Most DNA resides in the nucleus of a cell. However, some organelles in the cell cytoplasm⁠—such as chloroplasts and mitochondria⁠—also have their own DNA. These organelles replicate their DNA independently of the nuclear DNA of the cell in which they reside. Non-nuclear inheritance describes the inheritance of genes from structures other than the nucleus.

Mitochondria are present in both plants and animal cells. They are regarded as the “powerhouses” of eukaryotic cells because they break down glucose to form energy that fuels cellular activity. Mitochondrial DNA consists of about 37 genes, and many of them contribute to this process, called oxidative phosphorylation.

Chloroplasts are found in plants and algae and are the sites of photosynthesis. Photosynthesis allows these organisms to produce glucose from sunlight. Chloroplast DNA consists of about 100 genes, many of which are involved in photosynthesis.

Unlike chromosomal DNA in the nucleus, chloroplast and mitochondrial DNA do not abide by the Mendelian assumption that half an organism’s genetic material comes from each parent. This is because sperm cells do not generally contribute mitochondrial or chloroplast DNA to zygotes during fertilization.

While a sperm cell primarily contributes one haploid set of nuclear chromosomes to the zygote, an egg cell contributes its organelles in addition to its nuclear chromosomes. Zygotes (and chloroplasts in plant cells) typically receive mitochondria and chloroplasts solely from the egg cell; this is called maternal inheritance. Maternal inheritance is a type of non-nuclear, or extranuclear, inheritance.

Why do mitochondria and chloroplasts have their own DNA? The prevailing explanation is the endosymbiotic theory. The endosymbiotic theory states that mitochondria and chloroplasts were once independent prokaryotes. At some point, they joined host eukaryotic cells and entered a symbiotic relationship—one that benefits both parties.