The mitochondria (singular: mitochondrion) are organelles inside the cell (figure 2). They carry on the oxidative metabolism of the nutrients and provide the cell with most of the energy that it needs. Due to their specialized function, defects in the mitochondria could produce severe diseases. The origin of a mitochondrial disease could originate from a mutation in the mitochondrial DNA, but could also originate from a mutation in the nuclear DNA since there are very few genes in the mitochondrial DNA and part of the proteins and enzymes of the mitochondria are encoded in nuclear genes, synthesized in the cytoplasm and then imported by the mitochondria.
Figure 2. Diagram of a cell. See the nucleus and the mitochondria among other organelles.
When the mitochondrial disease is due to a mutation in a nuclear gene, it will be transmitted according to the other genetic inheritance patterns. However, if the mutation is in the mitochondrial DNA, it will be transmitted with a specific pattern that it is very simple: the mitochondria, and their DNA, are only inherited from the mother. So all the children of a normal (gray color in the figures) father and an affected (orange color in the figures) mother will be affected (figure 3), while none of the children of an affected father and a normal mother will be affected (figure 4).
Figure 3. Mitochondrial inheritance pattern when the mother is affected.
Figure 4. Mitochondrial inheritance pattern when the father is affected.
If you want to read about mitochondrial inheritance pattern in Spanish, you can follow the link herencia mitocondrial.