Half-siblings are individuals who share one biological parent, either a mother or a father, but not both. Understanding the amount of DNA half-siblings share involves exploring the fundamental principles of genetic inheritance. While the exact percentage of shared DNA can vary, there is a general range and average that helps define this biological relationship.
The Genetics of Relatedness
Humans inherit their genetic information from their biological parents, receiving half of their DNA from each. This genetic material is organized into structures called chromosomes, with most human cells containing 23 pairs, totaling 46 chromosomes. One set of 23 chromosomes comes from the mother and the other set of 23 comes from the father. Within these chromosomes are segments of DNA known as genes.
During reproduction, specialized cells called gametes—sperm from the father and egg from the mother—are formed. These gametes each contain only one set of 23 chromosomes, representing a random selection of half of the parent’s total genetic material. When a sperm and egg unite, they combine their single sets of chromosomes to form a new individual with a complete set of 46 chromosomes, half from each parent.
Shared DNA Between Half-Siblings
Because they each inherit a random 50% of their DNA from this shared parent, half-siblings statistically share approximately 25% of their total DNA with each other. This average percentage is a direct result of both half-siblings drawing from the same genetic pool.
The amount of shared DNA is often quantified in centimorgans (cM), a unit that estimates the genetic distance between DNA segments. For half-siblings, the average amount of shared DNA typically falls around 1759 cM. However, the actual range can vary, commonly observed between 1160 cM and 2436 cM. While full siblings typically share about 50% of their DNA, the 25% shared by half-siblings reflects their single shared lineage.
Variability and DNA Testing for Half-Siblings
The precise amount of DNA shared between half-siblings can vary from the 25% average due to genetic processes like recombination and random assortment. During the formation of gametes, chromosomes exchange segments of DNA in a process called recombination, and they are then randomly sorted into new combinations. This means that even from the same parent, each child inherits a unique, random assortment of genetic material.
Consequently, while 25% is the average, the actual shared DNA between half-siblings can range from approximately 17% to 34%. This natural variability means that commercial DNA testing services analyze the specific shared DNA segments and their centimorgan values to determine the likelihood of a half-sibling relationship. These tests identify shared genetic markers to provide a probability of relatedness.