Less than a decade after the rediscovery of Mendel's laws describing the inheritance of genes in the nucleus, hereditary traits were discovered that obey a different set of laws. The genes involved in this non-Mendelian pattern of inheritance reside outside the nucleus, in the cytoplasm of the cell. Specifically, they were found to reside in mitochondria, chloroplasts, or intracellular symbiotic bacteria. Those genes play important roles in the cell. Mutations in extranuclear genes are responsible for some hereditary diseases in humans and other organisms, are used in plant breeding, and are used to study population genetics and evolution.

Genes in Mitochondria and Chloroplasts

The cytoplasm of most eukaryotic cells contain organelles called mitochondria, where energy is extracted from food molecules and stored in ATP (adenosine triphosphate) for later use throughout the cell. Virtually all of the oxygen we use is consumed by our mitochondria.

Mitochondria contain...