Genetics: Mendelian Inheritance Block I

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Alleles are different but both are abnormal; may modify phenotype in contrast to a disease resulting from 2 copies of the same mutation

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Compound heterozygote

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Single gene, multiple effects

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Pleiotropy

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Phenotypic differences in individuals carrying the same allele

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Variable expressivity

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Probability that a gene will have any phenotypic expression:

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Penetrance

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An individual has the genotype but does not exhibt the disease phenotype

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Reduced (incomplete) penetrance

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Quantitative and qualitative differences in phenotype between individuals having the same allele

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Variable expressivity.

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Multiple phenotypic effects of a single gene:

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Pleiotropism

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Autosomal dominant fibrillinopathy resulting in musculoskeletal abnormalities, cardiovascular lesions (aortic root dilatation, valvular disease), and ocular defects (dislocated lens).

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Marfan Syndrome

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Progressive, clincally variable autosomal dominant disorder characterized by multiple cafe-au-lait spots, peripheral neurofibromas, Iris hamartomas (Lisch nodules), optic pathway glioma, and tibial hypoplasia.

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Neurofibromatosis 1

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This is a rare, multi-system genetic disease that causes benign tumours to grow in the brain and on other vital organs such as the kidneys, heart, eyes, lungs, and skin. A combination of symptoms may include seizures, developmental delay, behavioral problems, skin abnormalities, lung and kidney disease. TSC is caused by mutations on either of two genes, TSC1 and TSC2, which encode for the proteins hamartin and tuberin respectively. These proteins act as tumour growth suppressors, agents that regulate cell proliferation and differentiation.

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Tuberous Sclerosis Complex

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Major diagnostic criteria include facial angiofibroma, forehead plaque, periungual fibroma, hypomelanotic macules, multiple retinal hamartomas, and shagreen patches.

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Tuberous Scleosis Complex

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Autosomal recessive disorder, whose infantile form (Werdnig-Hoffman disease) presents early in life as a floppy infant and usually results in death before 2 years of age.

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Spinal muscular atrophy

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The two genes associated with this disorder are SMN1 and SMN2 -

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Spinal muscular atrophy

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X linked recessive disorder characterized by progressive muscle weakness, cardiomyopathy, usually presents in males, 2/3 of affected females are carriers with elevated CPK.

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Duchenne Muscular Dystrophy

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neurodevelopmental disorder affecting grey matter. The clinical features include small hands and feet and a deceleration of the rate of head growth (including microcephaly in some). Repetitive hand movements such as mouthing or wringing are also noted. Girls with this syndrome are prone to gastrointestinal disorders and up to 80% have seizures. They typically have no verbal skills, and about 50% of females are not ambulatory. Scoliosis, growth failure, and constipation are very common and can be problematic.

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Rett's Syndrome

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This genetic condition is caused by mutations in MECP2 resulting in inappropriate brain developement. There is normal devleopment in the first 6-12 months of life followed by loss of skills, poor growth, seizures, autism and sterotypic hand wringing motions- rarely seen in live males.

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Rett Syndrome

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One genetic change has a wide range of effects in multiple organ systems

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Pleiotropy

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This genetic condition is caused by a defect in the fibrillin 1 (FBN1) gene and characteristics include multiple skeletal abnormalities, ectopia lens, aortic root dilitation, and dural ectasia.

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Marfan's Syndrome

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Most common skeletal displasia resulting in short limb form of dwarfism with large head (may be associated with hydrocephalus) small midface, bowed legs, small hands, delayed motor skills but not cognition.

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Achondroplasia

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This genetic condition is caused by a defect in fibroblast growth factor receptor 3 (FGFR3)

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Achondroplasia

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This X-linked recessive disorder is a rare inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT), produced by mutations in the HPRT gene. The HGPRT deficiency causes a build-up of uric acid in all body fluids. This results in both hyperuricemia and hyperuricosuria, associated with severe gout and kidney problems. Neurological signs include poor muscle control and moderate mental retardation. These complications usually appear in the first year of life. Beginning in the second year of life, a particularly striking feature is self-mutilating behaviors, characterized by lip and finger biting,

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Lesch-Nyhan syndrome

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This X-linked recessive disorder is characterized by clumsiness and muscle weakness, psuedohypertropy of the calves due to fat and connective tissue in muscle, mild cognitive deficits, and progressive weakness always results in loss of ambulation by ag 12- as muscle cells die, serum CK becomes markedly elevated

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Duchenne Muscular Dystrophy