Mitochondrial DNA is inherited exclusively from the mother via the cytoplasm of the egg cell. A male's mitochondria, located in the midpiece of the sperm, do not enter the ovum during fertilization or are actively degraded. Consequently, a male cannot transmit his mitochondrial DNA to any of his offspring. The risk of transmission from an affected father is 0%.

Genomic imprinting is an epigenetic process that involves the differential marking of genes based on their parental origin. The primary molecular mechanism for establishing and maintaining these imprints is DNA methylation. Specific CpG islands within imprinting control regions (ICRs) are methylated in a sex-specific manner during gametogenesis. This methylation pattern is then maintained in somatic cells after fertilization, leading to the silencing of either the maternal or paternal allele.

UBE3A shows tissue-specific imprinting. In most somatic tissues, both maternal and paternal alleles are expressed, so the mother's functional paternal allele compensates for her mutated maternal allele. However, in specific neurons of the brain, the paternal allele is silenced (imprinted), and only the maternal allele is expressed. The child inherited the mother's mutated maternal allele, and in his brain neurons, the paternal UBE3A is silenced, leaving no functional UBE3A and causing Angelman syndrome.

The patient's symptoms (myoclonus, seizures, myopathy) and the presence of 'ragged-red fibers' on muscle biopsy are characteristic of Myoclonic Epilepsy with Ragged-Red Fibers (MERRF), a mitochondrial disorder. Mitochondrial diseases exhibit variable expressivity due to heteroplasmy. Heteroplasmy is the presence of a mixed population of normal and mutant mitochondrial DNA (mtDNA) within a single cell. The severity of the disease in an individual depends on the proportion of mutant to wild-type mtDNA in different tissues, which can vary significantly among family members due to random segregation of mitochondria during oogenesis and cell division.

This patient's presentation with subacute bilateral optic neuropathy (Leber hereditary optic neuropathy, LHON) and a family history consistent with maternal inheritance (affected mother, maternal uncle, but unaffected father) strongly suggests a mitochondrial disorder. Mitochondrial DNA (mtDNA) is inherited exclusively from the mother, as the ovum contributes the cytoplasm and mitochondria to the zygote. Therefore, affected mothers pass the mutation to all their offspring, while affected fathers do not.

The clinical presentation is classic for Prader-Willi syndrome (PWS). PWS is an imprinting disorder caused by the loss of expression of genes that are normally transcribed only from the paternal chromosome 15. The most common cause is a deletion on the paternal chromosome 15. The second most common cause is maternal uniparental disomy (UPD), where the individual inherits two copies of chromosome 15 from the mother and none from the father. Since the relevant genes are imprinted (silenced) on the maternal chromosome, inheriting two maternal copies results in the same lack of gene expression as a paternal deletion.

Mitochondrial disorders are transmitted exclusively through the maternal line. This is because the zygote's mitochondria are derived entirely from the ovum; the sperm's mitochondria are typically destroyed after fertilization. Therefore, an affected male cannot pass a mitochondrial DNA mutation to any of his children, regardless of their sex. The probability of his son or daughter inheriting the disorder from him is 0%.

Angelman syndrome results from the lack of expression of the maternal UBE3A gene. In this case, the child has inherited both copies of chromosome 15 from her father and none from her mother. This is known as paternal uniparental disomy (UPD). Because the paternal copy of UBE3A is normally imprinted (silenced) in the brain, the child has no functional UBE3A protein, leading to the Angelman syndrome phenotype. This is the second most common cause of AS after maternal deletion.

Beckwith-Wiedemann syndrome is an imprinting disorder, typically involving genes on the short arm of chromosome 11 (11p15), such as IGF2 and CDKN1C. The core pathogenetic mechanism is the dysregulation of parent-of-origin specific gene expression. For example, loss of maternal imprinting (silencing) of the IGF2 gene (a growth promoter) can lead to biallelic expression and overgrowth. This highlights how genomic imprinting, a form of non-Mendelian inheritance, dictates that the effect of a gene depends on whether it was inherited from the mother or the father.

Mitochondrial DNA is inherited maternally, so an affected mother will pass the mutation to all of her offspring (both sons and daughters). However, the clinical severity of mitochondrial disorders is highly variable due to heteroplasmy—the presence of both mutant and wild-type mtDNA. The proportion of mutant mtDNA inherited by each child is random and unpredictable, leading to a wide range of clinical phenotypes, as seen in her own family. Thus, all her children will inherit the mutation, but their clinical severity cannot be precisely predicted.