MRCOG UK Part-1 | Sample Lecture | Genetic Disorders | SsAcademy

MRCOG_UK_Part_1_|_Sample_Lecture_|_Genetic_Disorders_|_SsAcademy

Genetic_Disorders
Description:
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1. Genetic disorders consist of 3 type
• Autosomal
i. Autosomal dominant
ii. Autosomal recessive
• Sex linked
• Mitochondrial

2. Autosomal dominant
• The trait / condition is expressed in heterozygous individuals
• Males and females affected in equal numbers
• Vertical pedigree pattern - does not skip generations
• Males and females can transmit the condition to their male / female off-spring
• Unaffected individuals cannot transmit the disease (the exception is in conditions with variable or non-penetrance - where individuals with the mutant allele have a normal phenotype but can transmit the condition)
• Show variable expressivity - both in terms of severity of the condition and age of onset
• Affected individuals have a 1 in 2 risk of their offspring being affected
• There is a paternal age effect with respect to the incidence of new mutations
• Examples include: otosclerosis, von Willebrand disease, familial hypercholesterolaemia, Huntington’s disease, Tuberous sclerosis, Myotonic dystrophy, Neurofibromatosis, Hereditary spherocytosis, familial polyposis coli

3. Autosomal recessive
• The trait / condition is only expressed in homozygous individuals
• Males and females are affected equally
• Horizontal pedigree pattern
• Both males and females can transmit the trait to their male / female offspring
• If both parents are carriers, 1 in 4 offspring will have the disease, 1 in 4 will be normal homozygotes and 1 in 2 will be normal carriers
• The offspring of an affected individual must either have the disease or be carriers as there is no normal allele
• Variable penetrance and variable expressivity are features of autosomal dominant inheritance
• There is an increased risk of autosomal recessive conditions in the offspring of consanguineous marriages
• Examples include: Cystic fibrosis, sickle cell disease, phenylketonuria, adrenogenital syndrome, mucopolysaccharidoses, spinal muscular atrophy

4. X-Linked recessive
• Inheritance = 1 : 2 sons of carrier females
• Males are affected predominantly
• Male-male transmission never occurs. Unaffected males never transmit the condition
• Affected males transmit the allele to their daughters who become heterozygous carriers and are almost always unaffected. Lyonisation (random inactivation of one X chromosome) may result in some females having mild symptoms
• Knight’s move pedigree pattern
• The severity of the disease is uniform across generations
• A woman with an affected son and an affected brother is an obligate carrier as the possibility of two independent new mutations is extremely small. A woman with one affected son is not an obligate carrier
• Examples include: red-green colour blindness, Duchenne / Becker muscular dystrophy, Haemophilia A / B, X-linked agammaglobulinaemia
• Females can be affected by X-linked recessive conditions due to
• Atypical lyonisation - random inactivation of (predominantly) the normal X chromosome
• X-autosome translocation under which circumstances the normal X chromosome is preferentially inactivated (otherwise an autosomal monosomy would result)
• Turner syndrome (45X)
• A new mutation on the second X chromosome

5. X-Linked dominant
• Inheritance = 1 : 2 offspring of affected females
• Often manifest very severely in males , frequently leading to spontaneous loss or neonatal death of affected male pregnancies
• Includes
i. Incontinentiapigmenti
ii. Rett syndrome
iii. Vitamin D resistance rickets

6. Mitochondrial
• Mitochondrial DNA is inherited through the material line because sperm do not contribute to the zygote beyond their nuclear DNA
• A mitochondrially inherited condition can affect both sexes but is only passed on by affected mothers
• Includes
i. Leber’s hereditary optic neuropathy
ii. Leigh’s syndrome
1. Genetic disorders consist of 3 type
• Autosomal
i. Autosomal dominant
ii. Autosomal recessive
• Sex linked
• Mitochondrial

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