How are mitochondrial genetic disorders inherited from mothers?

Mitochondrial genetic disorders are inherited through a process known as maternal inheritance. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA (mtDNA) is contained within the cytoplasm of the egg cell. During fertilization, the mother’s egg provides the majority of the cytoplasm to the developing embryo, while mitochondria from the father’s sperm are typically destroyed. Consequently, mutations in the mitochondrial genome are passed exclusively from mothers to their offspring.123

The clinical expression of these disorders is influenced by the concept of heteroplasmy, which refers to the presence 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. A child may appear healthy or mildly affected if the number of normal mitochondria outweighs the abnormal ones, whereas a higher percentage of dysfunctional mitochondria can lead to severe impairment of cellular energy production. While maternal family history is a key diagnostic indicator, it is important to note that the majority of mitochondrial diseases are actually caused by mutations in nuclear genes rather than mtDNA.213