The amount of shared genetic material between relatives reflects how recently they shared a common ancestor. Every person inherits their entire genome from their biological parents, establishing a foundational level of genetic overlap within a family unit. Understanding the percentage of deoxyribonucleic acid (DNA) that different relatives share provides a clear measure of their biological distance. This measure is the basis for modern genetic testing, allowing individuals to map their family trees and confirm familial connections.
Defining Half Siblings and Shared Inheritance
A half-sibling relationship is defined by sharing exactly one biological parent, whether the mother or the father. While both individuals receive genetic material from the same person, they each receive genetic material from a different second parent. The principle of human inheritance dictates that every child receives an approximately equal contribution of genetic information from each parent: 50% from the biological mother and 50% from the biological father.
This equal distribution allows for calculating the theoretical genetic overlap between relatives. For half-siblings, the shared parent contributes 50% of their DNA to each child. The siblings are genetically connected only through the portion of the genome inherited from that single common ancestor. The non-shared parent contributes a distinct 50% of the genome to each child, which does not overlap between the two half-siblings.
The Average Percentage of Shared DNA
The average amount of shared DNA between half-siblings is calculated to be 25%. This figure is derived directly from the principle of 50% inheritance from each parent. Since the half-siblings share only one parent, they are expected to share half of the 50% of DNA that parent contributes, which is one-quarter of their total genome.
Genetic testing reveals that the actual amount of shared DNA is not a fixed number but rather a range due to the random nature of genetic transmission. The observed shared DNA between half-siblings falls within a range of approximately 17% to 34% of their total DNA. This variability stems from a process that occurs during the formation of reproductive cells, known as meiosis.
During meiosis, the parent’s chromosomes, which are themselves a mix of their own parents’ DNA, undergo recombination and crossing over. This action involves the swapping of segments between the paired chromosomes before they are passed on to the child. This random shuffling means that the 50% of DNA a parent passes to one child is not an identical copy of the 50% they pass to another child. Because the shared parent’s contribution is a unique, randomly assorted mix for each child, the half-siblings may inherit slightly more or less than the expected 25% average. The 25% is a statistical mean, representing the most likely outcome across a large population.
Comparing Genetic Relationships
Placing the 25% average for half-siblings into context requires comparing it to the genetic overlap of other close family relationships. A parent and child share a fixed 50% of their DNA, as the child is an immediate product of the parent’s entire genome contribution. This 50% connection is the highest possible genetic relationship outside of identical twins, who share 100% of their DNA.
Full siblings, who share both biological parents, average 50% shared DNA. Like half-siblings, the amount of shared DNA between full siblings is variable, with observed ranges falling between approximately 38% and 61%. This wider range of variability exists because they inherit genetic material from two different parents, doubling the potential for unique recombination events.
The 25% average for half-siblings is genetically equivalent to the shared DNA of a grandparent and grandchild or an aunt/uncle and a niece/nephew. First cousins, who share grandparents, are expected to share a theoretical average of 12.5% of their DNA. This difference illustrates the halving of genetic material with each step of distance in the family tree.