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SINGLE GENE DISORDER Learning Objectives • • • • • • Define pedigree Identify symbols required to make pedigree Define single gene disorders Enlist different types of single gene disorders Describe genetic changes which occur in these disorders Enlist characteristics of single gene which gives variation in expression of diseases Pedigree • • Family tree First person identified – Index case proband, propositus or proposita Definition • • • The single gene is responsible for the disorder Mendelian pattern of inheritance Over 10,000 identified -2 % of general population • • • • • • • • • Huntington Disease Myotonic dystrophy Hereditary motor and sensory neuropathy Neurofibromatosis Marfan syndrome Cystic fibrosis Inherited cardiac arrhythmias and cardiomyopathies Spinal muscular atrophy Duchenne muscular dystrophy • Hemophilia Huntington Disease • • • • 1872 Dr.George Huntington • • • Genetics AD, penetrance, low mutation rate, anticipation markers • located at 5’ region • • • • • Expansion of CAG Normal alleles 26 or few CAG repeats – new mutation -39 CAG late onset or complete absence of disease expression (non penetrance) • Myotonic Dystrophy • • • • Common MD 1 in 8000 Slow progressive weakness and myotonia, cardiac conduction defects, cataract Genetics consists of 37 repeats. Marfan Syndrome • • Beal syndrome, congenital contractual arachnodactyly AD Fibrous connective tissue type 1 fibrillin glycoprotein FBN1 gene • Cystic Fibrosis • • • • • Mucoviscidosis AR 1 in 2000 – 1 in 3000 Chronic lung disease, pancreatic function is impaired CFTR exons • • • • Mutation F 508 (70%) Over 1300 mutations (missences, framshift, splice site, nonsence & deletion) . Duchenne Muscular Dystrophy • • • • • BMD 1 in 3500 males DMD 1 in 20,000 males Usually slow progressively muscle weakness Genetics X linked recessive inheritance Dystrophin gene • • and promoter mutations Hemophilia • • Inherited coagulation disorder 1 in 5000 males activation of prothrombin to thrombin • • • • • 186 kb with 26 exons 9kb mRNA transcrip nonsense and missence mutation Factor IX gene 34 kb with 8 exons Variable Expression Expressivity is a term used in genetics to refer to variations in a phenotype among individuals carrying a particular genotype Variable expressivity occurs when a phenotype is expressed to a different degree among individuals with the same genotype. Example: Hemochromatosis Incomplete Penetrance • • • Penetrance: The frequency with which a heritable trait is manifested by individuals carrying the principal gene or genes conditioning it. when penetrance is less than 100%. incomplete penetrance Example: Familial cancer, retinoblastoma Pleiotrophy • • Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously Example : Marfan Syndrome. Locus hetrozygosity • • • It exists when the same disease phenotype can be caused by mutation in different loci It is especially important when genetic testing is performed by testing for mutation at specific loci. Example: Osteogenesis imperfecta type 2 Anticipation • • • Its a phenomenon whereby the symptoms of a genetic disorder become apparent at an earlier age as it is passed on to the next generation. In most cases, an increase of severity of symptoms is also noted. Anticipation is common in trinucleotide repeat disorders such as Huntington's disease and myotonic dystrophy where a dynamic mutation in DNA occurs. • Imprinting A small no. of genes are transcriptionally active only when transmitted by one of the two parents Example: Prader-Willi Syndrome • Referance Kaplan Biochemistry and Medical Genetics •