DNA, the blueprint of life, carries the unique instructions that make each individual distinct. This intricate molecule also serves as a biological record, connecting family members through shared genetic material. The amount of DNA shared between individuals varies, providing insights into the closeness of their biological relationship.
Shared DNA Percentage for Half-Siblings
Half-siblings, who share one biological parent, typically share approximately 25% of their DNA. This percentage is an average, with the actual amount varying between 17% and 34% due to random genetic inheritance. This shared genetic material is measured in centimorgans (cM), a unit that quantifies genetic distance. Half-siblings commonly share between 1,160 and 2,436 cM of DNA, with an average around 1,759 cM.
Understanding DNA Inheritance
Humans inherit roughly half their DNA from each parent, with each contributing one set of 23 chromosomes.
Before DNA is passed down, genetic recombination occurs. This process shuffles genes by exchanging segments between paired chromosomes, creating unique DNA combinations for each child. Half-siblings, sharing one common parent, inherit segments from that parent, while the rest comes from their different, unrelated parents.
Centimorgans (cM) measure the frequency of genetic recombination, indicating the likelihood of genetic markers separating during shuffling. A higher cM count signifies greater genetic distance or more opportunities for recombination between genes. Centimorgans quantify shared DNA segments, which is fundamental to understanding genetic relationships.
How Half-Sibling DNA Compares
Half-siblings share approximately 25% of their DNA. Full siblings and parent-child pairs typically share around 50% of their DNA. First cousins typically share about 12.5% of their DNA.
Other relationships can also exhibit comparable average percentages of shared DNA, which can sometimes lead to confusion in DNA test results. For instance, a grandparent and grandchild, or an aunt/uncle and their niece/nephew, also tend to share around 25% of their DNA, similar to half-siblings.
Despite similar average percentages, the specific patterns and lengths of shared DNA segments often differ. DNA testing companies can distinguish between full and half-siblings by detecting fully identical regions (shared from both parents) versus half-identical regions (shared from only one common parent).
DNA Testing and Half-Sibling Confirmation
Direct-to-consumer DNA tests analyze shared DNA segments to confirm biological relationships, including half-sibling connections. These tests measure total shared DNA in centimorgans (cM) to estimate relatedness. A shared cM value within the typical half-sibling range (1,160 to 2,436 cM, averaging 1,759 cM) strongly indicates this relationship.
Interpreting results requires more than just numerical values, as some relationships can have overlapping ranges of shared DNA. While a shared percentage near 25% strongly suggests a half-sibling relationship, genealogical information and other family context are often used with DNA data for definitive confirmation. DNA tests can distinguish between full and half-siblings by examining patterns of shared segments, such as fully identical regions versus half-identical regions.