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Basic Medical Genetics Concepts PowerPoint Presentation

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This medical is about the basic concepts in medical genetics.

Medical genetics is the branch of medicine that studies the role of genetics in health and disease. Here are some basic concepts in medical genetics:

DNA: DNA, or deoxyribonucleic acid, is the molecule that carries genetic information in all living organisms. It is composed of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C).

Genes: Genes are segments of DNA that encode instructions for making proteins. Proteins are the building blocks of cells and perform a variety of functions in the body.

Chromosomes: Chromosomes are structures in the cell nucleus that carry genes. Humans have 23 pairs of chromosomes, for a total of 46.

Alleles: Alleles are different versions of a gene. For example, there are different alleles for eye color, such as brown, blue, and green.

Genotype: An individuals genotype refers to the specific combination of alleles they have for a given gene.

Phenotype: An individuals phenotype refers to the observable characteristics that result from their genotype and interactions with the environment. For example, eye color is a phenotype that is influenced by genetics and other factors such as light exposure.

Inheritance patterns: Different genetic conditions are inherited in different ways. For example, autosomal dominant conditions are caused by a single copy of a mutated gene, while autosomal recessive conditions require two copies of the mutated gene (one from each parent).

Genetic testing: Genetic testing involves analyzing an individuals DNA to look for specific mutations or variations that may be associated with a particular condition.

Genetic counseling: Genetic counseling involves working with a healthcare professional who specializes in medical genetics to discuss the risks and implications of genetic testing and to help individuals make informed decisions about their healthcare.

Tags : medical genetics, DNA, deoxyribonucleic acid, Genes, Chromosomes, Alleles, Genotype, Phenotype, Inheritance patterns, Genetic testing, Genetic counseling, Medical Genetics, Medical Genetics

Published on : Jun 01, 2023 | Views : 1210 | Downloads : 1

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BASIC MEDICAL GENETICS CONCEPTS
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mutation effect on protein function phenotypic expression classes of genetic disease
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Mutations result in different alleles alleles are classified as “dominant” or “recessive” dominant phenotypes – observable in heterozygotes recessive phenotypes – observable only in homozygotes
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loss-of-function (most common) e.g. Decreased amount normal protein: Inborn errors of metabolism as in Tay-Sachs [recessive] Haploinsufficiency as in FH [dominant] gain-of-function e.g. Gene dosage effects as in trisomy 21 [dominant]; Dominant-negative effect as in OI [dominant] Abnormal protein properties as in HD [dominant] novel property e.g. HbS [recessive] inappropriate expression e.g. Oncogenes in cancer Mutations are classified by effect on protein function
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complementation as in XP, profound hearing loss penetrance (100% - achondroplasia - unusual) variable expression Variations in phenotypic manifestation of mutant alleles are due to:
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allelic heterogeneity: hemophilia variants locus heterogeneity: hyperphenylalanemias modifier loci: Waardenburg syndrome (methylation); Alzheimer’s (multiple genes); SNPs environment (XP, α-1 antitrypsin deficiency) Causes of variable expression:
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Common classes of genetic disease: enzyme defects (PKU; Lesch-Nyhan; Tay-Sachs; I-cell disease; XP) Almost always recessive. Pathophysiology due to substrate accumulation,product deficiency, or both. When substrate is diffusible, the pathophysiology is unpredictable; when substrate can’t diffuse, the cell in which it accumulates is damaged. Several enzyme functions can be affected.
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Defects in receptor proteins (Familial hypercholesterolemia) Transport defects (Cystic fibrosis) Disorders of structural proteins (Duchenne muscular dystrophy; Osteogenesis imperfecta) Neurodegenerative disorders (Alzheimer’s disease; triplet repeat disorders) Mitochondrial diseases (MELAS, MERRF) Pharmacogenetic diseases (malignant hyperthermia; G6DP) Common classes of genetic disease (cont.):
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Triplet Repeat Disorders Dynamic expansion of DNA triplet repeats Normal alleles polymorphic Inheritance dominant or recessive Presymptomatic, symptomatic expansion size varies Base sequence, location of repeat varies Parent-of-origin effects on repeat expansion varies (anticipation) Stability during meiosis and mitosis varies (variable expression)
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Huntington Disease (autosomal dominant) Spinobulbar muscular atrophy (X-linked recessive; androgen receptor) CAG repeat Anticipation: expansion occurs preferentially during male gametogenesis Variable expression: mitotic instability low (limited mosaicism) Protein aggregation, not loss-of-function Polyglutamine disorders
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Fragile X Syndrome X-linked recessive CGG repeat in 5’ untranslated region of FRA gene (posttranscriptional regulator; methylation effects) Most common form of hereditary mental retardation Anticipation: expansion occurs preferentially in female gametogenesis Variable expression: Mitotic instability high Disease caused by loss of function; very large expansions needed
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Myotonic Dystrophy Autosomal dominant CTG repeat in 3’ untranslated region of protein kinase gene; mechanism of pathophysiology unknown. Anticipation: either parent can transmit amplified copy; massive expansion occurs only in maternal gametogenesis Variable expression: mitotic instability high Abnormal transcript processing, not deletions, point mutations, etc.
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Freidreich ataxia Autosomal recessive GAA repeat in intron of mitochondrial gene frataxin (involved in iron metabolism). Anticipation: no parent of origin effects Variable expression: mitotic instability low Loss of function 4% are compound heterozygotes (expansion/point mutation)
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Mitochondrial Disorders 3 types of mutations missense mutations in coding regions of genes that alter activity of OXPHOS proteins (Leigh disease-ATPase) point mutations in tRNA or rRNA genes that impair mitochondrial protein synthesis (MELAS; MERRF) rearrangements that generate deletions/duplications in mtDNA ( not usually transmitted from affected mother to offspring; disorders occur as sporadic new cases-Kearns-Sayre syndrome) Maternal inheritance Usually heteroplasmic (phenotypic expression: reduced penetrance, variable expression, pleiotropy)
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Pharmacogenetic Diseases Unanticipated reactions to medications largely/entirely genetic (6.7% incidence in American hospitals; 0.3% fatal). Single gene defects or multifactorial Examples: Malignant hyperthermia (autosomal dominant-Ca+ release channel; other loci) Acute Intermittent Porphyria (autosomal dominant disease: drug-related alteration in gene expression of heme biosynthetic enzyme) G6PD (X-linked recessive; more than 400 variants; most common disease-producing single gene enzyme defect of humans) Acetylation polymorphism (slow or rapid drug inactivation)
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PRINCIPLES OF CLINICAL CYTOGENETICS
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Common chromosome structural disorders Chromosome banding Aneuploidies: nondisjunction Chromosome breakage syndromes; translocation Faulty DNA metabolism chromosome syndromes Genomic imprinting; UDP Sex reversal
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Banding
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Banding
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Sister chromatid exchanges in Bloom syndrome
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Banding (FISH)
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Aneuploidy: Nondisjunction
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The phenotypes associated with sex chromosome trisomies are less severe than those associated with autosomal trisomies.
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Clinical phenotype of Turner syndrome is due to haploinsufficiency.
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Structural chromosome abnormalities arising from chromosome breakage: Deletions Ring chromosomes Isochromosomes Translocation
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Cri du chat, 5p- Deletions
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Ring Chromosomes; Isochromosomes
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Translocation examples and consequences
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Translocation Reciprocal translocation Robertsonian translocation
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46,XY,t(11;16)(q24;q23)
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Translocation PAIRING AT MEIOSIS
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Translocation: non-random X-inactivation
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Structural chromosomal abnormalities arising from faulty DNA metabolism: Slipped mispairing during DNA replication Nonreciprocal recombination
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Slipped mispairing during DNA replication
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N = normal P = unmethylated premutation MN/P = methylated normal or premutation F = methylated full mutation EclXI: methylation sensitive MN/P
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Non-reciprocal recombination
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Non-reciprocal recombination
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Genomic Imprinting: mechanism distribution consequences
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Map of Imprinted Regions in Human Genome Maternally inherited homolog (left) Paternally inherited homolog (right)
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gene A
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Genomic Imprinting
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Sex Reversal: Role of the sry gene
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Sex reversal due to translocation of SRY from Y to X