Shared DNA provides a scientific measure of the relationship between two people, quantifying the genetic material inherited from common ancestors. In genetic genealogy, the focus is on specific DNA segments shared due to recent familial ties. The amount of shared DNA between relatives is not a fixed number, but a statistical probability following the rules of genetic inheritance. These probabilities are predicted based on the number of generations separating two people from their most recent shared ancestor.
The Average Shared DNA with a First Cousin
A first cousin relationship is defined by sharing a set of grandparents, meaning two individuals descend from a common pair of grandparents through different children. Statistically, first cousins are expected to share an average of 12.5% of their total DNA. This percentage is a theoretical average assuming a predictable inheritance pattern.
DNA testing companies present shared genetic material using the centimorgan (cM) unit. For first cousins, the average shared DNA is approximately 866 cM. The common range is broad, extending from about 396 cM to 1397 cM. This wide spread reflects that the exact amount of DNA passed down is subject to random chance. The overlap in shared cM ranges with other relationships, such as a great-great-grandparent or a half-aunt, shows why shared DNA alone does not always pinpoint a single relationship.
Understanding Centimorgans and Genetic Measurement
The centimorgan (cM) is the standardized unit used by genetic genealogists to measure the total amount of shared DNA segments between two individuals. It is a measure of genetic distance and the probability of recombination, not a physical measurement of length. One centimorgan represents a 1% chance that a specific chromosome segment will undergo a recombination event in a single generation.
The cM unit is a more precise tool for relationship prediction than a simple percentage. The total number of centimorgans reflects the actual, measured quantity of shared genetic material. DNA testing uses the combined length of all identical segments to produce a total cM value, which is plotted against statistical ranges for various family relationships.
Why Shared DNA Amounts Vary Greatly
The wide range of shared DNA observed between first cousins, which can span over 1000 centimorgans, is a direct result of the randomness inherent in Mendelian inheritance and genetic recombination. Each parent contributes exactly 50% of their DNA, but the specific segments inherited are randomly selected from the parent’s two chromosome copies.
Genetic recombination, or “crossing over,” shuffles the genetic material. During the formation of reproductive cells, homologous chromosomes exchange DNA segments, creating unique combinations of genes. The number and location of these crossover events are random, meaning two siblings will inherit different combinations of their parents’ DNA.
Cousins share their relationship through a pair of grandparents, so the degree of shared DNA depends on the random segments of the grandparents’ DNA passed down to both of the cousins’ parents. If both parents inherit large, overlapping segments from the grandparents, the first cousins will share a high amount of DNA. Conversely, if the random shuffling passes down mostly non-overlapping segments, their descendants will share a lower total amount of DNA, despite the same genealogical relationship. This explains why some first cousins share an amount near the 12.5% average, while others fall at the high or low ends of the predicted range.
Genetic Sharing with Other Cousin Relationships
The principles of random inheritance and recombination apply across all cousin relationships, leading to predictable average values that decrease as the common ancestor becomes more distant. Second cousins, who share a set of great-grandparents, have an expected average of about 3.4% shared DNA. This translates to an average of around 229 cM, with a typical range falling between roughly 41 cM and 592 cM.
Half-cousin relationships, where individuals share only one grandparent instead of a pair, result in a lower average of shared DNA, similar to the first cousin once removed category. A half-first cousin, for example, shares an average of approximately 449 cM, or about 6.25% of their DNA, with a range between 156 cM and 979 cM.
First cousins once removed (1C1R), where one person is a generation removed from the other, also share an average of about 6.25% of their DNA, which equates to approximately 433 cM, with a range of 102 cM to 980 cM. These predictable averages and their associated ranges are the primary tools used by genetic testing services to estimate familial connections.