TEMPLATE

Mitochondrial Inheritance Pedigree

Mitochondrial DNA is inherited almost exclusively from the mother, so an affected mother passes the trait to all of her children, and only her daughters can pass it to the next generation. This fictional example shows the pattern in a simplified form.

Inheritance pattern & biological mechanism

Mitochondrial inheritance (maternal inheritance) describes the transmission of genes encoded on the circular mitochondrial DNA (mtDNA). During fertilization, the sperm contributes only nuclear genetic material, while the ovum provides virtually all organelles, cytoplasm, and mitochondria to the zygote. Consequently, mitochondrial traits are transmitted strictly through maternal lines.

Key transmission rules

An affected female transmits the mitochondrial trait to 100% of her biological offspring, both sons and daughters.
Affected males never transmit the mitochondrial condition to any of their children.
The severity and clinical presentation of symptoms among siblings can vary widely due to heteroplasmy (the cellular mixture of mutated and wild-type mtDNA molecules).
The condition does not skip generations along maternal lineages.

Representative real-world conditions

Leber Hereditary Optic Neuropathy (LHON)

MT-ND1, MT-ND4, MT-ND6 mtDNA mutations

Bilateral, painless subacute central vision loss typically striking young adult males and females due to retinal ganglion cell degeneration.

MELAS Syndrome

MT-TL1 (m.3243A>G mutation)

Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes, featuring recurrent strokes before age 40, seizures, and muscle weakness.

MERRF Syndrome

MT-TK (m.8344A>G mutation)

Myoclonic Epilepsy with Ragged Red Fibers, characterized by myoclonus, ataxia, hearing loss, and characteristic ragged red muscle fibers on biopsy.

NARP Syndrome

MT-ATP6 mutation

Neuropathy, Ataxia, and Retinitis Pigmentosa, presenting with developmental delay, sensory neuropathy, and retinal pigmentation.

How to analyze this chart

1
Generation I: Affected mother (I-1) partners with unaffected father (I-2).
2
Generation II: All children of the affected mother (daughter II-1 and son II-2) are affected. Daughter II-1 partners with unaffected male; son II-2 partners with unaffected female.
3
Generation III: Affected daughter II-1 transmits the condition to all of her children (III-1 proband and III-2). In stark contrast, affected son II-2 has zero affected children.
4
The clear contrast between maternal continuation and paternal termination is the definitive signature of mtDNA inheritance.

Diagnostic checklist: confirming vs ruling out

Confirming clues

✓ All children of affected mothers inherit the altered mtDNA and express the phenotype.
✓ Children of affected fathers are never affected.
✓ Transmission persists unbroken along maternal lines.
✓ Both male and female offspring are affected from maternal transmission.

Caveats & ruling out

⚠ Any instance of father-to-child transmission immediately disproves mitochondrial inheritance.
⚠ If an unaffected mother produces multiple affected children, nuclear genetic disorders should be investigated instead.

Frequently asked questions

What is heteroplasmy and why does it matter in pedigrees?

Heteroplasmy is the presence of a mixture of mutated and normal mitochondrial DNA within cells. The proportion of mutant mitochondria determines whether tissue energy thresholds are breached and symptoms manifest, explaining why siblings in the same pedigree can have vastly different symptom severities.

Can a father ever transmit mitochondrial DNA to his child?

Under standard human biology, paternal mitochondrial transmission is prevented because sperm mitochondria are targeted for ubiquitin-mediated degradation in the fertilized egg.

Are all mitochondrial diseases inherited through maternal mtDNA?

No. Over 85% of proteins active in mitochondria are encoded by nuclear genes on autosomes or the X chromosome. Those follow Mendelian (autosomal recessive/dominant) patterns, not maternal inheritance.

Three-Generation Pedigree

Grandparents, parents, and children with a proband.

Autosomal Dominant Pedigree

Trait appears in every generation; one affected parent usually has affected children.

Autosomal Recessive Pedigree

Unaffected carrier parents can have an affected child; the trait may skip generations.