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Codominance
- Neither allele is dominant
- Ex: ABO blood groups
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Variable expression
- Severity of phenotype varies from one person to another
- Ex: neurofibromatosis
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Incomplete penetrance
Not all individuals with a mutant genotype show a mutant phenotype
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Pleiotropy
- One gene has more than one effect on the individual's phenotype
- Ex: PKU
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Imprinting
- Differences in phenotype depend on whether the mutation is of maternal or paternal origin
- Ex: Prader-Willi, Angleman's
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Anticipation
- Severity of disease worsens in successive generations
- Ex: Huntington's
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Loss of heterozygosity
- With a tumor suppressor gene, BOTH alleles must be mutated to turn cancerous
- Ex: Rb, p53
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Dominant negative mutation
- A heterozygote will produce a mutant protein that prevents the normal protein produced by the non-mutated allele from functioning
- Ex: mutation of a TF
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Linkage disequilibrium
Tendency for certain alleles at liked loci to occur together
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Mosaicism
- Different cells have different genetic makeup
- Ex: lyonization
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Locus heterogeneity
- Mutations at different loci produce the same phenotype
- Ex: Marfan's, MEN 2B and homocysteinuria all produce a marfanoid habitus
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Heteroplasmy
- Presence of normal and mutated mtDNA results in variable expression of a disease
- Ex: LHON
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Uniparental disomy
Offspring receives two copies of a chromosome from one parent and none from the other parent
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Trinucleotide repeat expansion disease
- Huntington's
- Friedreich's ataxia
- Fragile X
- Myotonic dystrophy
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