The population genetics of Duchenne: natural and artificial selection in Duchenne muscular dystrophy

Sleep pattern and spectral analysis of caregiver-mothers of sons with Duchenne muscular dystrophy, and an examination of differences between carriers and non-carriers

OBJECTIVES

Sleep is essential for physical and mental well-being. However, poor sleep is a common complaint among caregivers. The aim of the present study was to determine sleep patterns of caregiver-mothers (CM group) of sons with Duchenne muscular dystrophy (DMD) and also to examine the differences between non-carriers and carriers of the gene related to DMD within the CM group.

METHODS

Observational case-control study.

PARTICIPANTS

The CM and control (CTRL) groups were matched for age, body mass index and social class. Polysomnography was conducted in a sleep laboratory for one night. The discrete fast Fourier transformation method was used to calculate the electroencephalogram (EEG) power spectrum for the entire night and sleep stages.

RESULTS

The CM group presented higher sleep latency and N3 sleep stage compared with the CTRL. When carrier and non-carrier CM subgroups were analyzed, increased sleep latency and time awake, as well as reduced sleep efficiency and N2, were observed in the carrier group. Regarding respiratory parameters, carriers demonstrated higher hypopnea index values compared with non-carriers. Spectral analysis showed that carriers compared with non-carrier DMD caregiver-mothers presented lower spectral power in fast waves, mainly beta, during REM sleep in some EEG derivations.

CONCLUSIONS

There was an impairment of sleep pattern in the CM group compared with CTRL mothers; this was possibly associated with difficulty in initiating sleep. Being a DMD gene carrying caregiver further compromised some aspects of sleep microstructure during REM sleep. The data demonstrated the importance of sleep evaluation in caregiver-mothers, and the relationship between sleep and being a carrier of the gene associated with DMD, which was demonstrated as possibly impacting sleep quality.

Imperatives for DUCHENNE MD: a Simplified Guide to Comprehensive Care for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive, life-limiting muscle-wasting disease. Although no curative treatment is yet available, comprehensive multidisciplinary care has increased life expectancy significantly in recent decades. An international consensus care publication in 2010 outlined best-practice care, which includes corticosteroid treatment, respiratory, cardiac, orthopedic and rehabilitative interventions to address disease manifestations. While disease specialists are largely aware of these care standards, local physicians responsible for the day-to-day care of patients and families may be less familiar. To facilitate optimal care, a one-page document has been generated from published care recommendations, summarizing the key elements of comprehensive care for people living with DMD ("Imperatives for Duchenne muscular dystrophy). This document was developed through an international collaboration between Parent Project Muscular Dystrophy (PPMD), United Parent Projects Muscular Dystrophy (UPPMD) and TREAT-NMD.

Symptomatic female carriers of Duchenne muscular dystrophy (DMD): genetic and clinical characterization

Duchenne muscular dystrophy (DMD) is an X-linked recessive disease caused by mutations in the dystrophin gene and is characterized by muscle degeneration and death. DMD affects males; females being asymptomatic carriers of mutations. However, some of them manifest symptoms due to a translocation between X chromosome and an autosome or to a heterozygous mutation leading to inactivation of most of their normal X chromosome. Six symptomatic female carriers and two asymptomatic were analyzed by: I) Segregation of STRs-(CA)n and MLPA assays to detect a hemizygous alteration, and II) X chromosome inactivation pattern to uncover the reason for symptoms in these females. The symptomatic females shared mild but progressive muscular weakness and increased serum creatin kinase (CK) levels. Levels of dystrophin protein were below normal or absent in many fibers. Segregation of STRs-(CA)n revealed hemizygous patterns in three patients, which were confirmed by MLPA. In addition, this analysis showed a duplication in another patient. X chromosome inactivation assay revealed a skewed X inactivation pattern in the symptomatic females and a random inactivation pattern in the asymptomatic ones. Our results support the hypothesis that the DMD phenotype in female carriers of a dystrophin mutation has a direct correlation with a skewed X-chromosome inactivation pattern.

Molecular diagnosis of duchenne muscular dystrophy: past, present and future in relation to implementing therapies

Duchenne muscular dystrophy (DMD) is the commonest and best-known of the muscular dystrophies. Being an X-linked disorder, it affects mainly boys. The disease gene was identified in 1987, with the majority of mutations demonstrated to be large-scale deletions. Current best practice molecular diagnosis includes multiplex ligation-dependent probe amplification (MLPA) followed by direct sequencing of all exons at the genomic level, or from cDNA, in order to detect point and other small mutations. The difference between DMD and the allelic Becker muscular dystrophy (BMD) is whether the precise mutation in the gene is a null mutation or results in a modified still partially functional protein. Over the last few years, significant progress has been made in moving experimental therapies into clinical trials, with one of the most promising possible therapies being anti-sense oligonucleotide induced exon-skipping, which converts DMD to BMD. In order to maximise the benefit from future therapies, it will be necessary to start administering the therapies as early as possible in the life of the affected boys, before significant muscle loss occurs. This will require early diagnosis, which evidence suggests is best achieved through population screening. Population screening also allows the avoidance of multiple affected boys in families with no previous family history.

The role of polymorphic short tandem (CA)n repeat loci segregation analysis in the detection of Duchenne muscular dystrophy carriers and prenatal diagnosis

BACKGROUND

Duchenne and Becker muscular dystrophies (DMD/BMD) are X-linked diseases caused by mutations in the dystrophin gene at Xp21.2; they include gross deletions (60%), duplications (10%), and small mutations (30%). Since there is no cure or effective treatment for progressive muscular dystrophy, prevention of the disease is important and strongly depends on carrier-status information. Two-thirds of DMD/BMD cases are familial; thus, female relatives are candidates for carrier-risk assessment.

AIM

Segregation analysis of polymorphic short tandem (CA)n repeats [STR-(CA)n] was used to establish and compare the haplotypes of female relatives of patients with DMD/BMD with those of the patient in order to identify the mutant dystrophin gene and thus determine each female relative's carrier status.

METHODS

248 individuals from 52 families were studied through segregation of up to 11 STR-(CA)n loci. The assay was performed on leukocyte DNA by PCR amplification, polyacrylamide-gel electrophoresis and autoradiography. Haplotypes were established by determination of alleles on the autoradiography.

RESULTS

38 of 51 (75%) female relatives from familial cases were diagnosed as carriers or non-carriers with a 95-100% likelihood, and 18 out of 56 (32%) female relatives from sporadic cases could be excluded from the risk of being a DMD carrier with the same probability. In addition, STR studies detected gross deletions in 13 of the 52 (25%) families in both male and female individuals, four of which were de novo deletions. STR assays were also informative in families without an available DNA sample of an affected male and in two of seven symptomatic females. Determination of carrier status was particularly significant for prediction of DMD risk in prenatal analysis of five male chorionic villi. Other genetic events revealed by STR analysis were: (i) 11 recombinations identified in 6.6% of meiosis in the DMD families; (ii) germinal mosaicism detected in two female carriers; and (iii) changes in STR-(CA)n length during transmission from father to daughters, including three retractions and one elongation at an estimated rate of 0.004.

CONCLUSION

The STR assay is an excellent molecular tool for carrier-status identification and the detection of deletions and other genetic changes in families affected by DMD/BMD. Thus, it is useful in genetic counseling for the prevention of this disease.

Detection of germline mosaicism in two Duchenne muscular dystrophy families using polymorphic dinucleotide (CA)n repeat loci within the dystrophin gene

BACKGROUND

Approximately one-third of new cases of Duchenne muscular dystrophy (DMD) can be attributed to sporadically arising new mutations, however in the majority of cases the DMD mutation has been inherited from the mother. These female carriers can have either a constitutive or mosaic mutation.

AIM

The aim of this study was to determine the segregation of the at-risk haplotype and to find a deletion in the dystrophin gene of patients.

METHOD

We analyzed individuals from two families with a history of DMD in order to predict the carrier status of related females. In one of these cases the mother had two affected sons, while in the other one son and two grandchildren were affected; therefore we predict that the mother would be an obligatory carrier.

RESULTS

Haplotype analysis of the DMD loci revealed that in the two families both the healthy and affected brothers had inherited the same X maternal chromosome. However, the affected brother carried a deletion, which was absent in the unaffected sibling.

CONCLUSION

These findings suggested that the mothers in the two families were germline mosaics for the DMD gene. The results of this study demonstrate the usefulness of the methodology that combine the haplotype analysis with the identification of the mutation in order to detect hidden germline mosaicisms and, thus, improve genetic counseling.

Molecular genetic basis of tuberous sclerosis complex: from bench to bedside

Tuberous sclerosis complex is an autosomal dominant disease of benign tumors occurring in multiple organ systems of the body. Either of two genes, TSC1 or TSC2, can be mutated, resulting in the tuberous sclerosis complex phenotype. The protein products of the tuberous sclerosis complex genes, hamartin (TSC1) and tuberin (TSC2), have been discovered to play important roles in several cell-signaling pathways. Knowledge regarding the function of the tuberin-hamartin complex has led to therapeutic intervention trials. Numerous pathogenic mutations have been elucidated in individuals affected with tuberous sclerosis complex. Information on the type and distribution of nearly 1000 mutations in the two genes is discussed. Mosaicism for tuberous sclerosis complex mutations has been documented, complicating provision of genetic counseling to families. Emerging genotype-phenotype correlations should provide guidance for better medical care of individuals with tuberous sclerosis complex.

Direct deletion analysis in two Duchenne muscular dystrophy symptomatic females using polymorphic dinucleotide (CA)n loci within the dystrophin gene

Duchenne muscular dystrophy (DMD) is the most common hereditary neuromuscular disease. It is inherited as an X-linked recessive trait in which males show clinical manifestations. In some rare cases, the disease can also be manifested in females. The aim of the present study was to determine the molecular alteration in two cases of nonrelated DMD symptomatic carriers with no previous history of DMD. Multiplex PCR is commonly used to search for deletion in the DMD gene of affected males. This method could not be used in females because the normal X chromosome masks the deletion of the mutated one. Therefore, we used a set of seven highly polymorphic dinucleotide (CA)(n) repeat markers that lie within the human dystrophin gene. The deletions were evidenced by hemizygosity of the loci under study. We localized a deletion in the locus 7A (intron 7) on the maternal X chromosome in one case, and a deletion in the region of introns 49 and 50 on the paternal X chromosome in the other. The use of microsatellite genotyping within the DMD gene enables the detection of the mutant allele in female carriers. It is also a useful method to provide DMD families with more accurate genetic counseling.

Inherited skeletal muscle disorders

The field of inherited skeletal muscle disease research has advanced rapidly since the identification of mutations in the dystrophin gene as the cause of Duchenne muscular dystrophy in 1987. From that point, an ever-increasing number of the genes associated with inherited muscle diseases have been identified. These discoveries have led to much more accurate diagnosis of the individual diseases and have allowed prenatal diagnosis where this was not previously possible. The major challenges for the future are to understand the pathophysiology of the diseases, now that the genes are being identified, and then to develop successful therapies.

Germ-line mosaicism in tuberous sclerosis: how common?

Two-thirds of cases of tuberous sclerosis complex (TSC) are sporadic and usually are attributed to new mutations, but unaffected parents sometimes have more than one affected child. We sought to determine how many of these cases represent germ-line mosaicism, as has been reported for other genetic diseases. In our sample of 120 families with TSC, 7 families had two affected children and clinically unaffected parents. These families were tested for mutations in the TSC1 and TSC2 genes, by Southern blotting and by single-strand conformational analysis. Unique variants were detected in six families. Each variant was present and identical in both affected children of a family but was absent in both parents and the unaffected siblings. Sequencing of the variants yielded two frameshift mutations, one missense mutation, and two nonsense mutations in TSC2 and one nonsense mutation in TSC1. To determine which parent contributed the affected gametes, the families were analyzed for linkage to TSC1 and TSC2, by construction of haplotypes with markers flanking the two genes. Linkage analysis and loss-of-heterozygosity studies indicated maternal origin in three families, paternal origin in one family, and either being possible in two families. To evaluate the possibility of low-level somatic mosaicism for TSC, DNA from lymphocytes of members of the six families were tested by allele-specific PCR. In all the families, the mutant allele was detected only in the known affected individuals. We conclude that germ-line mosaicism was present in five families with mutations in the TSC2 gene and in one family with the causative mutation in the TSC1 gene. The results have implications for genetic counseling of families with seemingly sporadic TSC.

Carrier detection in Duchenne and Becker muscular dystrophy Argentine families

In order to offer carrier detection, genetic counseling, and prenatal diagnosis to families with Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) in our country, segregation analysis of highly polymorphic short tandem repeats (STR) (dC-dA)n: (dG-dT)n loci was utilized. The risks to females of 15 DMD BMD families (9 familial and 6 sporadic) were evaluated on STR, pedigree and serum creatine kinase (SCK) data. From the 36 females at risk of being carriers (not including 8 obligate carriers), results of STR analysis were compatible with carrier status in 7 and not compatible in 20. In 9 females, no information regarding carriership was derived from the STR analysis. Prenatal diagnosis is now possible on the carrier females. Previously identified deletions in the central part of the gene were confirmed by STR analysis in 3 families. Five new alleles were identified in Argentine individuals; allele frequencies differed from those of North American people. Results derived from this study are useful for carrier detection and genetic counseling in DMD/BMD. One case of probable mosaicism in an unaffected father was detected on a pedigree basis in a family with DMD patients.

Multipoint linkage analysis and homogeneity tests in 15 Dutch X-linked retinitis pigmentosa families

Linkage analysis and homogeneity tests were carried out in 15 Dutch families segregating X-linked retinitis pigmentosa (X L R P). The study included segregation data for eight polymorphic DNA markers from the short arm of the human X chromosome. The results of both multipoint linkage analysis in individual families and heterogeneity analysis support the view that there are only two X L R P loci on the short arm of the human X chromosome, with one locus near the OTC gene and one in the vicinity of DXS255. Furthermore, our data confirm the hypothesis that a tapetal reflex in female carriers can be observed more frequently, if not exclusively, in X L R P families of the R P 3 type.

Mosaicism with a normal cell line and an autosomal structural rearrangement

Over three decades, 12 cases of mosaicism for an autosomal rearrangement were recognised in the major cytogenetics laboratories in New Zealand, eight of which were studied between 1990 and 1992. One case inferentially involved the gonad, eight the soma, and three both gonad and soma. This mosaicism could have arisen as a postzygotic event either in a conceptus that was initially normal, with the generation of an abnormal cell line, or in a conceptus having a supernumerary chromosome which was lost at a subsequent mitosis, thereby restoring a normal cell line. Three of the 12 cases involved a presumed direct duplication, an otherwise very uncommon rearrangement. This may indicate a propensity for direct duplications to arise at mitosis rather than at meiosis; unequal sister chromatid exchange is a plausible mechanism. Mosaicism has clinical relevance for genetic counselling, as an intragonadal cell line carrying a rearrangement could generate multiple unbalanced gametes. Mosaicism for an autosomal rearrangement my be very much more common that is, or ever could be, recognised.

Parental somatic and germ-line mosaicism for a multiexon deletion with unusual endpoints in a type III collagen (COL3A1) allele produces Ehlers-Danlos syndrome type IV in the heterozygous offspring

Ehlers-Danlos syndrome (EDS) type IV is a dominantly inherited disorder that results from mutations in the type III collagen gene (COL3A1). We studied the structure of the COL3A1 gene of an individual with EDS type IV and that of her phenotypically normal parents. The proband was heterozygous for a 2-kb deletion in COL3A1, while her father was mosaic for the same deletion in somatic and germ cells. In fibroblasts from the father, approximately two-fifths of the COL3A1 alleles carried the deletion, but only 10% of the COL3A1 alleles in white blood cells were of the mutant species. The deletion in the mutant allele extended from intron 7 into intron 11. There was a 12-bp direct repeat in intron 7 and intron 11, the latter about 60 bp 5' to the junction. At the breakpoint there was a duplication of 10 bp from intron 11 separated by an insertion of 4 bp contained within the duplicated sequence. The father was mosaic for the deletion so that the gene rearrangement occurred during his early embryonic development prior to lineage allocation. These findings suggest that at least some of the deletions seen in human genes may occur during replication, rather than as a consequence of meiotic crossing-over, and that they thus have a risk for recurrence when observed de novo.

Germinal mosaicism and risk calculation in X-linked diseases

Germinal mosaicism is a major problem in risk estimation for an X-linked disease. A mutation can happen anytime in germ cell development, and the proportion of germ cells bearing the mutated gene is twice the probability of recurrence of the mutation. This proportion could be either very low in late mutations or very high in germinal and somatic mosaicism. When this heterogeneity is taken into consideration, the distribution of the recurrence risk is conveniently represented as a set of discrete classes that may be derived either from models of gametogenesis or from empirical data. A computer program taking into account germinal mosaicism has been devised to calculate the probability of a possible carrier belonging to any of these classes, in order to settle the origin of the mutation of a given family. Germinal mosaicism increases the probability of inheriting the mutation, but this effect is always lowered by the possibility of heterogeneity. When the mother of a possible carrier is not herself a carrier, the risk of her daughter being a carrier is approximately halved, even under the assumption of a high recurrence risk from mosaicism.

Somatic mosaicism for a deletion of the dystrophin gene in a carrier of Becker muscular dystrophy

Duchenne muscular dystrophy (DMD) and the allelic milder form of Becker muscular dystrophy (BMD) are caused by mutations of the dystrophin gene on the short arm of the X chromosome. One third of affected individuals are expected to result from de novo mutations. Genetic counselling of families with sporadic cases is complicated by the potential meiotic origin of the mutation in the mother resulting in germline mosaicism. Here we present direct evidence for combined somatic and germline mosaicism for a deletion of the dystrophin gene, thereby proving the mitotic origin of this deletion and pinpointing a further potential pitfall for genetic counselling. The mother of a BMD son and a BMD carrier daughter, both carrying a deletion of dystrophin cDNA 7 (0.5 kb Hind III fragment) and cDNA 8, was herself clinically healthy and had normal creatine kinase levels. A muscle specimen of the mother showed mild overall pathology as well as focal dystrophin deficiency. In contrast chromosomal in situ suppression (CISS) hybridization of metaphase chromosomes using a cosmid clone of the corresponding cDNA deleted in her son revealed no evidence of somatic mosaicism in their lymphocytes. These results emphasize the value of an approach correlating genetic and immunological data for the definition of a carrier state in BMD or DMD. The possibility of somatic mosaicism should be considered when genetic counselling of a family with a sporadic case of BMD or DMD is performed.

Parental origin and germline mosaicism of deletions and duplications of the dystrophin gene: a European study

Knowledge about the parental origin of new mutations and the occurrence of germline mosaicism is important for estimating recurrence risks in Duchenne (DMD) and Becker muscular dystrophy (BMD). However, there are problems in resolving these issues partly because not all mutations can as yet be directly detected, and additionally because genetic ratios are very sensitive to ascertainment bias. In the present study, therefore, analysis was restricted to currently detectable mutations (deletions and duplications) in particular types of families which tend to be rare. In order to obtain sufficient data we pooled results from 25 European centers. In mothers of affected patients who were the first in their family with a dystrophin gene deletion or duplication, the ratio between the paternal and the maternal origin of this new mutation was 32:49 (binomial test P = 0.075) for DMD. In five BMD families the ratio between paternal and maternal origin of new mutations was 3:2. Recurrence risk because of maternal germline mosaicism was studied in sisters or subsequent sibs of isolated cases with an apparently new detectable mutation. In 12 out of 59 (0.20; 95% CI 0.10-0.31) transmissions of the risk haplotype the DMD mutation was transmitted as well. No recurrences were found in nine BMD families.

Somatic mosaicism at the Duchenne locus

Results of testing a family for carrier status and prenatal diagnosis for Duchenne muscular dystrophy (DMD) are best explained by somatic mosaicism in the maternal grandfather. This genetic situation was identified using segregation analysis of intragenic DNA polymorphisms, a serum creatine phosphokinase assay, and physical examination of the patients. This event at the DMD locus represents one more potential source of error in carrier testing and prenatal diagnosis.

Segregation analysis of 1885 DMD families: significant departure from the expected proportion of sporadic cases

The proportion of sporadic cases of Duchenne muscular dystrophy has been estimated by classical segregation analysis in a pooled sample of 1885 sibships from 7 different countries. A significant departure from the theoretical expectations based on mutation-selection equilibrium is observed (segregation frequency = 0.439 +/- 0.017; frequency of sporadic cases = 0.229 +/- 0.026, at the maximum likelihood). The occurrence of germinal mosaicism in some of the mothers of Duchenne cases may account for this peculiar finding, although a possible role of inequality of mutation rates in the two sexes cannot be ruled out.

Localizing multiple X chromosome-linked retinitis pigmentosa loci using multilocus homogeneity tests

Multilocus linkage analysis of 62 family pedigrees with X chromosome-linked retinitis pigmentosa (XLRP) was undertaken to determine the presence of possible multiple disease loci and to reliably estimate their map location. Multilocus homogeneity tests furnished convincing evidence for the presence of two XLRP loci, the likelihood ratio being 6.4 x 10(9):1 in favor of two versus a single XLRP locus and gave accurate estimates for their map location. In 60-75% of the families, location of an XLRP gene was estimated at 1 centimorgan distal to OTC, and in 25-40% of the families, an XLRP locus was located halfway between DXS14 (p58-1) and DXZ1 (Xcen), with an estimated recombination fraction of 25% between the two XLRP loci. There is also good evidence for a third XLRP locus, midway between DXS28 (C7) and DXS164 (pERT87), supported by a likelihood ratio of 293:1 for three versus two XLRP loci.

Partial deletion of 14q and partial duplication of 14q in sibs: testicular mosaicism for t(14q;14q) as a common mechanism

The propositus and a subsequently born sister presented with multiple congenital anomalies. Chromosome analyses were performed initially on peripheral blood lymphocytes from the propositus and his parents: the propositus was found to have a deletion of chromosome 14 (q32.11- greater than qter); the parents' chromosomes were normal. When the sister of the propositus was born, she was determined to have a duplication of an equivalent segment of 14qter. Chromosome studies on the parents' fibroblasts demonstrated no structural abnormality or mosaicism. The parents have an older, phenotypically normal, healthy daughter, which supports mosaicism for intragonadal t(14;14)(q32.11;q32.33) in one parent. Chromosome polymorphism comparisons show that the normal number 14 chromosome in the propositus and his sister was inherited from the mother, thus indicating paternal testicular mosaicism. Clinical findings are compared to those of other reported cases of deletion 14q and duplication 14q.

Duchenne muscular dystrophy in Wales: impact of DNA linkage analysis and cDNA deletion screening

A register of families with Duchenne muscular dystrophy (DMD) has been maintained in Wales since 1973. Since 1986 we have attempted to refine carrier status, and when necessary offer prenatal diagnosis, for those at significant risk by using intragenic probes. cDNA probes were included from the beginning of 1988. Thirty-four (30%) of the 115 women tested were assigned a risk of carrying the DMD gene of less than 5%. Thirty-three (29%) of the women at 5% or greater risk are now able to have prenatal diagnosis using a molecular deletion; such deletions were detected in 50% of affected boys. The remaining women could have prenatal diagnosis using a linked intragenic probe with an error rate varying between 0.25% and 9%. In 19 cases DNA samples from DMD boys who were dead at the time of analysis were used, indicating that it is essential to bank DNA from all males affected by DMD. We conclude that a large proportion of women at risk of carrying the DMD gene can now be helped by DNA studies.

Familiarity, recessivity and germline mosaicism

In man evidence of autosomal recessive disease is usually based on a high sib risk, absence of parent-child transmission and increased consanguinity. Discrimination from what are sometimes termed multifactorial disorders and their associated environmental effects is usually based on the latter having a lower recurrence risk, an increased recurrence risk after a second affected child and no increase in consanguinity. Another cause of familial disorders with recurrence restricted to sibs which has received little attention is germline mosaicism for a mutation expressed as a dominant. If, for example, an embryonic mutation resulted in half the precursors of the germ cells carrying a mutation with dominant expression, then the proportion of haploid nuclei conveying the mutation, which is the recurrence risk, would be a quarter. If severity precluded reproduction the disorder would tend to be classified as a recessive. While germline mosaicism will rarely be expressed with such a high recurrence risk, the estimation of this risk in rare disorders is difficult due to extreme and unpredictable bias in ascertainment.

A simple method for calculating risks before DNA analysis

Calculation of carrier risk of an X linked disease may be performed on a small computer after DNA analysis, but a method for rapid hand estimation of the risk is still useful for a quick check of the results and weighing the relative importance of each element of information, such as the determination of a haplotype. Each risk estimation is a function of a prior risk and the product of likelihood ratios and these terms are derived themselves from parameters such as fitness or the relative mutation rate in male and female gametes. Even if it is often difficult to have strong experimental estimation of these variables, the existence of a normal father or grandfather must be considered whenever male fitness is not null. The likelihood ratio for a woman for not being a carrier, when her father is not affected and her mother has herself a likelihood R for not having the mutated gene, may be expressed as the ratio 2R/(CmR + 1), with Cm being a function of male fitness and relative mutation rate. Cm represents the odds ratio for the mother of a carrier not to be a carrier, given that the father of the known carrier is not affected. This formula can be used recurrently and reduces to 2R/(R + 1) in lethal X linked disease. When likelihood ratios are expressed as an algebraic function, maximum values are easily determined, hence fixing the limits of DNA analysis.

The analysis of close linkage in large families

In large families, if two closely linked loci both have rare alleles and several distant members are ascertained through one having rare alleles at both loci, simple estimates of their recombination fraction are possible. This information is free from errors due to both reduced penetrance and erratic paternity. Simple estimates based on counting will often have high efficiency and limited bias. Some problems of linkage analysis between loci with codominant expression in complete three generation families are also considered. The omission of individuals of uncertain genotype at the test locus will be more efficient than their inclusion.

Germinal mosaicism in Duchenne muscular dystrophy

We have identified a Duchenne muscular dystrophy (DMD) pedigree where the disease is associated with a molecular deletion within the DMD locus. We have examined the meiotic segregation products of the common female ancestor using marker restriction fragment length polymorphisms (RFLPs) detected by probes that lie within this deletion. These studies show that this female has transmitted three distinct types of X chromosome to her offspring. This observation may be explained by postulating that the mutation arose as a postzygotic deletion within this common ancestor, who was consequently germinally mosaic.

Noonan syndrome

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Gonadal mosaicism in pseudoachondroplasia

We report on a family in which a brother and sister have pseudoachondroplasia and normal parents. The brother married a normal woman, and they have 2 daughters; one of them has typical changes of pseudoachondroplasia, the other is normal. The most likely explanation in this family is gonadal (germinal cell) mosaicism in one of the grandparents. Other reports of possible autosomal recessive pseudoachondroplasia are reviewed. It is likely that gonadal mosaicism is responsible for a small percentage of cases with what appears to be a new mutation for pseudoachondroplasia.