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Updated: Jul 10 2018

Genetic Principles

  • Principles
    • Codominance
      • both allelic copies are expressed
        • e.g.) blood groups (A, B, AB)
    • Variable expression
      • nature and degree of phenotype vary from 1 individual to another with the same mutation
        • e.g.) 2 patients with neurofibromatosis may have varying disease severity
    • Incomplete penetrance
      • not all individuals with a mutant genotype have diseased phenotype
        • explanation for a dominant disease "skipping" a generation
      • penetrance can be calculated by ( # with symptoms) / (# with disease genotype)
        • must be figured into recurrance calculations
          • if parents have a 50% chance of giving defective gene but the penetrance is 50%
            • 0.5 x 0.5 x100 = 25% of recurrence
      • observed in recessive and dominant diseases
    • Pleiotropy
      • single mutation has diverse effects upon several organ systems
        • e.g.) PKU
    • Anticipation
      • changes in disease presentation in succeeding generations
        • ↑ severity
        • earlier onset
      • caused by trinucleotide expansion
        • region of repeating triplets expands in each generation
        • e.g.) Huntington's disease, fragile X, myotonic dystrophy, Friedreich ataxia
    • Loss of heterozygosity
      • "two-hit model"
      • individual inherits or develops a mutation in one copy of gene
        • disease occurs when the complementary allele is lost
      • e.g. tumor suppressor diseases (Li-Fraumeni, retinoblastoma)
    • Dominant negative mutation
      • mutant gene product antagonizes wild-type gene product
        • exerts a dominant effect
        • e.g.) common in multimeric proteins where one mutant subunit can change function of entire enzyme
    • De novo mutation
      • genetic disease in an individual with no familial history
      • recurrence risk for offspring of same parents is low
    • Locus heterogeneity
      • different mutations can produce the same phenotype
        • e.g., marfanoid habitus caused by
          • Marfan's syndrome, MEN 2B, homocystinuria
    • Heteroplasmy
      • presence of both normal and mutated mitochondrial (mt)DNA in the same cell
        • results in variable expression in mitochondrial inherited disease
    • Uniparental disomy
      • offspring receives both copies of a chromosome from 1 parent
        • no copies from the other parent
        • causes disease if the chromosome is usually imprinted
          • see Epigenetics topic
    • Polygenic inheritance
      • multiple genes are responsible for inheritance of a disease
      • e.g.) androgenic alopecia
    • Heritability
      • can measure the relative effect of genetic vs. environmental factors on a phenotype
        • calculated by phenotypic relationship between dizygotic (DZ) and monozygotic (MZ) twins
        • heritability = (CMZ-CDZ) / (1-CDZ)
          • where C = concordance
            • prevalence of disease in both twins
            • entirely environmental disease should have CMZ = CDZ
            • entirely genetic disease should have CMZ=1.0 and CDZ=0.5
              • siblings share 50% of their genes
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