Final answer:
Mitochondrial disorders are inherited through cytoplasmic inheritance, resulting from the transmission of mitochondrial DNA from mother to child. The variability in disease severity is due to differences in the mutation distribution among mitochondria within cells and subsequent cell divisions.
Step-by-step explanation:
In humans, disorders stemming from mutations in mitochondrial DNA exhibit a pattern of cytoplasmic inheritance, because mitochondria are transmitted from mother to child. This type of inheritance is a result of the larger amount of cytoplasm contained in the female gamete, which includes organelles such as mitochondria. Since mitochondria contain their own DNA, and this mitochondrial DNA is passed strictly through the maternal line, we can trace mitochondrial lineages directly back through our female ancestors.
Even within a single family, the severity of mitochondrial diseases can vary widely. This is due to the fact that not all mitochondria within a cell may have the same mutations, so different cells—and thus different individuals or even different tissues within an individual—may have varying amounts of healthy and affected mitochondria. Moreover, during cell division, the distribution of mitochondria to daughter cells can be uneven, contributing to the variability in symptoms expressed by individuals with mitochondrial diseases.