DNA is the blueprint for all living organisms, carrying genetic instructions that make each individual unique. This molecule forms the basis of family connections, as genetic material is passed down through generations. Understanding DNA inheritance provides insights into the biological ties and shared heritage within a family.
Shared DNA with First Cousins
First cousins typically share about 12.5% of their DNA. This is measured in centimorgans (cM), a unit reflecting the length of shared DNA segments. For first cousins, the typical range of shared centimorgans falls between 680 and 1300 cM.
These figures represent averages. The actual amount of DNA shared between any two first cousins can vary due to the random nature of genetic inheritance. This variability means some first cousins might share slightly more or less than the average, while still being biologically full first cousins.
The Science of DNA Inheritance
DNA is organized into chromosomes, with humans having 23 pairs in each cell. One set comes from the mother and the other from the father. Inheritance is not a simple halving of parents’ exact DNA; instead, a complex process of recombination shuffles genetic material.
During meiosis, homologous chromosomes exchange DNA segments through “crossing over,” creating new gene combinations before passing to offspring. This random shuffling means siblings inherit different DNA combinations from the same parents, explaining why even full siblings are not genetically identical. Cousins share DNA because they descend from common grandparents, inheriting portions of the same recombined genetic material. The varying extent of recombination in each generation contributes to the range of shared DNA among relatives.
Comparing DNA Sharing Across Relatives
Shared DNA decreases with increasing genetic distance. While first cousins share about 12.5% of their DNA, other relatives share different average percentages and centimorgan ranges. For instance, half-siblings share approximately 25% of their DNA, similar to grandparents, grandchildren, aunts, and uncles.
Half-first cousins, who share one grandparent, typically share about 6.25% of their DNA, with a cM range of approximately 156-979 cM. Double cousins, who share both sets of grandparents, often share around 25% of their DNA, more than typical first cousins. Aunts, uncles, nieces, and nephews generally share between 19-33% of their DNA, or 1300-2300 centimorgans. This broad range highlights why DNA testing companies use patterns of shared segments, in addition to total amounts, to estimate relationships.
Unlocking Family History with Shared DNA
Understanding shared DNA is fundamental to genetic genealogy and uncovering family history. Ancestry testing services analyze an individual’s DNA to identify shared segments with others in their database. These segments can indicate a common ancestor, allowing individuals to connect with previously unknown relatives.
By comparing the amount and patterns of shared DNA, these services estimate relatedness. Genetic genealogists use this information, alongside traditional research, to build and confirm family trees and identify common ancestors. Tools visualizing shared DNA segments across chromosomes help pinpoint ancestral lines, providing a method for exploring one’s heritage.