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The mitochondrial genome demonstrates strict maternal inheritance.
- Highest yield point on the USMLE about mitochondrial disorders.
- Disease is inherited strictly through mothers.
- Zero percent of fathers pass mitochondrial diseases to offspring.
Mitochondrial disorders share very characteristic pathologies that are easy to identify in Qs
- The combination of visual + hearing problems, hypotonia, and lactic acidosis is exceedingly HY and reflective of a mitochondrial disorder.
Lactic acidosis is common in mitochondrial diseases; tissues with high metabolic rates are affected first (i.e., muscle, eyes, ears, neural tissue). |
Heteroplasmy is exceedingly HY for mitochondrial genetics
- Heteroplasmy is the presence of more than one type of mitochondrial DNA in the same individual.
- This means there is a non-uniformity of mitochondrial genes inherited (i.e., some cells contain one type of mitochondrial genome; other cells contain a different type, similar to mosaicism).
- In turn, mitochondrial diseases frequently vary in severity among individuals because of varying ratios of diseased vs wild-type mitochondrial genes inherited.
Similar genetics terms:
Variable expressivity
Incomplete penetrance
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“Ragged red fibers” is buzz-wordy for mitochondrial inheritance
- Seeing “ragged red fibers” in a question for a mitochondrial inheritance is akin to seeing malar rash in an SLE question. Both too easy.
“Ragged red fibers” on microscopy is a phrase the USMLE likes to throw around for mitochondrial inheritance.
They’re classically associated with MERRF syndrome (myoclonic epilepsy with ragged red fibers). |
Myoclonic epilepsy with ragged red fibers (MERRF)
- This is a mitochondrial disorder characterized by seizures, ataxia, hearing loss, and myopathy.
- Ragged red fibers are abnormal mitochondria accumulating under the sarcolemma. They are red with trichrome stain.
MELAS
- MELAS stands for mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes.
Leber hereditary optic neuropathy
- Characterized by painless, subacute, usually adult-onset (classically males in their 20s) bilateral blindness w/ multiple sclerosis-like Sx, movement disorders, and arrhythmia.
Kearns-Sayre syndrome
- Characterized by progressive ophthalmoplegia, pigmentary retinopathy, cerebellar ataxia, and heart block.
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