Paternal Meiosis Research Articles

Molecular study of parental origin of extra chromosome 21 in regular and de novo translocation trisomies

The parental origin of the extra chromosome 21 (or extra 21q) was determined in seven informative families with a Down syndrome (DS) child by using molecular polymorphisms. Five DS patients had regular trisomy, one a de novo 14/21 translocation and another a de novo 21/21 translocation or isochromosome 21q. In four families with regular trisomy, the extra chromosome was of maternal origin, and in one family it was paternally derived. In the two families with a de novo aberration, both the 14/21 translocation and 21/21 rearrangement originated during maternal meiosis. For a better evaluation of the stage of meiotic error and the occurrence of crossovers between nondisjoined chromosomes, the regional map position of four of the nine informative DNA markers, used in this study, was refined, leading to useful localizations in both centromeric and distal regions. Recombination events were found in two families with regular trisomy, one occurring between chromosomes 21 that failed to disjoin at maternal meiosis I, the other prior to a paternal meiosis II nondisjunction.

Del(18)(q12.2q21.1) caused by a paternal sister chromatid rearrangement in a developmentally delayed girl

Monosomy of 18q12.3 has been reported in only 16 cases, in one as a mosaic with a normal cell line. Abnormal behaviour, developmental delay, normal measurements, and minor facial anomalies including ptosis, bilateral epicanthus, strabismus, short and slightly down-slanting palpebral fissures, and full cheeks are characteristic manifestations. We report on a 26-month-old girl with del(18)(q12.2q21.1) and typical phenotype. Microsatellite mediated haplotype analysis showed approximately 12 Mb deletion and demonstrated that the deletion was most likely formed during paternal meiosis by a rearrangement between the grandpaternal sister chromatids.

Studying the biological and technical sources of variation in telomere length of individual chromosomes

Consistent average length differences between species and chromosome arm differences within species indicate that telomere length is genetically determined. This seems to contradict an observed large variation in lengths of the same human telomere between metaphases of the same individual. We examined the extent to which the variation in the telomeres of the human X and Y chromosomes is heritable, induced, or technical in origin. Metaphase chromosomes were stained by fluorescence in situ hybridization with a telomere repeat-specific probe, and fluorescence intensities of the X and Y chromosomes were measured. If telomere length variation is predominantly genetically determined and a 50% probability of meiotic recombination between the pseudo-autosomal regions of Yp and Xp in the father is taken into account, one expects an equal chance that the Yp telomere of a son is derived from his father's Xp or Yp telomere. This implies that the Yp/Yq telomere ratios in fathers and sons will be identical in the absence of paternal meiotic recombination and different when recombination occurs. Among five father-son pairs, four showed similar Yp/Yq ratios (P > 0.05), whereas one pair exhibited a large difference in the Yp/Yq ratio that was attributable to a significantly longer Xp than Yp telomere in the father and a presumptive meiotic exchange between X and Y during paternal meiosis. Further, the Xq telomere exhibited a generally shorter telomere length than the others. The high variation in telomere length appeared to be intracellular (between sister chromatids) and, hence, technical in nature. We found no measurable induced variation in the cells studied, implying that, if induced variation exists, it is small compared with the technical variation.

Molecular and cytogenetic characterization of a non‐mosaic isodicentric Y chromosome in a patient with Klinefelter syndrome

We report on an adult male with Klinefelter phenotype and an isodicentric Y chromosome (47,XX,+idic(Y)(q12)), a combination which has to the best of our knowledge not been reported before. The patient was hospitalized in forensic psychiatry because of repeated delinquency, aggressive, aberrant and inappropriate behavior, and borderline intelligence. Molecular cytogenetic studies (FISH) showed that the SRY gene was present on both ends of the idicY, while there was only one signal for the Yq subtelomere probe. Molecular investigations by multiplex PCR, using STS markers covering the short and long arm of the Y chromosome did not indicate a deletion of Y chromosomal material. Molecular investigations of STR markers located on Xp22.3 and Xq28 indicated paternal origin of the additional X chromosome and an error in paternal meiosis I. Results of FISH analysis and molecular investigations are compatible with a phenotype as described for individuals with a 48,XXYY karyotype and support the findings that isodicentric Y chromosomes are frequently accompanied by other sex chromosomal abnormalities.

Recombination rate and reproductive success in humans.

Intergenerational mixing of DNA through meiotic recombinations of homologous chromosomes during gametogenesis is a major event that generates diversity in the eukaryotic genome. We examined genome-wide microsatellite data for 23,066 individuals, providing information on recombination events of 14,140 maternal and paternal meioses each, and found a positive correlation between maternal recombination counts of an offspring and maternal age. We postulated that the recombination rate of eggs does not increase with maternal age, but that the apparent increase is the consequence of selection. Specifically, a high recombination count increased the chance of a gamete becoming a live birth, and this effect became more pronounced with advancing maternal age. Further support for this hypothesis came from our observation that mothers with high oocyte recombination rate tend to have more children. Hence, not only do recombinations have a role in evolution by yielding diverse combinations of gene variants for natural selection, but they are also under selection themselves.

Clinical, cytogenetic, and molecular findings of prenatally diagnosed mosaic trisomy 4

To present the clinical, cytogenetic, and molecular findings of prenatally diagnosed mosaic trisomy 4. An amniocentesis was performed at 21 weeks' gestation because of maternal anxiety. Cytogenetic analysis revealed mosaicism for trisomy 4, 47,XX,+4[4]/46,XX[16]. Level II ultrasound demonstrated tetralogy of Fallot. Repeated amniocentesis at 23 weeks' gestation revealed 47,XX,+4[4]/46,XX[19]. The pregnancy was terminated. Phenotypic findings included tetralogy of Fallot, hypertelorism, micrognathia, abnormal ears, duplicated phalanges of the left thumb, clinodactyly, and overlapping of the toes. The karyotype of the cord blood was 46,XX. Cytogenetic analyses of the multiple tissue samplings showed a karyotype of 47,XX,+4 in 40/40 cells of the amniotic membrane (amnion), and 47,XX,+4/46,XX with various levels of trisomy 4 in the cells of the liver, lungs, placenta, skin, and umbilical cord. The levels of trisomy 4 were 11/40 in the liver, 8/40 in the lungs, 31/40 in the placenta, 9/40 in the skin, and 8/40 in the umbilical cord. The parental origin and meiotic origin of trisomy 4 were determined by examining the amniotic membrane using quantitative fluorescent polymerase chain reaction assays with polymorphic markers specific for chromosome 4. The result was consistent with a paternal meiosis I nondisjunction error. The cord blood showed a biparental inheritance. An extra paternal heterozygous allele with partial dosage increase was noted in other fetal and extraembryonic tissues studied. A diagnosis of trisomy 4 mosaicism in amniocytes indicates an increased risk for fetal abnormalities. Associated abnormal findings include congenital heart defects and anomalies of the digits and thumb. A confirmatory placental sampling may be helpful, whereas a fetal blood sampling is of a very limited value. A postnatal amnion sampling may provide additional clues to the fetal involvement of trisomy 4.

Linkage and QTL mapping for Sus scrofa chromosome 3

SummaryLinkage maps of Sus scrofa chromosome 3 (SSC3) were established on the basis of eight marker loci using three F2 families generated from crosses of Meishan (M), Pietrain (P) and Wild Boar (W) founders. The maps were similar for the three families and in agreement with the published linkage maps. Recombination was significantly higher in maternal than in paternal meioses. Quantitative trait loci (QTLs), explaining up to 6.0% of the F2 phenotypic variance, were mapped for daily gain, carcass traits and meat quality in proximal and central intervals of SSC3. The QTLs detected were often family‐specific. Pietrain QTL alleles were superior for muscle traits in comparison with Meishan alleles, and Wild Boar QTL alleles caused lower values for fatness than Pietrain alleles.

DNA microsatellite and linkage analysis supports the inclusion of LOCR in the Rh blood group system.

In 1994, a new low-incidence RBC antigen called LOCR was described. It was established that RBCs expressing LOCR had altered expression of Rh antigens (c or e). Unfortunately, because of an insufficient number of informative families, it was not possible to formally assign LOCR to the Rh blood group system by serology alone. Genomic DNA from 19 family members segregating for LOCR was analyzed for repeat polymorphisms of the chromosome 1p microsatellite markers D1S1612, D1S1597, D1S552, D1S247, and D1S2134. No evidence of recombination (in either paternal or maternal meioses) between LOCR and D1S1597, D1S552, or D1S247 was observed. Peak lods for combined paternal and maternal meioses were 2.41 for either LOCR:D1S552 or LOCR:D1S247. Lods for linkage between LOCR and D1S1597 peaked at 1.81 for maternal meioses alone. With serologic methods, a peak lod of 2.107 was determined previously between LOCR and RH. In this study, DNA analysis of the only informative family (with seven children) not segregating for RH yielded a peak lod of 1.81 between LOCR and D1S1597-D1S552-D1S247. By combining the results generated by each approach (lods of 3.917), evidence has been provided that supports the placement of LOCR in the Rh blood group system.

STR mutations in paternity investigations: a study of 1-year consecutive cases

A study of 1-year consecutive cases for STR mutations has been performed during 2000 with SGM Plus and PowerPlex 1.2 or PowerPlex 16. In paternity investigation cases, 18 STR mutations have been detected in several loci. Paternal and maternal mutation cases and a case with two paternal inconsistencies in D7S820 and CSF1PO are also presented. Maternal and paternal meioses were studied and the average mutation rates concerning several types of mutations were determined. During this 1-year study, multi-banded alleles and vWA genotype differences using different multiplex systems were also detected. The final probability paternity value (after inclusion of mutations) was always >99.99%, allowing to conclude confidently the presence of a mutation.

Paternal uniparental heterodisomy with partial isodisomy of chromosome 1 in a patient with retinitis pigmentosa without hearing loss and a missense mutation in the Usher syndrome type II gene USH2A.

To evaluate a form of nonmendelian inheritance in a patient with retinitis pigmentosa (RP). Direct DNA sequencing of the USH2A coding region and microsatellite analysis of polymorphic markers from chromosome 1 and other chromosomes. A patient with RP without hearing loss caused by the homozygous mutation Cys759Phe in the USH2A gene on chromosome 1q was found to be the daughter of a noncarrier mother and a father who was heterozygous for this change. Further evaluation with microsatellite markers revealed that the patient had inherited 2 copies of chromosome 1 from her father and none from her mother. The paternally derived chromosome 1's were heteroallelic from the centromere of chromosome 1 to the proximal short and long arms. The distal regions of the short and long arms of chromosome 1 were homoallelic, including the region of 1q with the mutant USH2A allele. This genetic pattern is compatible with a phenomenon of uniparental primary heterodisomy with regions of homozygosity arising through a nondisjunction event during paternal meiosis I and subsequent trisomy rescue or gamete complementation. A paternal second cousin of the patient also had RP and also had an identical heterozygous mutation in the USH2A gene in the same codon. However, the analysis of an isocoding polymorphism 20 base pairs away and closely linked microsatellite markers in the patient and family members indicated that the 2 mutant alleles are unlikely to be identical by descent and that the 2 relatives fortuitously had RP and a mutation in the same codon of the USH2A gene. This family illustrates that recessive RP without hearing loss can rarely be inherited from only 1 unaffected carrier parent in a nonmendelian manner. The genetic counseling of families with recessively inherited eye diseases must take into consideration the possibility that an unaffected heterozygous carrier can have an affected offspring homozygous for the same mutation, even if the carrier's spouse has wild-type alleles at the disease locus.

CTLA-4 49 A/G dimorphism and type 1 diabetes susceptibility: a French case-control study and segregation analysis. Evidence of a maternal effect.

Several studies have demonstrated an association of cytotoxic T lymphocyte-associated molecule 4 (CTLA-4) (IDDM 12) alanine 17 with type 1 diabetes, but we wished to study the parental effect of CTLA-4 49 A/G dimorphism in diabetic families. The CTLA-4 exon 1 polymorphism (49 A/G), HLA-DRB1 and insulin gene (INS) variable number tandem repeats (VNTR) were analysed in 134 type 1 diabetic patients vs. 273 control subjects. The segregation analysis for transmission was carried out in 70 informative diabetic families using the transmission distortion test (TDT). All genotyping was performed by PCR-RFLP. CTLA-4 49 G allele frequency was not increased in diabetic patients compared to controls (41 vs. 38%, not significant). The distribution of GG, AG and AA CTLA-4 genotypes was similar in the two groups (13, 57 and 30% vs. 11, 54 and 35%, respectively) and was independent of HLA-DRB1 or INS VNTR polymorphism. The CTLA-4 49 G allele showed weak distorted transmission to the diabetic offspring, whereas random transmission was observed in unaffected offspring. This distortion is attributable to a maternal effect (71% compared to the 50% expected ratio; tdt = 4.8; P < 0.03). The combined transmission of maternal CTLA-4 G with HLA-DRB1*03 (90%; tdt = 6.4; P < 0.01) and VNTR class I (80%; tdt = 5.4; P < 0.02) enhanced the susceptibility effect of each marker separately. We noted a slight CTLA-4 49 G and HLA-DRB1*04 distortion of transmission shared in paternal and maternal diabetic meiosis. In non-diabetic offspring, the CTLA-4 49 A allele confers a protective effect in the presence of maternal HLA-DRB1*03 and paternal HLA-DRB1*04 alleles. Despite the absence of a positive association of the CTLA-4 49 G allele with type 1 diabetes, our segregation analysis supports the hypothesis of a modulation by CTLA-4 49 G/A dimorphism of the susceptibility conferred by maternal HLA-DRB1*03 inheritance. This potential parental effect needs to be confirmed in a larger data set.

Evidence for linkage disequilibrium between the alpha 7-nicotinic receptor gene (CHRNA7) locus and schizophrenia in Azorean families.

Recent studies have suggested that the alpha 7-nicotinic receptor gene (CHRNA7) may play a role in the pathogenesis of schizophrenia. The alpha 7-nicotinic receptor gene (CHRNA7) is involved in P50 auditory sensory gating deficits, and the genomic locus for this gene lies in the chromosome 15q13-14 regions. The human gene is partially duplicated (exons 5-10) with four novel upstream exons. The marker D15S1360 has been shown to be significantly linked with the phenotype of abnormal P50 suppression in schizophrenia families. The marker L76630 is 3 kb in the 3' direction from the last exon of the CHRNA7 gene and is located in the duplicated region. The function of the two L76630 copies is unknown. We genotyped three polymorphic markers D15S1360, D15S165, and L76630 that are localized in a genomic fragment containing the CHRNA7 in 31 Azorean schizophrenia families/trios (including 41 schizophrenia individuals and 97 unaffected families members). An overall analysis utilizing the family-based association test revealed significant linkage disequilibrium between L76630 and schizophrenia (P = 0.0004). Using the extended transmission disequilibrium test and limiting the analysis to one triad per family, transmission disequilibrium of D15S1360 was near significance (P = 0.078). The 15q13 region overlaps with the location of two well-known genomically imprinted disorders: Angelman syndrome and Prader-Willi syndrome. Therefore, we investigated maternal and paternal meioses. We found significant transmission disequilibrium for D15S1360 through paternal transmission (P = 0.0006) in our schizophrenia families. The L76630 marker showed a significant disequilibrium in maternal transmissions (P = 0.028). No parent-of-origin effect was found in D15S165. Overall, our results suggest that the CHRNA7 may play a role in schizophrenia in these families. A parent of origin effect may be present and requires further study.

An amino acid substitution in the putative second extracellular loop of RBC band 3 accounts for the Froese blood group polymorphism.

The low incidence RBC antigen Fr(a) has been excluded from 17 of the 25 established blood group systems. Previous genetic analysis assigned the gene controlling Fr(a) expression to the same chromosomal region as the solute carrier family 4, anion exchanger member 1 gene (SLC4A1). Because SLC4A1 encodes RBC band 3 and controls the expression of Diego blood group system antigens, the possible relationship of Fr(a) to the Diego blood group system was investigated by molecular analysis of SLC4A1. Blood samples were obtained from the members of two unrelated Mennonite kindreds segregating for Fr(a). DNA was extracted, amplified by PCR using intronic primer sets flanking exons 11-20 of SLC4A1, and screened by single-strand conformation polymorphism (SSCP) analysis. Those exons displaying SSCPs were subjected to DNA sequence analysis. An exon 13 SSCP mobility shift was observed in the DNA from all Fr(a+) persons that was not seen in the DNA from Fr(a-) family members or control subjects. Linkage between the exon 13 SSCP and FR:(a) was established, with peak lods = 3.62 at theta = 0.00 for combined paternal and maternal meioses. DNA sequencing revealed a GAG --> AAG mutation that underlies a Glu480Lys substitution in RBC band 3. A point mutation in exon 13 of SLC4A1 accounting for a Glu480Lys substitution in band 3 controls Fr(a) expression. On the basis of these our results, the International Society of Blood Transfusion Working Party on Terminology for Red Cell Surface Antigens has assigned Fr(a) to the Diego blood group system, with the designation DI20.

Confirmation of the Assignment of the Dombrock Blood Group Locus (DO) to Chromosome 12p: Narrowing the Boundaries to 12p 12.3‐p13.2

Abstract Background and Objectives : Antigens belonging to the Dombrock blood group system have been well characterized serologically. Despite being reasonably polymorphic, chromosomal localization of the gene controlling Dombrock expression (DO) has proved difficult. In the past, DO had been provisionally assigned to chromosomes 1 and 4; more recently, DO was provisionally assigned to chromosome 12. The current study was undertaken in an attempt to confirm or refute this latest assignment. Materials and Methods: DNA from a series of families segregating for DO was amplified by polymerase chain reaction and analyzed for trinucleotide or tetranucleotide repeat polymorphisms of four anonymous DNA markers (D12S1042, D12S373, D12S391, D12S372) and of the von Willebrand factor gene (VWF). Lods for or against linkage were determined between DO and each marker. Results: DO is linked to all five markers. The closest linkage was established between DO and D12S373, and DO and D12S391, with peak lods for combined paternal and maternal meioses of 7.16 at peak recombination fraction ( ) = 0.08 and 7.69 at = 0.08, respectively. Conclusions: We have confirmed the assignment of the Dombrock blood group locus to chromosome 12p and have refined its regional localization to 12p 12.3–p 13.2.

Confirmation of the Assignment of the Dombrock Blood Group Locus (DO) to Chromosome 12p: Narrowing the Boundaries to 12p12.3–p13.2

Background and Objectives: Antigens belonging to the Dombrock blood group system have been well characterized serologically. Despite being reasonably polymorphic, chromosomal localization of the gene controlling Dombrock expression (DO) has proved difficult. In the past, DO had been provisionally assigned to chromosomes 1 and 4; more recently, DO was provisionally assigned to chromosome 12. The current study was undertaken in an attempt to confirm or refute this latest assignment. Materials and Methods: DNA from a series of families segregating for DO was amplified by polymerase chain reaction and analyzed for trinucleotide or tetranucleotide repeat polymorphisms of four anonymous DNA markers (D12S1042, D12S373, D12S391, D12S372) and of the von Willebrand factor gene (VWF). Lods for or against linkage were determined between DO and each marker. Results: DO is linked to all five markers. The closest linkage was established between DO and D12S373, and DO and D12S391, with peak lods for combined paternal and maternal meioses of 7.16 at peak recombination fraction (θcirc;) = 0.08 and 7.69 at θcirc; = 0.08, respectively. Conclusions: We have confirmed the assignment of the Dombrock blood group locus to chromosome 12p and have refined its regional localization to 12p12.3–p13.2.

Amino acid substitutions in human erythroid protein band 3 account for the low-incidence antigens NFLD and BOW.

The low-incidence red cell antigens NFLD (700.37) and BOW (700.46) were first described in 1984 and 1988, respectively. Recent investigations showed that antigens of the Diego blood group system (including a number of low-incidence antigens) are coded by SLC4A1 (solute carrier family 4, anion exchanger member 1 gene). Among these newly characterized Diego system antigens is Wu (designated DI9). Because a serologic relationship among Wu, NFLD, and BOW has been established, a series of genetic and molecular investigations of SLC4A1 in relation to NFLD and BOW were undertaken. By the use of exon-specific primers, single-strand conformational polymorphism (SSCP) analysis of SLC4A1 was performed on DNA isolated from an NFLD+ person from Japan, from the members of a Canadian kindred segregating for NFLD, and from two unrelated BOW+ persons. Exons displaying SSCPs were subjected to genetic linkage analysis (for NFLD only) and DNA sequencing. SSCPs in DNA amplified from exons 12 and 14 of SLC4A1 were observed for all NFLD+ subjects. Linkage between each of these polymorphisms and NFLD was established with peak lods = 4.82 at theta = 0.00 for combined paternal and maternal meiosis. DNA sequencing of exons 12 and 14 of SLC4A1 from NFLD+ persons identified A-->T and C-->G mutations that underlie Glu429Asp and Pro561Ala substitutions in human erythroid band 3 protein (band 3). DNA from the two unrelated BOW+ persons only exhibited an SSCP in exon 14 of SLC4A1. Subsequent DNA sequencing revealed a C-->T mutation that accounts for a Pro561Ser substitution in band 3. SLC4A1 codes for the low-incidence red cell antigens NFLD and BOW. In light of these findings, both antigens have been assigned to the Diego blood group system.

SRY gene transferred to the long arm of the X chromosome in a Y-positive XX true hermaphrodite

Yp-specific sequences, including the testicular determinant gene SRY, have been detected and located in a 46,XX true hermaphrodite individual, using PCR amplification and fluorescent in situ hybridization (FISH). Among different Y chromosome loci tested, it was only possible to detect Yp sequences. The Y-centromere and Yq sequences were absent. Unexpectedly, the Y fragment was translocated to the long arm of one of the X chromosomes, at the Xq28 level, and the derivative (X) chromosome of the patient lacked q-telomeric sequences. To our knowledge, this is the first Yp/Xq translocation reported. The coexistence of testicular and ovarian tissue in the patient may have arisen by differential inactivation of the Y-bearing X chromosome, in which Xq telomeric sequences are missing. The possible origin of the Yp/Xq translocation, during paternal meiosis or in somatic paternal cells, is discussed.

The origin of the extra Y chromosome in males with a 47,XYY karyotype.

The presence of an extra Y chromosome in males is a relatively common occurrence, the 47,XYY karyotype being found in approximately 1 in 1000 male births. The error of disjunction must occur either during paternal meiosis II or as a post-zygotic mitotic error, both of which are rare events for other chromosomes. It is therefore of interest to determine when errors of Y chromosome disjunction occur. It is possible to distinguish between the different mechanisms of non-disjunction by analysing DNA polymorphisms at the distal tip of the Xp/Yp pseudoautosomal region in 47,XYY males, their parents and in some cases paternal grandparents. A cohort of 28 non-mosaic 47,XYY males was analysed. The results show that there are at least two mechanisms causing non-disjunction of the Y chromosome. In 16 of the 19 cases from which parents were available, the extra Y was generated by non-disjunction at meiosis II after a normal chiasmate meiosis I. Three cases were due to either a post-zygotic mitotic error or non-disjunction at meiosis II after a nullichiasmate meiosis I. Of the nine cases with no parental DNA available, at least four were due to meiosis II non-disjunction following a normal chiasmate meiosis I.

Prenatally detected paternal uniparental chromosome 13 isodisomy

A 13q isodisomy in a balanced karyotype: 45,XY,-13,-13, + i(13)(q10) was found in cultured amniocytes studied because of advanced maternal age. The isochromosome was monocentric and a new mutation as both parents had normal chromosomes. Fetal blood was studied to exclude 13-trisomy mosaicism. All (100) lymphocytes studied had the same karyotype with i(13)(q10) as the amniocytes. To determine the origin of the isochromosome, six microsatellite markers from 13q were analysed: D13S175, D13S166, D13S162, AC224, COLAC1 and D13S122. The results indicated that the i(13)(q10) was of paternal origin and isodisomic. The father had a risk of 1/20 for being a carrier for an autosomal recessive, progressive brain disorder, variant late infantile neuronal ceroid lipofuscinosis (CLN5). The risk for the fetus for this disorder of chromosome 13 was excluded by haplotype analysis. A healthy child was born at week 40 of pregnancy, supporting the idea that there are no paternally imprinted genes on chromosome 13q. Analysis of extra embryonal tissue (four samples studied) revealed the same balanced karyotype with the i(13)(q10)pat chromosome. According to the cytogenetic and molecular studies, the origin of the isochromosome 13 could be a transverse centromere cleavage at the paternal meiosis II or at an early mitosis.

De Novo Rearrangements Found in 2% of Index Patients with Spinal Muscular Atrophy: Mutational Mechanisms, Parental Origin, Mutation Rate, and Implications for Genetic Counseling

Spinal muscular atrophy (SMA) is a relatively common autosomal recessive neuromuscular disorder. We have identified de novo rearrangements in 7 (approximately 2%) index patients from 340 informative SMA families. In each, the rearrangements resulted in the absence of the telomeric copy of the survival motor neuron (SMN) gene (telSMN), in two cases accompanied by the loss of the neuronal apoptosis-inhibitory protein gene . Haplotype analysis revealed unequal recombination in four cases, with loss of markers Ag1-CA and C212, which are near the 5' ends of the SMN genes. In one case, an interchromosomal rearrangement involving both the SMN genes and a regrouping of Ag1-CA and C212 alleles must have occurred, suggesting either interchromosomal gene conversion or double recombination. In two cases, no such rearrangement was observed, but loss of telSMN plus Ag1-CA and C212 alleles in one case suggested intrachromosomal deletion or gene conversion. In six of the seven cases, the de novo rearrangement had occurred during paternal meiosis. Direct detection of de novo SMA mutations by molecular genetic means has allowed us to estimate for the first time the mutation rate for a recessive disorder in humans. The sex-averaged rate of 1.1 x 10(-4), arrived at in a proband-based approach, compares well with the rate of 0.9 x 10(-4) expected under a mutation-selection equilibrium for SMA. These findings have important implications for genetic counseling and prenatal diagnosis in that they emphasize the relevance of indirect genotype analysis in combination with direct SMN-gene deletion testing in SMA families.