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Chapter: Medicine Study Notes : Genetics and Cancer

Patterns of inheritance

Autosomal Dominant o Single gene abnormalities expressed in heterozygotes

Patterns of inheritance:


·        Autosomal Dominant

o   =Single gene abnormalities expressed in heterozygotes

o   M = F, 50% risk of passing it to kids

o   1 abnormal gene causes disease 

o   Eg Huntington Disease, Marfan Syndrome, Achondroplasia (disturbance of epiphyseal chrondroblastic bone formation) 

o   But:

§  Variable expression, variable age of onset

§  Non-penetrance happens 

§  Gonadal mosaicism (esp if old paternal age) ® somatic genes normal, mutation in gonads

·        Autosomal Recessive:

o   = Only symptomatic when both alleles at a locus on homologous chromosomes are defective

o   Must have mutations in both genes Þ both parent‟s carriers

o   Shows up early (no normal genes)

o   Eg cystic fibrosis, phenylketonuria

·        X-linked Recessive:

o   Only affects males

o   Impact early (no normal gene) 

o   Females are carriers (random X inactivation should mean that 50% of cells are abnormal. But they‟re usually in the minority. Still may have some traits)

o  Eg haemophilia, Duchenne muscular dystrophy (wasting muscle disease – most dystrophies are X linked)

·        X-linked Dominant: eg Fragile X

·        Multifactorial:

o  Common diseases

o  Genetic predisposition plus environmental influence

o  Eg Cleft lip/palate

·        Others: mitochondrial, tumour predisposition


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