Humans often wonder about the extent of their genetic ties, especially with distant relatives. While immediate family connections are clear, the precise point at which genetic relatedness becomes negligible is a common curiosity. This exploration delves into the scientific mechanisms that determine our genetic links, revealing the diminishing nature of shared DNA as generations unfold.
How DNA is Passed Down Through Generations
Genetic inheritance is a biological process where DNA is transmitted from parents to offspring. Each child receives approximately 50% of their DNA from each biological parent. This transmission occurs through specialized reproductive cells called gametes, formed during meiosis.
During meiosis, recombination, or crossing over, shuffles segments of chromosomes between the paired chromosomes inherited from each parent. This reshuffling creates unique combinations of genetic material in each gamete, ensuring every child receives a distinct blend of their parents’ DNA. The specific segments received are random, which is central to understanding how shared DNA decreases over generations.
The Diminishing Share of Shared DNA
The amount of shared DNA decreases significantly with each successive generation of separation from a common ancestor. A child shares, on average, 50% of their DNA with each parent. Full siblings also share approximately 50% of their DNA, though this can vary due to the random nature of inheritance.
Individuals typically share about 25% of their DNA with a grandparent or grandchild. This percentage is an average, and the actual amount can range from about 18% to 32% with a grandparent. First cousins, who share a common set of grandparents, typically share about 12.5% of their DNA.
The percentage continues to halve with each step of genetic distance. Second cousins, who share great-grandparents, typically share about 3.125% of their DNA. Third cousins, who share great-great-grandparents, usually share around 0.78% of their DNA, with a possible range from 0.3% to 2.0%. By the time individuals reach fourth cousins, who share great-great-great-grandparents, shared DNA is generally around 0.20%. It becomes statistically insignificant or undetectable through standard genetic testing for many individuals, with only about 26% of fourth cousins sharing detectable DNA.
What “Not Related” Truly Means Genetically
From a broad genetic perspective, the concept of “not related” is nuanced because all humans share a common ancestry. This means everyone is distantly related. The “most recent common ancestor” (MRCA) for all humanity, the single individual from whom all living humans descend, is estimated to have lived only a few thousand years ago. Separate research has identified a Mitochondrial Eve and a Y-chromosomal Adam, representing the most recent common matrilineal and patrilineal ancestors, existing tens of thousands of years ago.
However, for practical purposes in areas like genealogy, health, or legal definitions, “not related” refers to the point where shared DNA is no longer detectable or statistically significant enough to establish a recent familial connection. Practical relatedness focuses on recent shared DNA segments, as genetic testing typically identifies relationships with measurable segments of shared DNA.
Beyond about 5 to 7 generations, the chance of sharing any detectable segment of DNA becomes very low, even if a genealogical connection is known. For example, third cousins may share no detectable DNA, and this likelihood increases for more distant relationships. Therefore, “not related” in a practical sense often means the genetic evidence of a common ancestor from recent generations has been diluted beyond recognition by the random nature of DNA inheritance and recombination.