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The pattern of inheritance of X-linked traits is not dominant or recessive, just X-linked.

UNLABELLED: Our modern concepts of genetic inheritance originated nearly a century ago. Early concepts of dominant and recessive inheritance were developed in insects and were subsequently applied to sex-linked inheritance in mammals. Years of clinical experience, however, suggest that the modern-day rules for X-linked dominant and recessive diseases do not explain why so many female carriers of X-linked 'recessive' disorders have an abnormal phenotype. In a review of 32 X-linked diseases we revealed an unexpectedly high degree of intermediate disease penetrance in females that cannot be explained by existing concepts. We recommend that the terms 'dominant' and recessive' be abandoned and that these disorders be referred to as X-linked. In this review we will present modified rules for X-linked inheritance and propose hypotheses related to the potential mechanisms that may explain differences in disease expression in females. CONCLUSION: Past assumptions regarding factors that may affect phenotype in heterozygous females do not capture the extraordinarily variable expressivity of X-linked disorders in females and need to be revisited.

Animals↗

Non-Mendelian inheritance of mosquito susceptibility to infection with Brugia malayi and Brugia pahangi.

The mode of inheritance of susceptibility or refractoriness of insect vectors to medically important pathogens such as those causing malaria or filariasis is usually believed to follow normal Mendelian laws and to involve a single pair of alleles. In this report, experiments are described that demonstrate another mode of inheritance of mosquito susceptibility to filarial parasites. Crosses were made between susceptibile and refractory species of the Aedes Scutellaris complex, and the hybrid and backcross progeny were tested for susceptibility to infection by Brugia malayi and Brugia pahangi. The data indicate that inheritance follows a non-Mendelian pattern indicative of extrachromosomal factors inherited through the maternal parent.

Aedes↗

Inheritance of extrachromosomal rDNA in Physarum polycephalum.

In the acellular slime mold Physarum polycephalum, the several hundred genes coding for rRNA are located on linear extrachromosomal DNA molecules of a discrete size, 60 kilobases. Each molecule contains two genes that are arranged in a palindromic fashion and separated by a central spacer region. We investigated how rDNA is inherited after meiosis. Two Physarum amoebal strains, each with an rDNA recognizable by its restriction endonuclease cleavage pattern, were mated, the resulting diploid plasmodium was induced to sporulate, and haploid progeny clones were isolated from the germinated spores. The type of rDNA in each was analyzed by blotting hybridization, with cloned rDNA sequences used as probes. This analysis showed that rDNA was inherited in an all-or-nothing fashion; that is, progeny clones contained one or the other parental rDNA type, but not both. However, the rDNA did not segregate in a simple Mendelian way; one rDNA type was inherited more frequently than the other. The same rDNA type was also in excess in the diploid plasmodium before meiosis, and the relative proportions of the two rDNAs changed after continued plasmodial growth. The proportion of the two rDNA types in the population of progeny clones reflected the proportion in the parent plasmodium before meoisis. The rDNAs in many of the progeny clones contained specific deletions of some of the inverted repeat sequences at the central palindromic symmetry axis. To explain the pattern of inheritance of Physarum rDNA, we postulate that a single copy of rDNA is inserted into each spore or is selectively replicated after meiosis.

Extrachromosomal Inheritance↗

The inheritance of genes in mitochondria and chloroplasts: laws, mechanisms, and models.

The inheritance of mitochondrial and chloroplast genes differs from that of nuclear genes in showing vegetative segregation, uniparental inheritance, intracellular selection, and reduced recombination. Vegetative segregation and some cases of uniparental inheritance are due to stochastic replication and partitioning of organelle genomes. The rate and pattern of vegetative segregation depend partly on the numbers of genomes and of organelles per cell, but more importantly on the extent to which genomes are shared between organelles, their distribution in the cell, the variance in number of replications per molecule, and the variance in numerical and genotypic partitioning of organelles and genomes. Most of these parameters are unknown for most organisms, but a simple binomial probability model using the effective number of genomes is a useful substitute. Studies using new cytological, molecular, and genetic methods are shedding some light on the processes involved in segregation, and also on the mechanisms of intracellular selection and uniparental inheritance in mammals. But significant issues remain unresolved, notably about the extent of paternal transmission and mitochondrial fusion in mammals.

Animals↗

[Analysis of the mode of inheritance in familial bronchial asthma].

To study the mode of inheritance of familial bronchial asthma and to understand the population genetics laws of bronchial asthma, Families with a family history of bronchial asthma in Handan region were investigated using group research method. The xi2 test of the degree of coincidence between the expected and observed values was analyzed by pedigree analysis and the "Smith" agonic revise method. The incidence rate within the 72 families pedigree with familial bronchial asthma, including 109 core pedigrees, is 0.46. Analysis shows a tendency towards single gene inheritance. Pedigree analysis reveals that it is consistent with autosome dominant inheritance. Analysis of the D- x dd marriage with the "Smith" analytical method supports this conclusion (xi2 = 3.181, P > 0.05) and further hints there exists genetic heterogeneity, i.e. different modes of inheritance among different marriage types. Our results can offer the reference to the prevention, diagnosis and treatment of familial bronchial asthma.

Adult↗

Inherited thrombophilia: an update.

Inherited thrombophilia can be defined as a genetically determined predisposition to develop thromboembolic complications. Inherited prothrombotic risk factors include antithrombin deficiency, protein C and protein S deficiencies, activated protein C resistance due to factor V Leiden mutation, inherited hyperhomocysteinemia, prothrombin G20210A variant, dys- and hyperfibrinogenemia and elevated factor VIII levels. In this review we briefly analyze, from an epidemiologic, laboratory and clinical point of view, the main inherited prothrombotic risk factors. Finally, we discuss the synergism between genetic and acquired prothrombotic risk factors in some conditions such as pregnancy and cardiovascular diseases.

Blood Coagulation Disorders, Inherited↗

Increased risk of late posttraumatic seizures associated with inheritance of APOE epsilon4 allele.

BACKGROUND: Late posttraumatic seizures are a common complication of moderate and severe traumatic brain injury. Inheritance of the apolipoprotein E (APOE) epsilon4 allele is associated with increased risk of Alzheimer disease, progression to disability in multiple sclerosis, and poor outcome after traumatic brain injury. OBJECTIVE: To determine whether inheritance of APOE epsilon4 is associated with increased risk of developing late posttraumatic seizures. DESIGN: Prospective study. SETTING: Neurosurgical service at an urban level I trauma center.Patients Patients admitted with a diagnosis of moderate and severe traumatic brain injury were enrolled. METHODS: Six months after injury, patients were contacted to determine functional outcome (according to the Glasgow Outcome Scale-Expanded [GOS-E]) and the presence of late posttraumatic seizures. Genotype at the APOE locus was determined by restriction fragment length polymorphism analysis. RESULTS: DNA and outcome information was obtained from 106 subjects. Six months after injury, 31 (29%) had a poor outcome (GOS-E score, 1-4), 47 (44%) had an intermediate outcome (GOS-E score, 5-6), and 28 (26%) had a favorable outcome (GOS-E score, 7-8). Twenty-one patients (20%) had at least 1 late posttraumatic seizure. The relative risk of late posttraumatic seizures for patients with the epsilon4 allele was 2.41 (95% confidence interval, 1.15-5.07; P =.03). In this cohort, inheritance of APOE epsilon4 was not associated with an unfavorable GOS-E score 6 (P =.47). CONCLUSIONS: Inheritance of the APOE epsilon4 allele is associated with increased risk of late posttraumatic seizures. In this cohort, this risk appears to be independent of an effect of epsilon4 on functional outcome. A better understanding of the molecular role of APOE in neurodegenerative diseases may be helpful in developing antiepileptogenic therapies.

Adult↗

Tamoxifen and breast cancer incidence among women with inherited mutations in BRCA1 and BRCA2: National Surgical Adjuvant Breast and Bowel Project (NSABP-P1) Breast Cancer Prevention Trial.

CONTEXT: Among cancer-free women aged 35 years or older, tamoxifen reduced the incidence of estrogen receptor (ER)-positive but not ER-negative breast cancer. The effect of tamoxifen on breast cancer incidence among women at extremely high risk due to inherited BRCA1 or BRCA2 mutations is unknown. OBJECTIVE: To evaluate the effect of tamoxifen on incidence of breast cancer among cancer-free women with inherited BRCA1 or BRCA2 mutations. DESIGN, SETTING, AND PARTICIPANTS: Genomic analysis of BRCA1 and BRCA2 for 288 women who developed breast cancer after entry into the randomized, double-blind Breast Cancer Prevention Trial of the National Surgical Adjuvant Breast and Bowel Project (between April 1, 1992, and September 30, 1999). MAIN OUTCOME MEASURE: Among women with BRCA1 or BRCA2 mutations, incidence of breast cancer among those who were receiving tamoxifen vs incidence of breast cancer among those receiving placebo. RESULTS: Of the 288 breast cancer cases, 19 (6.6%) inherited disease-predisposing BRCA1 or BRCA2 mutations. Of 8 patients with BRCA1 mutations, 5 received tamoxifen and 3 received placebo (risk ratio, 1.67; 95% confidence interval, 0.32-10.70). Of 11 patients with BRCA2 mutations, 3 received tamoxifen and 8 received placebo (risk ratio, 0.38; 95% confidence interval, 0.06-1.56). From 10 studies, including this one, 83% of BRCA1 breast tumors were ER-negative, whereas 76% of BRCA2 breast tumors were ER-positive. CONCLUSION: Tamoxifen reduced breast cancer incidence among healthy BRCA2 carriers by 62%, similar to the reduction in incidence of ER-positive breast cancer among all women in the Breast Cancer Prevention Trial. In contrast, tamoxifen use beginning at age 35 years or older did not reduce breast cancer incidence among healthy women with inherited BRCA1 mutations. Whether tamoxifen use at a younger age would reduce breast cancer incidence among healthy women with BRCA1 mutations remains unknown.

Adult↗

Congenital vocal cord paralysis with possible autosomal recessive inheritance: case report and review of the literature.

We describe an infant with congenital vocal cord paralysis born to consanguineous parents. While autosomal dominant and X-linked inheritance have been previously reported in this condition, we conclude that the degree of parental consanguinity in this case strongly suggests autosomal recessive inheritance. Although we cannot exclude X-linked inheritance, evidence from animal studies demonstrates autosomal recessive inheritance and provides a possible molecular basis for congenital vocal cord paralysis.

Adult↗

Autosomal dominant inheritance of Barber-Say syndrome.

We report on a mother-to-son transmission of the Barber-Say syndrome, a finding that strongly supports dominant inheritance of this rare disorder. The characteristic facial changes, small ears, hirsutism, and redundant skin of our patients are consistent with the findings of five reported cases. The mother also had cleft palate and mild conductive hearing loss. Her son had a shawl scrotum, primary hypospadias, and mild hearing loss by report. The inheritance of this rare disorder has not been established. The parent-to-child transmission in this family suggests X-linked or autosomal dominant inheritance. The parents of the patient reported by Santana et al. [1993: Am. J. Med. Genet. 47:20-23] were consanguineous, suggesting autosomal recessive inheritance in other cases.

Abnormalities, Multiple↗

Meiotic inheritance of functional Gal80S gene product in Saccharomyces cerevisiae.

Transcription factors inherited during meiosis play a crucial role directing subsequent gene activity. Factors of maternal origin have been shown to influence the pattern of early zygotic transcription during Drosophila and Xenopus embryogenesis. Nevertheless, little is known regarding the meiotic inheritance of the vast majority of transcription factors. In the case of yeast meiosis, for example, it is not yet known whether any of the multitude of transcription factors expressed in the diploid are transmitted to haploid spores in functional form. Here we use a GAL1-STE4 reporter whose activity is detectable in single living cells to identify a transcription factor inherited during sporulation in Saccharomyces cerevisiae. We show that functional Gal80S repressor is meiotically inherited at levels reflecting its expression in the diploid parent.

Diploidy↗

The inheritance of factors associated with joint mobility.

From a sample of 1,500 individuals belonging to 442 migrant nuclear families from northeastern Brazil, information on the interphalangial mobility was obtained: (a) the grades of extension of both the right and left thumbs and (b) the angle (in degrees) formed by the distant and proximal phalanx of the thumb. The first principal component of these variables was estimated and called "extensibility." A negative association of extensibility and age, as well as with inbreeding, was detected. Complex segregation analysis was applied to extensibility and both a multigenic mechanism and an extra transmissible component were detected. Mendelian inheritance was rejected, while a model with multifactorial inheritance, together with a factor that is inherited with a transmission probability different from 1/2 (tau = 0.63), was not rejected. These findings were supported by path analysis, which showed an important biologic inheritance (h2 = 0.675) and the existence of a small but significant cultural component (c2 = 0.003). The observed "inbreeding" effect, therefore, could not be attributed to a genetic mechanism and probably is the effect of concomitant environmental variability.

Brazil↗

Inheritance of the shared epitope and long-term outcomes of rheumatoid arthritis among community-based Caucasian females.

Multiple HLA-DRB1 alleles encoding a shared epitope (SE) at amino acid positions 70-74 are associated with susceptibility and severity of rheumatoid arthritis (RA). We examined the relationship between the number and DRB1 genotype of SE alleles inherited and long-term outcomes of 180 community-based, Caucasian female RA patients followed annually for up to 12 years. Outcomes examined were physician assessment of RA course; annual measures of pain, function, and number of painful joint groups; history of joint surgery; and resource utilization. Models accounted for correlation among serial observations for the same patient and adjusted for patient age and disease stage. We examined two genetic models: a SE model in which patients were classified according to the number of SE copies inherited and a genotype model in which patients were categorized into one of six groups based on the inherited DRB1 genotype. We used likelihood ratio tests to compare these genetic models and to compare alternative model specifications. Our results demonstrate strong associations between inheritance of the SE and long-term outcomes of community-based Caucasian females with RA. However, the pattern of results is not consistent across the outcomes. An additive model of risk is apparent for history of joint surgery and RA hospitalization. In contrast, a near reversal of this pattern is apparent for function, joint pain, pain rating, and RA physician visits. Finally, although the genotype model did not appear to be a better predictive model for RA outcomes overall, it did reveal some striking heterogeneity of SE alleles that was masked by the more parsimonious SE model. For example, the odds ratio (OR) for joint surgery for patients with 2 SE copies (OR = 3.16) reflects an average of 2 very different ORs when patients are further categorized according to genotype groups 4 and 5 (OR = 1.3 and 11.9, respectively).

Alleles↗

Genetic inheritance of body mass index in African-American and African families.

Numerous studies have shown recessive major gene inheritance of body mass index (BMI) in white populations; few have examined the inheritance of BMI in the African-American population where obesity is more prevalent, nor in African populations where obesity is comparatively rare. To evaluate the inheritance of obesity in two different populations of African origin, we used segregation analysis to determine the transmission of BMI in 95 African-American families and 400 Nigerian families. Probands were selected from participants in the population-based International Collaborative Study on Hypertension in Blacks. Using class D regressive models, results from the segregation analysis of the African-American data showed evidence of a major gene effect on BMI. The Nigerian results were strikingly similar, with comparable estimates for the genotype frequencies and means and strong evidence for a major effect in the transmission of BMI. The high BMI allele frequency estimate of 24% is consistent with estimates in other studies, but the mode of transmission appeared codominant, which differs from studies involving predominantly white populations. In the Nigerian analysis, however, the probability of a high BMI homozygote parent transmitting a low BMI allele to his/her offspring was significantly different from the Mendelian expectation of zero (estimated tau(BB) = 0.45), suggesting that additional complexities exist in the major gene inheritance of BMI in this population. The strong similarity of the genotype frequencies and means obtained from the African-American and Nigerian samples suggests that a common codominant major gene effect may contribute to the variation in BMI in both populations.

Alleles↗

Family study of serum carcinoembryonic antigen in inherited medullary carcinoma of the thyroid.

Serum carcinoembryonic antigen (CEA) and calcitonin were assayed in 8 patients with medullary carcinoma of the thyroid (MCT) and 14 unaffected family members, from 4 pedigrees of Sipple's syndrome and one pedigree with inherited MCT. Serum CEA ranged from 5.4 to 44.5 ng/ml in the patients, but less than 2.5 ng/ml in the unaffected. This is in contrast with retinoblastoma and cancer family syndrome, where not only the patients but also unaffected family members show high serum CEA, and cytoplasmic or selective maternal inheritance of serum CEA level is presumed. In inheritable MCT, serum CEA increases in association with the development of MCT, and serum CEA level per se is not inherited.

Adolescent↗

Patterns of inheritance of ovarian cancer. An analysis from an ovarian cancer screening program.

BACKGROUND: A variety of inheritance patterns for familial ovarian cancer have been proposed including an autosomal dominant inheritance, a breast-ovary cancer syndrome and Lynch Cancer Family Syndrome (involving breast, bowel, ovary, and endometrial cancers). METHODS: Women participating in an ovarian cancer screening study completed a questionnaire concerning their family history of ovarian and other malignancies (in particular breast, bowel, and endometrial cancer). Confirmation of the diagnosis was sought when there was uncertainty. RESULTS: Two hundred forty women with a first-degree relative with ovarian cancer participated in the study. Nine percent of these women (representing 13 families) gave a definite history of two or more affected first-degree relatives. Two families had a pedigree consistent with an autosomal dominant inheritance. A breast-ovary cancer family and a Lynch cancer family syndrome were suspected in one family each, although 34% of all women gave a history of at least one other first-degree relative with either breast, bowel, or endometrial cancer. CONCLUSIONS: Only a small number of women with a family history of ovarian cancer fit into the recognized hereditary patterns. Difficulty in recognizing the inheritance patterns and the lack of definitive genetic markers poses problems in providing adequate counseling regarding screening and prophylactic oophorectomy.

Adenocarcinoma↗

Electrophysiological features of inherited demyelinating neuropathies: A reappraisal in the era of molecular diagnosis.

The observation that inherited demyelinating neuropathies have uniform conduction slowing and that acquired disorders have nonuniform or multifocal slowing was made prior to the identification of mutations in myelin-specific genes which cause many of the inherited disorders involving peripheral nerve myelin. It is now clear that the electrophysiological aspects of these disorders are more complex than previously realized. Specifically, certain mutations appear to induce nonuniform slowing of conduction which resemble the findings in acquired demyelinating neuropathies. It is clinically important to recognize the different electrodiagnostic patterns of the various inherited demyelinating neuropathies. In addition, an understanding of the relationship between mutations of specific genes and their associated neurophysiological findings is likely to facilitate understanding of the role of these myelin proteins in peripheral nerve function and of how abnormalities in myelin proteins lead to neuropathy. We therefore review the current information on the electrophysiological features of the inherited demyelinating neuropathies in hopes of clarifying their electrodiagnostic features and to shed light on the physiological consequences of the different genetic mutations.

Electrophysiology↗

Unusual inheritance patterns due to dynamic mutation in fragile X syndrome.

Fragile X syndrome is the most common form of familial mental retardation and is one of the world's most common genetic diseases. The inheritance patterns of the disease have many unusual features. It is an X-linked disorder yet there are asymptomatic carrier males. The disease is expressed only when the gene is inherited from the mother. The risk of a carrier woman having a child with the syndrome depends upon her position in the pedigree (the Sherman paradox) and her own intellectual status. The discovery that the disease is due to dynamic mutation (which is a multistage process) that inactivates FMR1 has provided an explanation for the unusual inheritance patterns. The finding of linkage disequilibrium between the fragile X mutations and closely linked DNA markers (haplotype) has required a reinterpretation of this phenomenon for dynamic mutations. Only a small number of normal alleles at the fragile X locus have long stretches of perfect repeat (2% with more than 24 copies) and these form a reservoir of alleles that can increase in length into the premutation range. Dynamic mutation is, so far, an exclusively human phenomenon, but this is probably because it has yet to be discovered in other species. Unusual inheritance patterns are a hallmark of dynamic mutation diseases.

Alleles↗