunction, namely, reduced genetic recombination.

Mechanisms and targets involved in maternal and paternal age effects on numerical aneuploidy

Trisomy in the human appears to be predominantly associated with maternal age. The maternal-age effect, however, shows considerable variability across affected chromosomes. Chromosome-specific variation has been reported in the shapes of the maternal-age-effect curves, including very small effects for the large chromosomes (groups A and B), linear increases (chromosome 16), and exponential increases (chromosome 21). There is also variation among chromosomes in whether the segregation errors occur predominantly at maternal meiosis I, meiosis II, and/or postfertilization mitotic divisions. There is also limited epidemiological evidence for a paternal-age effect, which was recently supported by the findings of age-related increases in sperm aneuploidy using fluorescence in situ hybridization methods. The paternal-age effect is considerably smaller than the maternal and is more likely to involve meiotic II errors of the sex chromosomes, whereas the maternal-age effect is more likely to arise from meiotic I errors producing autosomal trisomies. These and other differences suggest that constitutional aneuploidy arises by multiple mechanisms that may affect (1) the nature and timing of an initiating lesion affecting the oocyte or sperm; (2) the cellular physiology of the time of the nondisjunction event at meiosis I, II, or postfertilization; and (3) the selection against specific chromosomal aneuploidies during embryonic development. Multidisciplinary research is needed to understand the maternal and paternal-age effects on aneuploidy, to (1) identify and characterize the genes that control meiosis, recombination, and segregation; (2) identify the micro-environmental factors around the oocyte and mole germ cells that are involved in the age effects; (3) develop a laboratory animal model for the age effects; (4) characterize the role of genetics, physiology, and environmental toxicology for the paternal-age effects; and (5) identify cohorts of men and women of differing ages who have been exposed to high doses of candidate aneugens and conduct epidemiological investigations of aneuploidies transmitted to their offspring.

Etiology of nondisjunction in humans

Aneuploidy is the most common class of chromosome abnormality in humans, occurring in at least 0.3% of newborns and approximately 50% of spontaneous abortions. Considered as a class, it is the most common known cause of mental retardation and the leading cause of pregnancy loss. Despite the high frequency of aneuploidy, its obvious clinical importance, its severe impact on human reproduction, and the 35 years of research since the first human chromosome abnormality was described, we still know very little about its causes, let alone the contribution of environmental exposures. Recently, however, with the advent of molecular and molecular cytogenetic techniques and advances in reproductive biology, a body of evidence has been generated that is beginning to shed light on the incidence, origin, and etiology of human aneuploid conditions. The bulk of this evidence comes from two sources: 1) studies of the incidence of aneuploidy in the cells of origin, namely oocytes and sperm; and 2) examinations of meiotic stage, parent of origin, and meiotic recombination in trisomic conceptuses, both liveborn and abortuses. Using a multicolor fluorescence in situ hybridization (FISH) approach, it is now possible to screen on extremely large number of human sperm to determine chromosome-specific rates of disomy. Likewise, because of the introduction in the past decade of in vitro fertilization technology, a population of human oocytes suitable for aneuploidy screening became available. The examination of the cells of origin of aneuploidy, the sperm and oocytes, has provided data on the incidence of chromosome aberrations and valuable insight into possible mechanisms of nondisjunction. Additionally, the recent identification of multiple, highly informative DNA polymorphisms on all human chromosomes has made the determination of parental origin and the analysis of recombination a straightforward matter. We now know that the vast majority of trisomic conceptuses are maternal in origin, that increased maternal age is associated with nondisjunction, and that the amount and position of recombination on nondisjoined chromosomes is altered. In this review we will restrict discussions to these recent developments and to new models of the mechanism(s) of human nondisjunction based on the molecular cytogenetic analyses. Additionally, we will discuss the direction of future epidemiological research made possible through the development of molecular and molecular cytogenetic techniques. These technological advances have now allowed for a systematic search for genetic and environmental components to human nondisjunction.