Molecular analysis of nondisjunction in mice heterozygous for a Robertsonian translocation

A Robertsonian translocation results in a metacentric chromosome produced by the fusion of two acrocentric chromosomes. Rb heterozygous mice frequently generate aneuploid gametes and embryos, providing a good model for studying meiotic nondisjunction. We intercrossed mice heterozygous for a (7.18) Robertsonian translocation and performed molecular genotyping of 1812 embryos from 364 litters with known parental origin, strain, and age. Nondisjunction events were scored and factors influencing the frequency of nondisjunction involving chromosomes 7 and 18 were examined. We concluded the following: 1. The frequency of nondisjunction among 1784 embryos (3568 meioses) was 15.9%. 2. Nondisjunction events were distributed nonrandomly among progeny. This was inferred from the distribution of the frequency of trisomics and uniparental disomics (UPDs) among all litters. 3. There was no evidence to show an effect of maternal or paternal age on the frequency of nondisjunction. 4. Strain background did not play an appreciable role in nondisjunction frequency. 5. The frequency of nondisjunction for chromosome 18 was significantly higher than that for chromosome 7 in males. 6. The frequency of nondisjunction for chromosome 7 was significantly higher in females than in males. These results show that molecular genotyping provides a valuable tool for understanding factors influencing meiotic nondisjunction in mammals.     

Meiotic studies of translocations causing male sterility in the mouse. II. Double heterozygotes for Robertsonian translocations.

Unusual meiotic behavior of the XY chromosome pair was observed in sterile male mice doubly heterozygous for two Robertsonian translocations, Rb(16.17)7Bnr and Rb(8.17)1Iem. Nonrandom association between the X chromosome and the translocation configuration, ascertained from the frequencies of relevant C-band contacts, was found in 9 of 10 sterile males. Besides the nonrandom association, the XY chromosomes showed signs of impaired condensation, as judged by measurement of their lengths at diakinesis/MI of the first meiotic division. In contrast, neither nonrandom contact nor decondensation of the XY chromosomes pair was found in fertile males heterozygous for a single Robertsonian translocation, Rb1Iem or Rb7Bnr. The present observations lend indirect support to the working hypothesis advanced previously, the assumption that interference with X-chromosome inactivation is a possible cause of spermatogenic breakdown in carriers of various male-sterile chromosomal transloations. Alternative explanations of the available data, which cannot be ruled out, are briefly discussed.     

Sperm dysfunction in the Rb(6.16)- and Rb(6.15)-bearing mice revisited: involvement of Hyalp1 and Hyal5

Earlier we showed that Sperm adhesion molecule1 (Spam1), the best studied sperm hyaluronidase, is involved in the sperm dysfunction associated with Robertsonian translocations (Rb). The dysfunction results in reduced fertility in mice homozygous for the Rb(6.16) or the Rb(6.15) translocation and transmission ratio distortion (TRD) in heterozygous males. This conclusion was based on the finding that Spam1 in the Rbs harbors multiple point mutations and a genomic alteration at the locus [in the case of Rb(6.16)]; and is accompanied by reduced steady-state levels of the RNA and protein. Here we show that closely linked family members in the hyaluronidase gene cluster on mouse chromosome 6, Hyalp1 and Hyal5, also harbor point mutations in these Rbs, leading to nonconservative substitutions in both the encoded proteins. To test if Spam1 by itself is capable of producing TRD we analyzed the transmission of wild-type and null alleles of the gene in the progeny of carriers and show that there is no significant TRD. This lack of TRD in null carriers argues for only a contributory role of Spam1 in the TRD seen in the Rb-bearing mice, and supports the involvement of Hyalp1 and/or Hyal5 in the sperm dysfunction and the resulting TRD. It is proposed that the clustering of point mutations in all three genes results from the cumulative effect of spontaneous mutations that do not disperse in the population due to suppression of recombination that occurs at Rb junctions.     

Evidence for differential maturation of reciprocal sperm segregants in the murine Rb(6.16) translocation heterozygote.

The fertilizing ability of unaged sperm and those aged experimentally in the cauda by surgically ligating the corpus epididymis in males carrying the Rb(6.16) translocation was studied. Chromosomally normal females were inseminated with unaged sperm delivered by males mating at 3-day intervals, and aged sperm were studied after matings on 6-14 postoperative days. The sperm chromosome complement was analyzed in first-cleavage metaphase zygotes after sequential G- and C-banding of the chromosomes. Of 283 metaphasic zygotes in the control group, 183 (or 64.7%) were analyzed and showed a ratio of 2.7:1 for chromosomally normal and balanced segregants of the translocation, deviating significantly (P less than 0.001) from the expected 1:1. The ratio of X- to Y-bearing sperm also deviated from expected (P less than 0.01) mostly due to a significant deficiency (P less than 0.05) of balanced sperm that were X-bearing. Fertilized oocytes were recovered from matings of 10 males on days 6-8 postoperatively, and, of 139 metaphasic one-cell zygotes, 101 (or 72.3%) were analyzed. These showed a Mendelian ratio of 1:1 for normal and balanced segregants. The sex ratio in the aged group (57Y:41X) also showed no deviation from 1:1. The results, which reveal significant physiological distortions for both the segregation and the sex ratios in males heterozygous for the Rb(6.16) translocation, suggest that differential maturation of the translocation-bearing sperm and the chromosomally normal reciprocal exists. The findings further support the concept that sperm chromosomal complement affects their maturation and function, and that factors on chromosome 6 and the X or Y chromosome additively affect sperm function.     

Aneuploid epididymal sperm detected in chromosomally normal and Robertsonian translocation-bearing mice using a new three-chromosome FISH method

We present a new method to detect epididymal sperm aneuploidy (ESA) in mice using simultaneous fluorescence in situ hybridization (FISH) with DNA probes specific for mouse chromosomes X, Y and 8. The method was applied to Robertsonian (Rb) translocation (8.14) heterozygotes and homozygotes as well as the chromosomally normal B6C3F1. The sex ratios of sperm did not differ from the expected 1∶1 and the hybridization efficiencies were ≈99.7% for over 60 000 sperm analyzed. Mice heterozygous for Rb (8.14) produced about tenfold higher rates of sperm with chromosome 8 hyperhaploidy than did Rb (8.14) homozygotes or chromosomally normal mice, while frequencies of sperm with hyperhaploidies for chromosomes X and Y were unaffected in all three lines of mice. Hyperhaploid frequencies obtained with the ESA method were consistent with those of the previous testicular FISH method and were validated by published data obtained by conventional cytogenetic analyses (meiotic metaphase II and first cleavage). Thus, the mouse three-chromosome ESA assay together with the previously developed aneuploidy assay for human sperm constitute a promising pair of interspecific biomarkers for comparative studies of the genetic and physiologic mechanisms of the induction and persistence of aneuploidy in male germ cells. Edited by: T. Hassold     

Nondisjunction, disturbances in spindle structure, and characteristics of chromosome alignment in maturing oocytes of mice heterozygous for Robertsonian translocations

To correlate the chromosomal constitution of meiotic cells with possible disturbances in spindle function and the etiology of nondisjunction, we examined the spindle apparatus and chromosome behavior in maturing oocytes and analyzed the chromosomal constitution of metaphase II-arrested oocytes of CD/Cremona mice, which are heterozygous for a large number of Robertsonian translocation chromosomes (18 heterobrachial metacentrics in addition to two acrocentric chromosomes 19 and two X chromosomes). Spreading of oocytes during prometaphase 1 revealed that nearly all oocytes of the heterozygotes contained one large ring multivalent, apart from the bivalents of the two acrocentric chromosomes 19 and the X chromosomes, indicating that proper pairing and crossing-over between the homologous chromosome arms of all heterobrachial chromosomes took place during prophase. A large proportion of in vitro-matured oocytes arrested in metaphase II exhibited numerical chromosome aberrations (26.5% hyperploids, 40.8% hypoploids, and 6.1% diploids). In addition, some of the oocytes with euploid chromosome numbers (26.5% of the total examined) appeared to be nullisomic for one chromosome and disomic for another chromosome, so that aneuploidy levels may even be higher than expected on the basis of chromosome counts alone. Although oocytes of the complex heterozygous mice seemed able initially to form a bipolar spindle during first prometaphase, metaphase I spindles were frequently asymmetrical. Chromosomes in the multivalent did not align properly at the equator, centromeres of neighboring chromosomes in the multivalent remained maloriented, and pronounced lagging of chromosomes was observed at telophase I in oocytes obtained from the Robertsonian translocation heterozygotes. Therefore, disturbance in spindle structure and chromosome behavior appear to correlate with the chromosomal constitution in these oocytes and, ultimately, with failures in proper chromosome separation. In particular, reorientation appears to be a rare event, and malorientation of chromosomes may remain uncorrected throughout prometaphase, as we could not find many typical metaphase I stages in heterozygotes. This, in turn, could be the basis for malsegregation at anaphase and may ultimately induce a high rate of nondisjunction and aneuploidy in the oocytes of CD/Cremona mice, leading to total sterility in heterozygous females.     

Synapsis and chiasma distribution in mice heterozygous for translocations in chromosomes 16 and 17

Electron microscopic analysis of synaptonemal complexes and analysis of chiasmata distribution in male mice heterozygous for Robertsonian translocation T(16; 17)7Bnr - (Rb7), for synaptonemal reciprocal translocation T(16;17)43H - (T43), in double heterozygotes for these translocations and in males with partial trisomy of the proximal region of chromosome 17 was carried out. Synaptic disturbances around the breakpoints of the translocations, such as asynapsis of homologous regions of partners and non-homologous synapsis of centromeric regions of acrocentric chromosomes, were revealed. Synaptic regularity in the proximal part of the chromosome 17 appeared to be affected by no t12 haplotype. Good coincidence between sizes of mitotic chromosomes and corresponding lateral elements of synaptonemal complexes was found for all chromosomes, with the exception of Rb7 in trisomics. In the latter karyotype, the proximal part of chromosome 17 involved in Robertsonian fusion seems to be shortened in the course of zygotene and never synapted with homologous segment of neither the acrocentric chromosome 17 nor large product of reciprocal translocation. Drastic increase in chiasmata frequency in the proximal part of chromosome 17 was revealed in heterozygotes for T43H and in trisomics, as compared with the double heterozygotes Rb7/T43. The latter finding was explained by the existence of two independent pairing segments in the former karyotypes.     

Chromosome segregation in mice heterozygous for Robertsonian translocations. II. Changes in the structure of homologs as a cause of abnormal disjunction in females

It was demonstrated that mutations T, Fu, Ki, t6 of chromosome 17 cause preferential transmission of the acrocentric homologues to the progeny from female Rb heterozygotes. The results indicate that the effects of these mutations on segregation are restricted to the Robertsonian translocations involving chromosome 17. Substitution of the parts of chromosome 17 distal or proximal to the T-locus did not alter the effect, of this chromosome on the transmission rate of the homologue. The transmissions effects of these mutations, whether cis or trans with Rb, were the same. It was observed that mothers Rb7/T43H transmitted the chromosome with the reciprocal translocation T43H to 70.9% of their progeny. Data were obtained supporting the idea that structural changes of the chromosomes caused by mutations affect segregation of the homologues in Rb heterozygous females. The possible mechanism of this influence is discussed.     

Immunocytogenetics. III. Analysis of trivalent and multivalent configurations in mouse pachytene spermatocytes by human autoantibodies to synaptonemal complexes and kinetochores

Mice heterozygous for one or more Robertsonian (Rb) translocation chromosomes have been used to analyze synaptonemal complex (SC) configurations and kinetochore arrangements in trivalents and multivalents. Rb heterozygosity without arm homologies leads to the formation of heteromorphic trivalents in meiosis I; alternating homology of the chromosome arms produces ringlike or chainlike multivalents. Immunofluorescence double-labeling with human antibodies to SCs and kinetochores was performed on surface-spread pachytene spermatocytes. Both Rb bivalents and Rb trivalents clearly showed that metacentrics possess only one centromere. In heteromorphic trivalent SCs, the nonhomologous kinetochores of the two acrocentrics were closely paired in a cis-configuration and juxtaposed opposite the kinetochore of the metacentric; the latter appeared to be an integral part of the longitudinal SC axis. Meiotic multivalents of interpopulation hybrids included up to 36 chromosome arms. In multivalent SCs, the kinetochores always lay together, with the SC arms arranged away from the central centromere cluster. The paracentromeric regions of the Rb chromosomes appeared to remain unsynapsed on both sides of the centromeres. The SC arms were often linked by end-to-end associations. Following desynapsis of the multivalent SC, the kinetochores of the Rb metacentrics showed a highly nonrandom topologic distribution within the nucleus, reminiscent of their arrangement during synapsis.     

Meiotic drive of t haplotypes: chromosome segregation in mice with tertiary trisomy

The properties of the t haplotypes, specific mutant states of the proximal region of chromosomes 17 in the house mouse, are of continuing interest. One such property is increased transmission of the t haplotype by heterozygous t/+ males to offspring. Using the reciprocal translocation T(16;17)43H we have constructed males with tertiary trisomy of chromosome 17 (+T43/+ +/Rb7+) carrying the Robertsonian translocation Rb(16.17)7Bnr. Only the progeny of these males which had inherited either T43/+ or Rb7 from their male parent were viable. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that, when the t12 haplotype is in the normal acrocentric (males+ +T43/+ t12 + /Rb7+ +), its presence in the gamete +t12+/+ + T43 does not produce meiotic drive. However, when t6 is in Rb7, meiotic drive was observed: 80% of offspring carried the t haplotype. It is concluded that the meiotic drive is probably inhibited by the presence of a normal homologue of chromosome 17 in the same sperm. Possible mechanisms for the t haplotype effect are discussed.     

Effects of zero to four copies of chromosome 15 on mouse embryonic development

Intercrosses of mice doubly heterozygous for Rb(6.15)1A1d and Rb(4.15)4Rma (thus are characterized by monobrachial homology for chromosome 15) produced embryos with zero to four copies of chromosome 15 in their expected frequencies at the first cleavage division. By 3 1/2 days' gestation, nullisomy 15 embryos were missing. At 8 1/2 to 9 1/2 days, no monosomy 15 embryos were found, although trisomy 15 and tetrasomy 15 embryos were still present in their expected numbers. Tetrasomics were more severely affected than trisomics at this gestational age; the former were severely retarded "streak" embryos, while the latter had open neural tubes and were 2/3 the size of euploid embryos. The functional activity of chromosomes during the embryonic development of autosomal aneuploids is discussed in light of these findings.     

Defective imprint resetting in carriers of Robertsonian translocation Rb (8.12)

Meiotic silencing of unsynapsed chromatin (MSUC) occurs in the germ cells of translocation carriers and may cause meiotic arrest and infertility. We hypothesized that if bypassing meiotic checkpoints MSUC may cause epigenetic defects in sperm. We investigated the meiotic behavior of the Robertsonian translocation Rb (8.12) in mice. The unsynapsed 8 and 12 trivalent was associated with the XY body during early and mid-pachynema in heterozygous Rb (8.12) carriers, suggesting possible silencing of pericentromeric genes, such as the Dnmt3a gene. In wild-type mice, DNMT3A protein showed a dramatic accumulation in the nucleus during the mid-pachytene stage and distinct association with the XY body. In translocation carriers, DNMT3A was less abundant in a proportion of pachytene spermatocytes that also had unsynapsed pericentromeric regions of chromosomes 8 and 12. The same mice had incomplete methylation of the imprinted H19 differentially methylated region (DMR) in sperm. We propose that impaired H19 imprint establishment results from lack of synapsis in chromosomes 8 and 12 probably through transient silencing of a chromosome 8 or 12 gene during pachynema. Furthermore, our findings support the notion that imprint establishment at the H19 locus extends into pachynema.     

Deletion analysis of male sterility effects of t-haplotypes in the mouse

We present data on the effects of three chromosome 17 deletions on transmission ratio distortion (TRD) and sterility of several t-haplotypes. All three deletions have similar effects on male TRD: that is, Tdel/tcomplete genotypes all transmit their t-haplotype in very high proportion. However, each deletion has different effects on sterility of heterozygous males, with TOr/t being fertile, Thp/t less fertile, and TOrl/t still less fertile. These data suggest that wild-type genes on chromosomes homologous to t-haplotypes can be important regulators of both TRD and fertility in males, and that the wild-type genes concerned with TRD and fertility are at least to some extent different. The data also provide a rough map of the positions of these genes.     

Chromosomal polymorphism in the house mouse (Mus domesticus) of Greece and Yugoslavia

A total of 88 wild mice from the Dalmatian coast of Yugoslavia (35 animals), and Peloponnesus (30 animals) and Thebes (23 animals) on mainland Greece were karyotyped. In all but five animals Robertsonian translocations were found. Mice from the Dalmatian region were homozygous for translocations Rb(5.15), Rb(6.12), Rb(8.17), Rb(9.13), and Rb(10.14); they were homo-or heterozygous for the translocation Rb(1.11). Some of them lacked the Rb(1.11) translocation altogether so that the diploid numbers in the Yugoslavian mice were 2n=28, 29, 30, or 40. The mice from the vicinity of Olympia in northwestern Peloponnesus were homozygous for eight Robertsonian translocations: Rb(1.3), Rb(2.5), Rb(4.6), Rb(8.12), Rb(9.16), Rb(10.14), Rb(11.17), and Rb(13.15). Their diploid chromosome number was therefore 2n=24. Mice from the vicinity of Patras in northwest Peloponnesus carried all except the first three of these eight translocations; their chromosome number was 2n=30. Finally, the mice from Thebes were homozygous for translocations Rb(2.15), Rb(4.14), Rb(5.12), and Rb(10.13). They were homo- or heterozygous for Rb(6.9), Rb(8.17), and Rb(1.11); some mice lacked the Tb(1.11) translocation altogether. The translocations Rb(6.9)40Tu and Rb(10.13)42Tu represent new arm combinations not found previously in any wild mouse population. the remaining translocations have previously been found in different Mediterranean countries, in Scotland and in southern Germany. The findings suggest that each translocation arose only once and that different translocations have come together in different populations to generate a unique karyotype characterizing this population.     

Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

The frequency of aneuploidy in the secondary spermatocytes of normal and Robertsonian translocation-carrying rams.

A detailed analysis was made of the chromosomes in 1008 M II figures from three different types of heterozygous Robertsonian translocation-carrying rams (53,xy,t1; 53,xy,t3) and 225 M II figures from homozygous Robertsonian translocation-carrying rams (52,xy,t1t1; 52,xy,t3t3) and rams of normal karotype (54,xy). No hypermodal cells were recorded in either the normal or the homozygous rams, but from 4-5% to 9-2% of M II cells from the heterozygous rams were hypermodal. The heterozygous rams also produced a significantly higher level of hypomodal cells suggesting that, in addition to non-disjunction, lagging at anaphase I may have occurred. There were also distinct differences in M II aneuploid spermatocyte frequency between heterozygous versus normal and homozygous rams. Fewer balanced translocation X-carrying M II cells were recorded than expected in three of the four 53,xy,t2 rams. This coincides with mating data which suggest that 26,x,t2 gametes may occur less frequently than expected. Since ewes of normal karotype mated to 53,xy,t rams conceive to first service at a rate equal to or better than normal mating groups, and because no blastocysts with unbalanced karotypes associated with the t1 translocation have been recorded, it is suggested that only euploid spermatozoa are involved in fertilization. In the sheep, aneuploid spermatocytes probably degenerate before sperm maturation.     

Unequal nondisjunction frequencies of trivalent chromosomes in male mice heterozygous for two Robertsonian translocations

Robertsonian (Rb) translocation heterozygosity may cause pairing problems during prophase and segregation irregularities at anaphase of meiosis I. These stages of meiosis I were studied in male mice doubly heterozygous for the two Rb chromosomes Rb(9.19)163H and Rb(16.17)8Lub. At pachytene both Rb chromosomes similarly showed pairing irregularities like unpaired segments. However, highly different nondisjunction frequencies of chromosomes forming the respective trivalents were found. The nondisjunction frequency of the Rb8Lub trivalent chromosomes was about 40%, whereas a very low frequency of nondisjunction was found in combination with the Rb163H trivalent. Since both trivalents were together in the same cell, differences in kinetochore function are assumed to be responsible for the diverse frequency of nondisjunction.     

Contribution of chromosomal imbalance to sperm selection and pre-implantation loss in translocation-heterozygous Chinese hamsters

Chinese hamster stocks with various structurally abnormal chromosomes have been produced by X irradiation. Among these stocks, 18 with various reciprocal translocations were used to investigate the participation of unbalanced gametes in fertilization and the development of unbalanced embryos. Among males as well as females heterozygous for the same translocation, there is no difference in the frequency of each disjunctional class. The participation of chromosomally unbalanced gametes in fertilization was investigated by chromosomal analysis of meiotic cells in heterozygotes for the 18 reciprocal translocations and pronuclei of fertilized ova obtained from crossing these heterozygotes. Compared with the expected frequencies from MII scoring, the frequencies of male pronuclei having a common deficiency of chromosome 1 (1q17-->1q42) or chromosome 3 (3p23-->3q31) decreased significantly in one-cell embryos. However, the frequencies of male pronuclei with other abnormalities were all consistent with those expected from MII scoring. In contrast, the frequencies of female pronuclei with any karyotype including the same abnormalities as those decreased in male pronuclei from the translocation heterozygotes were all consistent with those estimated from MII scoring. These results revealed clearly that most gametes with nullisomies as well as disomies for any chromosomal segments may participate in fertilization, whereas only male gametes nullisomic for certain segments of chromosomes 1 and 3 failed to participate in fertilization. The zygotic selection of chromosomal imbalance was also investigated by direct chromosomal and morphological analyses of preimplantation embryos from crosses between karyotypically normal females and male heterozygotes from the 18 stocks with various reciprocal translocations. These analyses revealed that some embryos were arrested in development at the two-cell stage. The karyotype of these two-cell embryos had a common deficiency in a segment of chromosome 1 or chromosome 2. Embryos with partial monosomy including chromosomes 1, 3, 4 and 5 showed arrested development at four- to eight-cell stages. Among day 4 embryos, some chromosomally unbalanced embryos, mainly with a deficiency of segments of chromosomes 1p, 1q, 2q, 5q, 7q and 8, had fewer blastomeres than karyotypically normal and balanced embryos. The homology between Chinese hamster and mouse chromosomes relating to abnormal embryogenesis at early stages has been partially confirmed from reported maps of chromosomes. The Chinese hamster is useful for further cytogenetic studies during the stages of meiosis and early embryogenesis.     

The influence of the Robertsonian translocation Rb(X.2)2Ad on anaphase I non-disjunction in male laboratory mice

A Robertsonian translocation in the mouse between the X chromosome and chromosome 2 is described. The male and female carriers of the Rb(X.2)2Ad were fertile. A homozygous/hemizygous line was maintained. The influence of the X-autosomal Robertsonian translocation on anaphase I non-disjunction in male mice was studied by chromosome counts in cells at metaphase II of meiosis and by assessment of aneuploid progeny. The results conclusively show that the inclusion of Rb2Ad in the male genome induces non-disjunction at the first meoitic division. In second metaphase cells the frequency of sex-chromosomal aneuploidy was 10.8%, and secondary spermatocytes containing two or no sex chromosome were equally frequent. The Rb2Ad males sired 3.9% sex-chromosome aneuploid progeny. The difference in aneuploidy frequencies in the germ cells and among the progeny suggests that the viability of XO and XXY individuals is reduced. The pairing configurations of chromosomes 2, Rb2Ad and Y were studied during meiotic prophase by light and electron microscopy. Trivalent pairing was seen in all well spread nuclei. Complete pairing of the acrocentric autosome 2 with the corresponding segment of the Rb2Ad chromosome was only seen in 3.2% of the cells analysed in the electron microscope. The pairing between the X and Y chromosome in the Rb2Ad males corresponded to that in males with normal karyotype. Reasons for sex-chromosomal non-disjunction despite the normal pairing pattern between the sex chromosomes may be seen in the terminal chiasma location coupled with the asynchronous separation of the sex chromosomes and the autosomes.(ABSTRACT TRUNCATED AT 250 WORDS)