Sperm chromosome complements from two human reciprocal translocation heterozygotes

Summary Using the hamster oocyte/human sperm fusion technique, we studied sperm chromosome complements in two male reciprocal translocation heterozygotes, 46,XY,t(11;17)(p11.2;q12.3) and 46,XY,t(1;11) (p36.3;q13.1). For the t(11;17) carrier, 202 sperm chromosome complements were obtained, but 18 karyotypes were not included in the segregation data because of multiple breaks and rearrangements. The alternate and adjacent I types, adjacent II, and 31 segregations accounted for 38.6%, 32.1%, 26.6%, and 2.7% of the sperm analyzed from the t(11;17) carrier. A total of 575 sperm chromosome complements was obtained using sperm from the t(1;11) heterozygote, and 27 karyotypes were excluded from the segregation data because of multiple breaks and rearrangements. For the t(1;11) carrier, the alternate and adjacent I types, adjacent II, and 31 segregations were responsible for 31.4%, 42.9%, 15.9%, and 8.0% of the analyzed sperm chromosome complements. Chromosomal abnormalities unrelated to the translocation, particularly the conservative estimate of aneuploidy frequency, were within the range observed in normal men. Hence, there was no evidence for an interchromosomal effect causing meiotic nondisjunction, despite the large sample sizes studied.     

Assessment of autosome and gonosome disomy in human sperm nuclei by chromosome painting

Disomy and diploidy frequencies for autosomes 1–22 and the gonosomes were assessed in 299,442 sperm nuclei from four normal fertile men by chromosome painting. This novel approach allowed us to perform a specific and sensitive detection of each chromosome. A minimum of 5000 sperm nuclei per subject were evaluated for each chromosome by dual colour fluorescence in situ hybridization. The disomy rate proved to be similar for all the autosomes (0.24%) and the diploidy rate varied from 0.12% to 0.15%. No interchromosomal or interindividual differences in the frequency of disomic and diploid sperm nuclei were observed between the four subjects. The mean frequency of XX-, YY- and XY-bearing spermatozoa was estimated to 0.17%, 0.17% and 0.32%, respectively. This strategy constitutes a new approach for detecting aneuploidy in human sperm nuclei and suggests an equal repartition of non-disjunction among chromosomes in male gametes. Received: 7 October 1997 / Accepted: 13 January 1998     

Aneuploidy in human sperm: the use of multicolor FISH to test various theories of nondisjunction.

While it is known that all chromosomes are susceptible to meiotic nondisjunction, it is not clear whether all chromosomes display the same frequency of nondisjunction. By use of multicolor FISH and chromosome-specific probes, the frequency of disomy in human sperm was determined for chromosomes 1, 2, 4, 9, 12, 15, 16, 18, 20, and 21, and the sex chromosomes. A minimum of 10,000 sperm nuclei were scored from each of five healthy, chromosomally normal donors for every chromosome studied, giving a total of 418,931 sperm nuclei. The mean frequencies of disomy obtained were 0.09% for chromosome 1; 0.08% for chromosome 2; 0.11% for chromosome 4; 0.14% for chromosome 9; 0.16% for chromosome 12; 0.11% for chromosomes 15, 16, and 18; 0.12% for chromosome 20; 0.29% for chromosome 21; and 0.43% for the sex chromosomes. Data for chromosomes 1, 12, 15, and 18, and the sex chromosomes have been published elsewhere. When the mean frequencies of disomy were compared, the sex chromosomes and chromosome 21 had significantly higher frequencies of disomy than that of any other autosome studied. These results corroborate the pooled data obtained from human sperm karyotypes and suggest that the sex chromosome bivalent and the chromosome 21 bivalent are more susceptible to nondisjunction during spermatogenesis. From these findings, theories proposed to explain the variable incidence of nondisjunction can be supported or discarded as improbable.     

Analysis of sex chromosome aneuploidy in sperm from fathers of Turner syndrome patients

Numerical sex chromosome abnormalities were analyzed in sperm from four fathers of Turner syndrome patients of paternal origin to determine whether there was an increased frequency of sex chromosome aneuploidy and to elucidate whether meiotic malsegregation mechanisms could be involved in the origin of Turner syndrome. Determination of the parental origin of the single X chromosome (maternal in all four cases) and exclusion of X and Y mosaicism were carried out by polymerase chain reaction amplification of five X chromosome polymorphisms and three Y chromosome segments. A total of 45,299 sperm nuclei from Turner fathers and 85,423 sperm nuclei from eight control donors was analyzed by three-color fluorescence in situ hybridization. The four patients showed a significant increase in the percentages of XY sperm (mean 0.22%; range 0.20% to 0.22%) compared with control donors (mean 0.11%; range 0.06% to 0.18%). These results suggest that the four individuals have an increased frequency of nondisjunctional errors in meiosis I, resulting in the production of an increased proportion of XY spermatozoa and of sperm lacking a sex chromosome. Received: 24 November 1998 / Accepted: 2 February 1999     

The arrangement of chromosomes in nuclei of sperm from plethodontid salamanders

Plethodontid salamanders have n = 13 or 14 large metacentric or sub-metacentric chromosomes. Sperm nuclei from Plethodon cinereus measure 72×1 m. The nucleoprotein of spermatids is at first finely granular. In elongate spermatids it clumps into larger granules, which then fuse to form the compact nucleoprotein of the mature sperm. The nuclei of mature sperm are negatively birefringent with respect to their length. — ³H RNA complementary to high-density satellite DNA of centromeric heterochromatin in P. cinereus has been hybridized in-situ to spermatids and sperm, and its site of binding to these cells has been examined by autoradiography. Labelling of round spermatid nuclei is localized in a single patch. Elongate spermatid nuclei are labelled only over the rear quarter of the nucleus. Label over the nuclei of mature sperm is localized in a region extending 10–20 m forwards from the rear of the nucleus. — In P. cinereus the ribosomal genes are located near the centromere on the short arm of chromosome 7. ³H ribosomal RNA hybridizes to a single patch in round spermatid nuclei. Elongate spermatid nuclei show label over a short segment of the rear half of the nucleus. In spermatids nearing maturity the labelled region is never more than 20 m long. — These results indicate that in P. cinereus each chromosome is arranged in a U formation with its centromere at the base of the sperm nucleus, and its arms extended forwards along the length of the nucleus. — Among plethodontids, increase in C value and corresponding increase in chromosome size is accompanied by increase in the length rather than the width of the sperm nucleus. — ³H ribosomal RNA hybridizes to a short segment in spermatid and sperm nuclei from Xenopus and Triturus. In these animals, the position of the labelled segment varies from sperm to sperm.     

Analysis of sperm chromosome complements from a man heterozygous for a pericentric inversion of chromosome 1

Human sperm chromosomes were studied in a man heterozygous for a pericentric inversion of chromosome (1)(p31q12). Q-banded pronuclear chromosomes were analyzed after in vitro penetration of golden hamster oocytes. A total of 159 sperm were examined: 54% bearing the inverted chromosome 1 and 46% the normal chromosome 1. These frequencies are not significantly different from the theoretical 11 ratio. There were no recombinant sperm with duplications or deficiencies, suggesting that a pairing loop failed to form or that crossing-over was suppressed. The frequency of abnormalities unrelated to the inversion was 5% for numerical, 8.8% for structural, 2.5% for numerical and structural, values not significantly different from control donors studied in our lab. The frequencies of X- and Y-bearing sperm were 46% and 54%, respectively, not significantly different from the expected value of 50%. This is the fifth pericentric inversion studied by human sperm chromosome analysis; recombinant chromosomes have been observed in two of the five cases. Some of the factors associated with an increased risk of recombinant sperm appear to be inversion size greater than 30% of the chromosome and chromosome breakpoints in G-light bands.     

Sperm chromosome analysis of a man heterozygous for a pericentric inversion of chromosome 20

Summary The sperm chromosomes of a man heterozygous for inv(20)(p13q11.2) were analyzed. Twenty-six sperm chromosome complements were examined, of which fourteen contained the normal chromosome, and twelve the inverted chromosome. None of the sperm complements contained a recombinant chromosome 20. The frequency of structural chromosomal aberrations unrelated to the inversion was 11.5% (3/26). Numerical aberrations were not observed. The percentages of X- and Y-bearing sperm were 56% and 44%, respectively, which was similar to the expected 11 ratio.     

Incidence of chromosome 3, 7, 10, 11, 17 and X disomy in mature human sperm nuclei as determined by nonradioactive in situ hybridization

In situ hybridizations were performed on mature human sperm cells with biotin-labeled -satellite DNA probes specific for chromosomes 3, 7, 10, 11, 17, and X in order to reveal the disomy rate for each of these chromosomes. A total of 76 253 sperm nuclei from seven healthy probands aged 23–57 years were analyzed. An average of 12 000 sperm nuclei (at least 1500 per donor) showing hybridization were scored with each probe. The disomy rate as indicated by two distinct hybridization signals turned out to be similar for all chromosomes, ranging from 0.31% to 0.34%. There were no significant interindividual differences and no age correlation in the frequency of disomic sperm cells between the donors.     

Chromosome segregation in a man heterozygous for a pericentric inversion, inv(9)(p11q13), analyzed by using sperm karyotyping and two-color fluorescence in situ hybridization on sperm nuclei

Analysis of sperm karyotypes and two-color fluorescent in situ hybridization (FISH) on sperm nuclei were carried out in a man heterozygous for the pericentric inversion inv(9)(p11q13). Sperm chromosome complements were obtained after in vitro fusion of zona-free hamster oocytes and donor sperm. A total of 314 sperm complements was analyzed: 153 (48.7%) carried the inverted chromosome 9 and 161 (51.3%) carried the normal one. None of the sperm complements contained a recombinant chromosome 9, suggesting that no chiasmata were formed in the heterochromatic region. The frequency of structural chromosome aberrations unrelated to the inversion (8.3%) and the frequency of conservative aneuploidy (3.2%) were within the limits observed in our control donors. The proportions of X-bearing (47.3%) and Y-bearing sperm (52.7%) were not significantly different from the expected 1:1 ratio. The percentage of disomy for chromosome 21 was analyzed by two-color FISH in 10 336 sperm nuclei. The disomy rate for chromosome 21 (0.30%) was not significantly different from that found in our controls. These results suggest that the risk for this man of producing chromosomally abnormal offspring or spontaneous abortions was not increased, and do not support the existence of an interchromosomal effect for chromosome 21. Received: 28 October 1996     

Frequency and distribution of chromosome abnormalities in human spermatozoa

This study reviews the frequency and distribution of numerical and structural chromosomal abnormalities in spermatozoa from normal men obtained by the human-hamster system and by multicolor-FISH analysis on decondensed sperm nuclei. Results from large sperm karyotyping series analyzed by chromosome banding techniques and results from multicolor FISH in sperm nuclei (of at least 10(4) spermatozoa per donor and per probe) were reviewed in order to establish baseline values of the sperm chromosome abnormalities in normal men. In karyotyping studies, the mean disomy frequency in human sperm is 0.03% for each of the autosomes, and 0.11% for the sex chromosomes, lower than those reported in sperm nuclei by FISH studies using a similar methodology (0.09% and 0.26%, respectively). Both types of studies coincide in that chromosome 21 and sex chromosomes have a greater tendency to suffer segregation errors than the rest of the autosomes. The mean incidence of diploidy, only available from multicolor FISH in sperm nuclei, is 0.19%. Inter-donor differences observed for disomy and diploidy frequencies among FISH studies of decondensed sperm nuclei using a similar methodology could reflect real differences among normal men, but they could also reflect the subjective application of the scoring criteria among laboratories. The mean frequency of structural aberrations in sperm karyotypes is 6.6%, including all chromosome types of abnormalities. Chromosome 9 shows a high susceptibility to be broken and 50% of the breakpoints are located in 9q, between the centromere and the 9qh+ region. Structural chromosome aberrations for chromosomes 1 and 9 have also been analyzed in human sperm nuclei by multicolor FISH. Unfortunately, this assay does not allow to determine the specific type of structural aberrations observed in sperm nuclei. An association between advancing donor age and increased frequency of numerical and structural chromosome abnormalities has been reported in spermatozoa of normal men.     

Segregation of sex chromosomes into sperm nuclei in a man with 47,XXY Klinefelter’s karyotype: a FISH analysis

Meiotic segregation of the sex chromosomes was analysed in sperm nuclei from a man with Klinefelter’s karyotype by three-colour FISH. The X- and Y-specific DNA probes were co-hybridized with a probe specific for chromosome 1, thus allowing diploid and hyperhaploid spermatozoa to be distinguished. A total of 2206 sperm nuclei was examined; 958 cells contained an X chromosome, 1077 a Y chromosome. The ratio of X : Y bearing sperm differed significantly from the expected 1 : 1 ratio (χ² = 6.96; 0.001 < P < 0.01). Sex-chromosomal hyperhaploidy was detected in 2.67% of the cells (1.22% XX, 1.36% XY, 0.09% YY) and a diploid constitution in 0.23%. Although the frequency of 24,YY sperm was similar to that detected in fertile males, the frequencies of 24,XX, 24,XY and diploid cells were significantly increased. A sex-chromosomal signal was missing in 4.26% of the spermatozoa. This percentage appeared to be too high to be attributed merely to nullisomy for the sex chromosomes and was considered, at least partially, to be the result of superposition of sex-chromosomal hybridization signals by autosomal signals in a number of sperm nuclei. The results contribute additional evidence that 47,XXY cells are able to complete meiosis and produce mature sperm nuclei. Received: 6 November 1996     

Risk assessment and segregation analysis in a pericentric inversion inv6p23q25 carrier using FISH on decondensed sperm nuclei

Fluorescent in situ hybridization (FISH) in decondensed sperm nuclei has been used to determine the percentage of normal/balanced or unbalanced spermatozoa produced by an inv(6)(p23q25) carrier, and the possible interchromosomal effect (ICE) of the reorganized chromosomes on other chromosome pairs. A dual color FISH with specific subtelomeric probes for the 6p and 6q regions was performed to determine the segregation pattern of the inverted chromosome. ICE on chromosomes 18, X and Y was assessed using a triple color FISH assay. In the segregation analysis 10,049 spermatozoa were analyzed, and only 45.7% of them were normal/balanced. The high number of unbalanced gametes in our carrier could be the consequence of the large size of the inverted segment. This situation could facilitate the formation of an inversion loop, where formation of an odd number of chiasmata (usually one) result in the production of 50% normal and 50% unbalanced sperm. Furthermore, an increase in the disomy rate for chromosome 6 was also observed. In the screening for ICE, 10,007 spermatozoa were analyzed. The disomy rate for the sex chromosomes and chromosome 18 were not significantly different from those found in our controls, suggesting no evidence of interchromosomal effects in this patient. The use of FISH in decondensed sperm nuclei has proved once more to be an accurate approach to determine the chromosome anomalies in sperm and could help to better establish a reproductive prognosis.     

Relationship between clinical phenotype, semen parameters and aneuploidy frequency in sperm nuclei of 50 infertile males

The purpose of this study was to analyse the frequency of disomy for chromosomes 1, 13, 14, 18, 21, 22, X and Y in sperm nuclei of 50 infertile men and 10 healthy probands of proven fertility. Semen parameters (sperm count, global motility and morphology), urological clinical examination, genital ultrasound and lymphocyte karyotyping were performed for each patient. Disomy frequency was established by fluorescence in situ hybridization by using whole chromosome paint probes. The mean rate of disomy for the various autosomes studied was higher in infertile males than in subjects of proven fertility. Interchromosomal and interindividual differences in the disomy frequency were observed between the 50 patients. The mean frequency of homodisomy YY and heterodisomy XY was increased in spermatozoa of patients with low semen quality parameters (0.24% and 0.54%, respectively). The disomy frequency in infertile males was directly correlated with the severity of oligospermia. However, no relationship was established between aneuploidy rate, sperm motility, morphology or clinical phenotype. These results support the hypothesis that, during spermatogenesis of males with sperm parameter alterations, a decreased frequency of meiotic chromosome pairing and crossing over may lead to spermatogenesis arrest at the meiosis stage and/or to an increase of meiotic nondisjunctions. Meiotic arrest in some germ cells may be responsible for oligospermia and nondisjunctions in other cells for aneuploidy in mature male gametes.     

Analysis of aneuploidy for chromosomes 13, 21, X and Y by multicolour fluorescence in situ hybridisation (FISH) in a 47,XYY male.

The frequency of aneuploid sperm was assessed by fluorescence in situ hybridisation (FISH) in a 47,XYY male previously studied by sperm karyotyping. A total of 20,021 sperm were studied: 10,017 by two-colour FISH for chromosomes 13 and 21 and 10,002 by three-colour FISH for the sex chromosomes using chromosome 1 as an autosomal control for diploidy and lack of hybridisation. Results were compared with more than 500,000 sperm from 18 normal men. The frequencies of X-bearing (49.4%) and Y-bearing sperm (49.8%) were not significantly different from 50% as shown in our sperm karyotyping study. There was no significant increase in the frequency of diploid sperm compared with control donors. There was a significant increase in the frequency of disomy for chromosome 13 (p < 0.0001) and XY disomy (p = 0.0008) compared with control donors. However, since the frequency of disomy was 0.40% for chromosome 13 and 0.55% for XY disomy, it is not surprising that these increases were not discovered previously in our analysis of 75 sperm karyotypes. Our results suggest that the extra Y chromosome is eliminated during spermatogenesis in the majority of cells but that there may be a small but significant increase in the frequency of aneuploid sperm in these men.     

Nondisjunction in human sperm: comparison of frequencies in acrocentric chromosomes.

Acrocentric chromosomes may be particularly predisposed to nondisjunction because of the frequency of trisomy for these chromosomes in human spontaneous abortions and liveborns. Studies of aneuploidy in human sperm have provided data on only a few acrocentric chromosomes, with evidence that chromosome 21 has a significantly increased frequency of disomy. To determine whether other acrocentric chromosomes have a higher frequency of nondisjunction or if chromosome 21 is anomalous, disomy frequencies for chromosomes 13 and 22 were studied by fluorescence in situ hybridization (FISH) analysis of 51,043 sperm nuclei from five normal men for whom the frequency of disomy for chromosomes 15 and 21 was known. The mean frequency of disomy for chromosome 13 (0.19%) did not differ significantly from that for other autosomes; however, the frequency of disomy 22 (1.21%) was significantly elevated (P < 0.001, Mantel-Haenszel chi(2) test). The G-group chromosomes (Nos. 21 and 22) also showed a significantly increased frequency of disomy (0. 75%) compared to acrocentric D-group chromosomes (viz., chromosomes 13 and 15; 0.15%) (P < 0.001, Mantel-Haenszel chi(2) test) and other autosomes (chromosomes 1, 2, 4, 9, 12, 13, 15, 16, 18, and 20; 0. 13%) studied in the same men (P < 0.001, Mantel-Haenszel chi(2) test).     

Sperm dysfunction in sex ratio males of Drosophila subobscura

Drosophila subobscura has 128 spermatids per cyst, enclosed by two cyst cells. At beginning of elongation in control males the spermatid nuclei surround the head cyst cell nucleus, in sex ratio males nuclei are found throughout the cyst. Spermatid nuclei can elongate in any position in the cyst. Nuclei can be eliminated during individualization or degenerate after individualization. The number of sperm in any wrong position in the cyst varies in control males from 0 to about ten, in sex ratio males from 0 to more than 50. Two cyst sizes are distinguishable. At beginning of elongation small cysts have homogeneously stained spherical nuclei which later on are rod like. Large cysts have granulated nuclei which at first become spindle shaped and then slender. The length of the DNA containing part of elongated sperm heads of the long class is about 33 m in sex ratio and control males. The small sperm heads are 15 m in sex ratio but 20 m in control males. The complete DNA-containing-sperm-length is about 10% less in short sperm and 5% less in long sperm of sex ratio males than in those of control. Sex ratio males have more cysts per testis than control males. In sex ratio we counted 53.8%, in control males 49.4% short cysts.The work was supported by the grants N.S.F. GB-43209 and NIH GM 21732 and the Schweizerischer Nationalfonds zur Foerderung der wissenschaftlichen Forschung grant Nr. 3.815.72.     

Male-Sex-Ratio Trait in Drosophila Pseudoobscura: Frequency of Autosomal Aneuploid Sperm

Males with the SR X chromosome show the "sex-ratio" (sr) phenotype in which they produce almost entirely daughters. The few sons (about 1%) are invariably sterile X/O males and result entirely from nullo-XY sperm. The "male-sex-ratio" (msr) phenotype is a modified form of sr in which SR/Y males produce a higher frequency of sterile X/O sons. The msr trait is due to the presence of the SR X-chromosome in males which are also homozygous for one or more autosomes from the L116 strain. Here the frequency of nullo-3 and diplo-3 sperm from msr males was measured by crossing to a compound-3 strain and found to be 13.8% and 3.2%, respectively, of the total viable sperm. The sr males produced very low levels of nullo-3 sperm at a frequency not different from control X/Y males and a slightly elevated frequency of diplo-3 sperm over X/Y males. The msr males were found to have only 12% the fecundity of sr males and in matings to cause a high frequency of brown inviable eggs. These results indicate that high rates of autosomal aneuploidy are not restricted to chromosome 3 but also occur for chromosomes 2, 4 and 5. The overall frequency of autosomal aneuploid sperm is estimated to be approximately 50%. Microscopic studies of meiosis in testes from msr males indicates meiotic nondisjunction and meiotic chromosome loss are responsible for the msr phenotype. Last, microscopic studies of sperm cysts from msr males reveal high levels of spermiogenic failure.     

A comparison of the frequency of sperm chromosome abnormalities in men with mild,moderate, and severe oligozoospermia

Infertile men undergoing intracytoplasmic sperm injection have an increased frequency of chromosome abnormalities in their sperm. Men with low sperm concentration (oligozoospermia) have an increased risk of sperm chromosome abnormalities. This study was initiated to determine whether men with severe oligozoospermia (<10(6) sperm/ml) have a higher frequency of chromosome abnormalities in their sperm compared with men with moderate (1-9 x 10(6) sperm/ml) or mild (10-19 x 10(6) sperm/ml) oligozoospermia. Multicolor fluorescence in situ hybridization analysis was performed using DNA probes specific for chromosomes 13, 21, X, and Y (with chromosome 1 as an autosomal control for the sex chromosomes). Aneuploidy and disomy frequencies were assessed from a total of 603,011 sperm from 30 men: 10 in each of the categories. The mean frequencies of disomy for the patients with mild, moderate, and severe oligozoospermia were 0.17%, 0.24%, and 0.30%, respectively, for chromosome 13 and 0.22%, 0.44%, and 0.58%, respectively, for chromosome 21. For the sex chromosomes, the mean frequencies of disomy for mild, moderate, and severe oligozoospermia were 0.25%, 1.04%, and 0.68%, respectively, for XY, 0.047%, 0.08%, and 0.10%, respectively, for XX, and 0.04%, 0.06%, and 0.09%, respectively, for YY. The frequencies for diploidy also increased from 0.4% for mild to 1.20% for moderate to 1.24% for severe oligozoospermia. There was a significant inverse correlation between the frequency of sperm chromosome abnormalities and the sperm concentration for XY, XX, and YY disomy and diploidy. These results demonstrate that men with severe oligozoospermia have an elevated risk for chromosome abnormalities in their sperm, particularly sex chromosome abnormalities.     

The male-derived genome after sperm-egg fusion: Spatial distribution of chromosomal DNA and paternal-maternal genomic association

After fusion with an egg, the haploid genome of a mammalian sperm expands from a volume of a few cubic microns to a volume of several thousand cubic microns in a fully developed zygote pronucleus. Using fluorescent in situ hybridization we studied two aspects of the chromatin organization of the sperm-derived genome during this process in a model system involving fusion of human sperm with hamster eggs. We found that (a) from the beginning of sperm head decondensation to early pronuclear stages, the hybridization signal of a probe targeted to the satellite III heterochromatin of chromosome 1 appeared as an irregularly shaped domain. In fully developed pronuclei, the signal became a fiber as much as 20-fold more extended compared with interphase somatic nuclei. The signal appeared as a string of non-uniformly distributed beads with interspersed gaps. A similar spatial distribution and appearance recurred after the first cleavage division. (b) Male-and female-derived genomes assembled non-randomly on the first mitotic spindle, and continued to be sequestered from each other in the nuclei of two-cell stages.by J.B. Rattner     

The synaptonemal complexes of Caenorhabditis elegans: the dominant him mutant mnT6 and pachytene karyotype analysis of the X-autosome translocation

The dominant X-autosome heterozygous translocation mutant mnT6 of the nematode Caenorhabditis elegans has an X chromosome that has been reduced in size by 40%, yet the remainder of the bivalent pairs effectively at pachytene and has a synaptonemal complex (SC) that has a normal appearance. Six SCs are present in pachytene nuclei of this mutant which correspond to a haploid value of n=6. Nondisjunction of the X chromosome occurs at a rate of 37% and there are no Disjunction Regulator Regions (DRR) in this him (high incidence of males) mutant. This is consistent with the notion that DRRs either promote disjunction or inhibit nondisjunction of the X chromosome. Their occurrence in pachytene nuclei is independent of the mechanism responsible for nondisjunction, i.e. point mutations as in him-8 versus chromosomal aberrations as in mnT6. Although an SC is present along the entire length of the X chromosome, crossover suppression is observed in mnT6.