A mitochondrion contains its own small genome and is usually inherited from the mother.
Mitochondria are energy-producing organelles found in most eukaryotic cells. Their DNA, called mitochondrial DNA or mtDNA, is separate from the much larger genome inside the cell nucleus. Human mitochondrial DNA is a circular molecule containing 37 genes.
Maternal inheritance occurs because the egg contributes most of the cytoplasm and organelles to the early embryo. Sperm normally contributes nuclear DNA, while its mitochondria are usually excluded or destroyed after fertilisation. As a result, mtDNA is commonly passed through the maternal line.
Mitochondria are thought to descend from bacteria that entered an ancient host cell and formed a long-term partnership. This endosymbiotic origin helps explain their circular DNA and bacterial-like features. Maternal inheritance is the usual pattern, though rare exceptions and complex inheritance patterns have been documented.