Human mitochondrial DNA that has an important role in the cycle of energy inside the cell, Contrary to nucleus DNA, has a cyclic form, exists in  the cytoplasm and 16kb long. Although it changes according to the function of the cell, there are hundreds of mitochondrions and inside the mitochondrion there are more than one cyclic DNA. Inside the Ovum cytoplasm there also are hundreds of thousand mitochondrions, however this number of mitochondrion is very less in the sperm cytoplasm (a few hundreds). Hence mitochondrial DNA mutations are inherited maternally.

All the children whose mother has a mitochondrial disease will have the same disease. But the children whose father has the disease have no risk at all.