Our genetic makeup connects us to relatives across generations, holding a detailed record of our family history. This connection is visible in “shared DNA,” which refers to identical segments of genetic material inherited from common ancestors. The amount of shared DNA directly reflects the closeness of a biological relationship, offering a scientific basis for understanding familial ties.
The Shared DNA of First Cousins
First cousins, on average, share about 12.5% of their DNA. This percentage translates to an average of 866 centimorgans (cM). However, the actual amount of shared DNA can vary, commonly ranging from 396 to 1397 cM.
A centimorgan is a unit of genetic measurement used to quantify the amount and length of specific DNA segments shared between relatives. Unlike physical units of distance, a centimorgan represents a probability related to genetic recombination events. A higher number of shared centimorgans indicates a closer genetic relationship. This genetic sharing occurs because first cousins have a set of common grandparents, from whom they both inherited portions of their genetic material through their respective parents, who are siblings.
Understanding DNA Inheritance and Variation
The reason for the range in shared DNA among first cousins, rather than a fixed amount, lies in the complex process of DNA inheritance. DNA is not passed down in exact, unchanging blocks from one generation to the next. Instead, a process called meiosis shuffles genetic material.
During meiosis, the cell division creating sperm and egg cells, two key events introduce variation. First, “crossing over” occurs, where homologous chromosomes exchange DNA segments, creating new genetic combinations.
Second, “independent assortment” randomly distributes chromosomes into developing gametes. This ensures each child receives a unique DNA assortment from their parents. These mechanisms explain why even full siblings, and their children (first cousins), share different amounts of DNA from common ancestors.
What Shared DNA Reveals
Knowing the amount of shared DNA between individuals holds practical significance, particularly in genetic genealogy. This information assists in confirming or discovering family relationships, combining DNA analysis with traditional historical records to build comprehensive family trees.
DNA testing can verify suspected first cousin relationships by comparing the amount of shared genetic material against established averages and ranges. While testing companies use shared centimorgan values to estimate relationships, certain centimorgan ranges can overlap across different relationship types. Therefore, combining DNA results with genealogical research, such as family trees, can help clarify the precise nature of the relationship. This data also helps identify unknown relatives, enabling connections with previously unknown biological family members, especially with multiple DNA matches and established family trees.