Mitochondria are specialized subunits (organelles) within cells. In humans, mitochondria are responsible for cell respiration and for producing energy. They evolved into their current state from separate organisms that form a mutually beneficial (symbiotic) relationship with the larger cell. Because they were once independent, they have their own mitochondrial DNA (mtDNA) genome.
Both men and women have mitochondria and mtDNA in their cells, but only women pass it on to their children. Because of this unique maternal inheritance, we can use mtDNA to trace your direct maternal line. Your mtDNA can trace your mother, her mother, her mother’s mother, and so forth and offers a clear path from you to a known or likely direct maternal ancestor.
Your Direct Maternal Line
Your direct maternal lineage is the line that follows your mother’s maternal ancestry. Fathers do not pass on their mtDNA to their children, so this line consists entirely of women. Your mtDNA can trace your mother, her mother, her mother’s mother, and so forth. It offers a clear path from you to a known or likely direct maternal ancestor.
For genealogists, this clear line means that they can trace two or more descendants of a single woman many generations back and compare their mtDNA results with the expectation of a match. For those interested in deeper ancestry, tracing the modern geographic origins of exact matches means that they can discover the anthropological origins of their own line.
There are two scientific baseline sequences to which scientists compare changes in mtDNA: the Reconstructed Sapiens Reference Sequence (RSRS) and the revised Cambridge Reference Sequence (rCRS). By comparing your mtDNA changes to each sequence, we can distinguish the differences in your DNA to the original values in both the RSRS and the rCRS. FamilyTreeDNA provides a separate list of each of these differences along with your other results when you take an mtDNA test.
Parts of mtDNA
mtDNA has two major parts: the control region and the coding region.
- Control Region - The control region is often called the hypervariable region (HVR). There are two human hypervariable regions: HVR1 and HVR2. Hypervariable means fast-changing. They do not contain genes. Therefore, they have a faster change (mutation) rate than the coding part of the mitochondrial genome.
- Coding Region - The coding region (CR) is the part of your mtDNA genome that contains genes. Because it does contain some genes, the coding region is believed to be slower mutating than the control region. Often, the mutations that are found in the coding region are used to define haplogroups. Haplogroups are defined in the section below.
Hypervariable Region (HVR)
A hypervariable region (HVR) is a part of the mitochondrial genome. There are two human hypervariable regions: HVR1 and HVR2. They do not contain genes. Therefore, they have a faster change (mutation) rate than the coding part of the mitochondrial genome.