How are mitochondrial DNA disorders passed from parent to child?

Mitochondrial DNA (mtDNA) disorders are passed from parent to child through a process known as maternal inheritance.21

Because mitochondria are located in the cytoplasm of the egg cell, they are inherited exclusively from the mother. While spermatozoa also contain mitochondria, these are typically destroyed upon fertilization and do not contribute to the embryo's mitochondrial population. Consequently, a mother with a mitochondrial mutation can pass the disorder to all of her children, whereas fathers do not transmit these conditions.231

The clinical expression of these disorders is often influenced by heteroplasmy, which refers to the presence of a mixture of both normal and mutant mtDNA within a single cell. Because mitochondria are randomly distributed to daughter cells during cell division, the severity of the disease can vary significantly among individuals, ranging from asymptomatic to severely compromised, depending on the proportion of abnormal mitochondria present in the affected tissues. It is important to note that while some mitochondrial diseases are caused by mtDNA mutations, the majority are actually the result of mutations in nuclear genes.132