DNA serves as the biological blueprint connecting individuals across generations. This genetic material, passed down from parents to children, forms the basis of our shared heritage. The amount of DNA two people share directly reflects their genetic closeness, providing a measurable link to common ancestors. Understanding these genetic connections helps reveal how individuals are related through their inherited DNA.
Defining Second Cousins
A second cousin is a relative who shares a common set of great-grandparents with you. This means that your grandparent and their grandparent were siblings. Your parents, in turn, are first cousins to the parents of your second cousin, placing both you and your second cousin in the same generation.
Quantifying Shared DNA
Second cousins typically share a measurable amount of DNA, offering a genetic confirmation of their familial bond. On average, second cousins share approximately 3.125% of their DNA. This translates to an average of around 212.5 centimorgans (cM), which is a unit used by genetic genealogists to measure the length of shared DNA segments. However, the actual amount of shared DNA can vary significantly, ranging from about 75 to 360 cM.
This variability in shared DNA is a direct result of genetic recombination, a process that shuffles DNA during its inheritance. Each child receives a random half of their DNA from each parent, and this mixing means that the exact segments passed down differ even among full siblings. Consequently, the amount of DNA inherited from more distant ancestors, like great-grandparents, can fluctuate. While the amount of shared DNA between second cousins can vary, it is statistically impossible for full second cousins to share no DNA at all.
Practical Applications of Shared DNA
Knowing the amount of DNA shared with second cousins is particularly useful in genetic genealogy and family history research. DNA testing services compare an individual’s genetic data with millions of samples in their databases to identify shared DNA segments. These comparisons help in identifying potential relatives and estimating the closeness of their relationship. The higher the centimorgan value shared, the more closely related two individuals are likely to be.
This genetic information can confirm suspected family connections or help individuals discover previously unknown relatives. For those researching their family tree, shared DNA with a second cousin can provide a valuable clue to “break through” genealogical “brick walls.” By analyzing these shared segments, researchers can identify common ancestors and expand their understanding of their family’s lineage.