By: Katy Rowe-Schurwanz
Editor’s Note: This is part two of a five-part series about what mtDNA is, what mtDNA can tell you, and how to apply mtDNA results to your genealogy. Continue reading the series here:
- Part 1: What is mtDNA?
Have you ever considered the journey of your maternal ancestors across generations and continents? Within human ancestry, mitochondrial DNA (mtDNA) reveals the unbroken line of maternal heritage.
Discover the significance of mitochondrial DNA, the unique inheritance of the mitochondrial genome, and the potential insights into our ancestral origins with mtFull Sequence.
Why is mtDNA only passed from mother to child?
Mitochondrial DNA has a unique inheritance pattern compared to other types of DNA. Mothers pass down their mtDNA to their children. While both sons and daughters will inherit their mother’s mtDNA, only daughters will continue to pass that down to their children.
The exclusive maternal inheritance of mitochondrial DNA (mtDNA) is rooted in the unique biology of egg and sperm cells, as well as the cellular machinery involved in fertilization.
During sexual reproduction, both egg cells (ova) and sperm cells contribute genetic material to the resulting offspring. However, the distribution of genetic material differs between these two types of gametes.
Mitochondrial DNA in the ova
Egg cells are relatively large compared to sperm cells and contain abundant cytoplasm, which includes mitochondria. When an egg cell is fertilized by a sperm cell, the mitochondria in the egg cytoplasm are retained, while the mitochondria in the sperm cell’s tail are typically not able to enter the egg during fertilization due to size constraints.
This means that the fertilized egg, or zygote, inherits its mitochondria exclusively from the mother.
Mitochondrial DNA in the sperm
In contrast, sperm cells are small and streamlined, containing only the genetic material necessary for fertilization. While sperm cells do contain mitochondria, these are typically located in the tail (flagellum) of the sperm and are not involved in fertilization. Therefore, when fertilization occurs, only the genetic material from the sperm’s nucleus is contributed to the zygote, while the sperm’s mitochondria are not passed on.
This mechanism ensures that the mitochondrial genome is transmitted exclusively through the maternal line. This means that your mitochondrial DNA can be used to trace an unbroken line of women in your family tree going back dozens of generations—your mom, her mom, her, mom, and so on, stretching so far back that ancestors’ names have been lost to time.
The regions of mitochondrial DNA
The mitochondrial genome is a circular molecule consisting of approximately 16,569 base pairs in humans. Mitochondrial DNA is made of two major parts: the control region and the coding region.
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- The control region, often called the hypervariable region (HVR), is made of three human hypervariable regions: HVR1, HVR2 and HVR3.
- The coding region includes genes that encode proteins essential for various mitochondrial functions.
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The control region
HVR1 and HVR2 are typically included in most genetic genealogy tests, while HVR3 may be analyzed in more specialized or comprehensive testing panels, like the mtFull Sequence test from FamilyTreeDNA. For testing purposes, we combine HVR2 and HVR3.
HVR1 and HVR2 are adjacent to each other in the control region. The control region is a non-coding region of DNA that plays a crucial role in the replication and transcription of mitochondrial DNA.
The nucleotide positions for the control region are as follows:
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- HVR1 starts at nucleotide position 16024 and ends at position 16383.
- HVR2 starts at nucleotide position 57 and ends at position 372.
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The reason for the difference in numbering is likely historical and based on conventions established by researchers and genetic testing companies. Different studies or testing protocols may use slightly different ranges for defining HVR1 and HVR2 within the control region.
While positions 1 through 56 and 16384 through 16569 may include regulatory elements and sequences important for mitochondrial function, they are not typically considered hypervariable regions (HVRs) commonly analyzed in genetic genealogy studies.
The coding region
The coding region (CR) is the part of your mitochondrial genome that contains genes. Because it does contain some genes, the coding region tends to be slower to mutate than the control region.
The nucleotide positions for the coding region start at position 373 and end at position 16023 and contain 37 genes.
Unlike the hypervariable regions (HVRs) located within the control region, the coding region is less variable and is typically not used for direct ancestry analysis. However, variations within the coding region can still provide valuable information for understanding mitochondrial genetic diversity and evolutionary relationships among populations.
Introduction to mtFull Sequence
The mtFull Sequence test uses Next Generation Sequencing (NGS) to read and report your mitochondrial DNA sequence. It covers all 16,569 base pairs.
Your results are compared to both the revised Cambridge Reference Sequence (rCRS) and Reconstructed Sapiens Reference Sequence (RSRS). These results determine your placement on the mtDNA Tree of Humankind (your haplogroup).
mtDNA matches
Your results are also compared to our database of mtDNA testers. The amount of your mtDNA and another tester’s mtDNA that is the same determines who your matches are in our database. The more of your mtDNA that is the same, the more recently in time your most recent common ancestor with that tester lived.
These results can help you determine where your matrilineal ancestors were from and how they got there, as well as help you discover new ancestors on your direct maternal line.
Matching levels
There are three sections of mtDNA used for matching: HVR1, HVR2, and the Coding Region. Our mtFull Sequence test examines the HVR1, HVR2, and Coding Region of mitochondria and is the highest resolution test available to genealogists. The closeness of a mtDNA match depends on the matching level. Matches at higher levels are more likely to be recent.
Matching is optional
Viewing your matches and sharing your information with them is optional. You can opt in and out of matching at any time.
About the Author
Katy Rowe-Schurwanz
Product Manager at FamilyTreeDNA
Katy Rowe-Schurwanz has always been interested in genealogy, inspired by her maternal grandparents, who told her stories about their family and family history when she was little. After studying anthropology and history in college, she joined FamilyTreeDNA in 2015 and became the Trainer for Customer Support. Katy created and improved training processes and was fundamental in the creation of the Big Y Specialist team. In September 2021, she became Product Manager and has focused closely on improving FamilyTreeDNA’s genetic genealogy products.