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Classification of Genetic disorders: I. Classical Genetic Diseases: 1. Chromosomal (Cytogenetic) disorders. 2. Single gene (or unifactorial) disorders (Mendelian Disorders). 3. Multifactorial disorders. II. Non-Classical Diseases "or the single gene disorders with atypical pattern of inheritance": i. Diseases caused by mutations in mitochondrial genes. ii. Triplet repeat mutations. iii. Uniparental disomy / Genomic imprinting. iv. Gonadal mosaism. Added to that, is the group of congenital malformations (Birth Defects). 2. Defects of Single Genes with Large Effect (Unifactorial or Mendelian Disorders) The number of known Mendelian disorders has grown to more than 5000. Although individually some are rare, altogether they account for about 1% of all adult hospital admissions and about 6-8% of all pediatric hospital admissions. They are caused by a mutation in a single gene. A mutation is a disturbance in the sequence of the nucleotide arrangement in the DNA molecule, or it is simply a permanent change in the DNA. Mutations affecting the germ cells are transmitted to the progeny and may give rise to inherited disorders. Those occurring in the somatic cells are important in the causation of cancers and some congenital malformation. A gene is that part of the DNA that codes for a polypeptide chain (or RNA). 30,000 genes (in contrast to what was previously though of about 100,000). Exons Introns (Intragenic or intergenic) Localization and structure of the factor VIII gene, located about 1000kb from the Xq telomere (Xqter). The gene is 186kb long and contains 26 exons . Dominant genes Recessive genes Autosomal X-linked Codominant Pleiotropy Genetic heterogeneity. Marfan syndrome is a connective tissue disorder, so affects many structures, including the skeleton, lungs, eyes, heart and blood vessels. The disease is characterized by unusually long limbs, and is believed to have affected Abraham Lincoln and Ausama Bin Laden. Marfan syndrome is an AD disorder that has been linked to the FBN1 gene on chromosome 15. FBN1 encodes a protein called fibrillin, which is essential for the formation of elastic fibres found in connective tissue. Without the structural support provided by fibrillin, many tissues are weakened, which can have severe consequences, for example, ruptures in the walls of major arteries. ANY QUESTION? NOTHING COMES EASILY A tree needs to be nurtured for a long time to become green, strong and fruitful Why some genes act in a dominant manner while others behave in a recessive one? Gene = Protein X Gene = Polypeptide (true to some extent) A single gene is responsible for the formation of a single type of polypeptide The types of proteins are varied; they could be structural proteins, like fibrous tissue, elastic tissue; they could be immunoglobulins; they could be signal proteins; they could be receptors, enzymes, hormones, … etc. Therefore, the action of the gene being dominant or recessive is determined by the type of protein it produces and its function. (not by the size or structure or no. of exons of the gene) Dominant genes usually produce two types of proteins, either: 1. Major structural (or key non-enzymatic) proteins (e.g. collagen, spectrin, etc.); examples are cases of achondroplasia & Ehler Danlos syndrome (lax joints and skin). 2. A key enzyme in a complex metabolic pathway usually under feedback control (AD porphyria) 3. a membrane receptor regulating a metabolic pathway or a membrane transport protein (AD familial hypercholesterolemia). HMG-CoA reductase (3-hydroxy-3-methylglutaryl coenzyme A reductase), forms cholesterol from fatty acids, ACAT (acyl-CoA:cholesterol) transferase, hydrolyzes cholesterol into ester rendering it inactive As for recessive genes, they code for proteins, enzymes which usually share in catabolic pathways and when both alleles are defective, there is no protein, i.e. no enzyme and therefore the catabolic pathway is obstructed with the accumulation of the biochemical substrate. Accumulation of the substrate in sensitive tissues will results in disease state. Examples of those diseases are some of diseases of mucopolysaccharidosis, lipidosis, phenylketonuria (PKU), albinism, and many others. Thyronine 4 1 2 Diet Phenylalanine Tyrosine L-DOPA Melanin 3 phenylpyruvic acid Homogenistic acid (excr. In urine in PKU) Acetoacetic acid 1 = Phenyl alanine hydroxylase PKU 2 = Tyrosinase albinism 3 = Homogenistic acid oxidase Alkaponuria (dark urine) 4= enzyme deficiencies that interfere with thyroxine biosynthetic pathway Cretinism ANY QUESTION? ALL HAS WEAKNESSES & STRENGTHS A butterfly is fragile but beautiful, useful and free Sex-linked diseases “X”-linked [both AD & AR] Lyon’s hypothesis deviate from expected pattern for AD or AR (female manifesting carriers) Lyon’s hypothesis states that in a female’s autosomal cells, all the “X” chromosomes will be inactivated during interphase except one which remains active. • It takes place early in the post-fertilization period, 19-20 days P.F. • It is random. • All the daughter cells that descend from the inactive X, the same “X” will remain inactive. This means that about 50% of the “X” chromosomes are inactivated. Therefore, the body of the female is a mosaic concerning the active “X” functioning. So, the female is considered heterozygote in regard to the origin of the Xchromosome. X-linked dominant disorders: Vitamin – D resistant rickets ANY QUESTION? Although SHE sits in the dark, but SHE looks to the light Etiology: All single gene diseases are due to mutations, which are of different types: 1. Single point mutation. 2. Addition – Deletion mutation. 3. Unequal crossing over. 1. Single point mutation, which is the commonest type. They usually result from a change in one of the nucleotide bases that form the trios (three bases), each of which codes for a specific amino acid in the protein molecule. The sequences are the following: 20 amino acids 4 bases in triplets (43 = 64 possible codes) 3 of them are stop codons 61 possibilities for only 20 a.a. = genetic code redundancy 1. Silent mutation (Redundancy of the code) 2. Neutral mutation 3. Missense mutation: new code coding for a different amino acid in the protein changing its character and behaviour resulting in disease, e.g. some cases of thalassemia, sickle cell disease, PKU. 4. Mutation of the termination codon, 1. e.g. Constant Spring type of haemoglobin (HbCS), where the termination codon of α-polypeptide of Hb is mutated resulting in addition of 31 amino acids to the original 14 amino acids of the normal chain 2. Non-sense mutation. Consequences of base pair substitutions (A) Silent mutation: The base pair substitution in the DNA codon does not change the coding specificity. (B) Neutral mutation. The base pair substitution changes the amino acid specified by the DNA codon, but the replacement amino acid has physicochemical properties similar to the original one. (C) Missense mutation. The base pair substitution changes the amino acid specified by the DNA codon. (D) Nonsense mutation. Base pair substitution changes a DNA codon that codes for an amino acid into one that specifies a stop codon. 2. Addition – deletion mutations. They could be one of four types: a. Addition or deletion of a single base. This will result in a shift in the reading frame changing the whole reading of the trio creating a new type of protein or sometimes it creates a termination code in the center of the molecule and the resulting polypeptide chain is shorter or called truncated proteins. These types of mutations are called frame-shift mutations. Consequences of frameshift mutations (A) A portion of a coding region of a structural gene with the expected transcribed and translated sequences. (B) The insertion of a base pair (bold letters) after the second nucleotide site of the DNA sequence presented in a changes the reading frame. (C) The deletion of the base pair at the third nucleotide site of the DNA sequence presented in A changes the reading frame. b. Much less commonly addition / deletion of two bases same consequences. c. Addition or deletion of 3 bases or the multiple of 3, i.e. 6, 9, 12, 15 … etc. This will lead to addition of 1, 2, 3, 4, … etc amino acid(s) in the protein molecule leading to abnormal protein, i.e. Frieberg Hb, where 5 amino acids (i.e. 15 bases) is added between amino acids 78-79 sequence in β-Hb polypeptide. d. Addition of deletion of a large piece of DNA inside the gene (intragenic) or in between the gene (intergenic). Again this creates variability and may lead to a disease state but it is used for genetic testing and diagnosis of some genetic diseases. 3. Unequal crossing over: This case takes place in sites of the DNA where there are grouping of genes of similar DNA structure with very slight variation so one gene is mistaken for a different gene as being its allele and if crossing over occurs. Then a defect will result leading to the formation of two unbalanced homologues, one containing more genes and another less genes and both containing a hybrid (mixed) new gene. ANY QUESTION? Be like our home planet, active and alive although your surrounding is not !! Multifactorial Inheritance (MFI) Multifactorial (also called polygenic) inheritance is involved in many of the physiologic characteristics (e.g. weight, height, blood pressure, hair color, etc.). A multifactorial physiologic or pathologic trait may be defined as a trait governed by the additive effect of two or more genes of small effect but conditioned by environmental, non-genetic influences. Even monozygotic twins reared separately may achieve different heights because of nutritional or other environmental influences. This form of inheritance is believed to underlie such common diseases as diabetes mellitus, hypertension, gout, schizophrenia, bipolar disorders and certain forms of congenital heart disease as well as some skeletal abnormalities. It is of value to mention that multifactorial inheritance differs from congenital malformation. In the latter, environmental factors, genetic causes (chromosomal disorders, single gene disorders, or multifactorial inheritance), physical agents (heart, pressure, radiation, etc.), maternal disorders, prenatal infection, etc. all are causes of congenital malformations, but each one is a sole cause at a time, and the disease is not the result of the additive effect of more than one of those factors. In multifactorial inheritance, it is the additive effect of more than one gene of small effect PLUS a suitable environment causes such disorders. In single gene disorders, individuals in regard to the abnormal gene are one of 3 groups: a heterozygote (carrying one mutated and one normal gene and thus affected in AD and not affected in AR disorders), a homozygote for the mutated gene (and thus affected in all cases), or a homozygote normal. There is no gradient in between these 3 groups. In MFI, we could group individuals in a community into many different grades, which have a normal distribution curve (Gaussian distribution) with a threshold point, which when exceeded, the disorder is expressed. The facts that MFI are affected by many genes (not just one) and that the additive effect of both genes and the environment determine the expression of MF disorder], are called Genetic Liability (or genetic predisposition) of the individual and this liability can be measured. So, MFI is only partially genetic (unlike other types of inheritance) and needs environmental factors to act for the disorder to appear. Some examples of MFI most relevant to the clinical geneticist: 1. Neural tube defects. 2. Cleft lip and/or palate. 3. Heart defects (PDA, VSD, ASD, etc.). 4. Pyloric stenosis. 5. Late-onset conditions such as hypertension, diabetes mellitus, schizophrenia, Alzheimer disease, ...etc. ANY QUESTION? An eaten apple is a symbol of a powerful worldwide corporation