Understanding shared DNA provides insight into how relatives are linked. The amount of DNA individuals share offers a scientific measure of their genetic relationship. This shared genetic material indicates common ancestry, with closer relatives typically sharing more DNA. The proportion of shared DNA generally decreases as the genealogical distance between individuals increases.
The Basics of Shared Genetic Material
Every person inherits half of their DNA from each parent. The specific segments of DNA passed down are a random assortment from each parent’s genetic material. This random inheritance forms the basis of genetic diversity within families.
Genetic genealogists use a unit called a centimorgan (cM) to measure shared DNA segments. A centimorgan represents the likelihood that two genetic markers on a chromosome will become separated during the process of genetic recombination. On average, one centimorgan corresponds to approximately one million base pairs of DNA. Higher cM values indicate a more recent common ancestor.
Typical Shared DNA for Cousin Types
The amount of shared DNA among cousins varies, but average ranges offer a guide to their genetic relationship. First cousins, who share a set of grandparents, typically share an average of 866 cM, with a range between 396 and 1397 cM. This amount averages around 12.5% of their total DNA.
First cousins once removed, meaning the child of a first cousin, share a grandparent and a great-grandparent. They typically share an average of 433 cM, with a range from 102 to 980 cM. Second cousins, who share a set of great-grandparents, generally share between 75 and 360 cM, with an average of about 246 cM. This corresponds to approximately 3.4% of their DNA. Third cousins, connected through shared great-great-grandparents, typically share around 53 cM, with a range up to about 100 cM. It is possible for third cousins to share no DNA due to the random nature of inheritance.
Why Shared DNA Amounts Vary
The variability in shared DNA amounts, even among relatives of the same type, is largely due to genetic recombination. During the formation of reproductive cells, chromosomes exchange segments of DNA in a process called crossing over. This shuffling creates unique combinations of genetic material for each offspring, ensuring that siblings, for example, do not inherit identical DNA from their parents. This random process explains why some first cousins might share more DNA than others.
Certain family structures can also lead to higher-than-average shared DNA. Half-cousins, who share only one common grandparent, share less DNA than full cousins. Double cousins occur when two siblings from one family marry two siblings from another family. For example, if two brothers marry two sisters, their children are double first cousins and average around 25% of their DNA.
Using Shared DNA for Family Connections
Direct-to-consumer DNA testing services utilize shared centimorgan amounts to predict relationships between individuals. These services compare DNA segments to identify matches. The total number of shared centimorgans, along with the length and number of shared DNA segments, helps predict the most likely relationship.
Individuals can use this information to confirm known family relationships or to discover previously unknown relatives. DNA matches can also aid genealogical research. While the amount of shared DNA is a strong indicator of relationship, overlapping cM ranges mean multiple relationship types are possible for a given amount. For more distant relationships, the predicted relationship is an estimate, and genealogical research can confirm the exact connection.