Deoxyribonucleic acid (DNA) is the instruction manual for all living things. The study of genetic relatedness quantifies the amount of shared hereditary material between two individuals, providing a scientific measure of familial connection. Genetic testing has made it possible to move beyond simple genealogical records, offering precise data that confirms how closely two people are linked through their shared ancestry. By analyzing DNA sequences, scientists can determine the precise percentage of genetic material relatives have in common.
The Average Shared DNA Percentage
The average shared DNA between first cousins is 12.5%. This percentage represents the theoretical amount of DNA inherited from their most recent common ancestors: the shared grandparents. Genetic testing companies typically express this shared material in centimorgans (cM), a unit of measure for genetic distance. Centimorgans correlate to the likelihood of recombination occurring between two points on a chromosome.
The average shared cM value for first cousins is approximately 874 cM. However, the actual measured amount varies significantly due to the randomness of genetic inheritance. The typical range of shared DNA often falls between 485 cM and 1,486 cM. This broad range highlights why the 12.5% figure is a theoretical average.
How Inheritance Creates Shared DNA
The 12.5% average results from the probabilistic nature of DNA inheritance across generations. First cousins are related through one set of grandparents, meaning the shared DNA passes through two separate inheritance events. Each parent passes on a random half (50%) of their DNA to their child.
The parents of the cousins are siblings, and they each received an average of 50% of their DNA from the shared grandparents. When these siblings have children, the cousins receive a random half of their parent’s DNA. This means the cousins receive 25% of their parent’s DNA that originated from the shared grandparents. On average, the cousins share half of that 25%, resulting in the 12.5% shared average.
Why the Shared Percentage Varies
The actual percentage of shared DNA rarely lands exactly on 12.5% due to genetic recombination. Recombination occurs during the formation of sperm and egg cells, where chromosome pairs randomly swap segments before being passed on. This shuffling is not perfectly uniform, meaning parents pass on slightly varying amounts of a specific grandparent’s DNA to their child.
Because this process is random for each child, two first cousins receive different total amounts of their shared grandparents’ DNA, leading to variation in shared percentages. Genetic testing detects these shared segments, often called “Identical by Descent” (IBD), to quantify the relationship. For example, a first cousin sharing 14% of their DNA received more shared segments than one who shares 11%. This randomness explains why the centimorgan range for first cousins is so broad.
Comparing First Cousins to Other Relationships
Comparing the 12.5% figure to other common family relationships helps contextualize the amount of shared DNA. This comparison illustrates the distinct genetic distance of first cousins from the shared ancestral source.
Common Shared DNA Percentages
Parent and child share a highly predictable 50% of their DNA.
Full siblings share an average of 50%, though this figure can vary due to recombination.
Relationships one generation closer, such as an aunt/uncle and niece/nephew or half-siblings, share an average of 25%.
Second cousins, who share half the DNA of first cousins, have an average shared percentage of 3.125%.
A unique case is the double first cousin, where the two individuals are descended from two pairs of siblings, resulting in an average shared DNA of 25%.