It is common for full siblings to receive different ancestry results from commercial DNA testing companies. These tests do not deliver a precise, fixed measurement of a family’s lineage; rather, they provide an estimate of an individual’s regional heritage based on the specific segments of DNA inherited. Every child receives a unique genetic blueprint, and the slight variations in which segments are passed down are the primary reason for the differences seen in sibling ancestry reports. The results simply reflect which portions of the parents’ diverse genetic makeup each child happened to inherit.
The Random Selection of Parental DNA
Humans inherit 23 pairs of chromosomes, receiving one set of 23 chromosomes from each parent. While siblings share approximately 50% of their DNA, the specific segments that make up that 50% are randomly selected during the formation of the sperm and egg cells. This process is often described as a genetic lottery, where each parent’s two copies of a chromosome are shuffled and dealt to their offspring.
Consider a parent who has genetic markers for three different regions distributed across their two sets of chromosomes. One child may randomly inherit the chromosome copy that contains a greater proportion of markers for one region, while a sibling may inherit the other copy with more markers for a different region. This random assortment of chromosomes ensures that unless they are identical twins, no two siblings receive the exact same 50% genetic material from their parents.
Chromosomal Recombination and Unique DNA Kits
The mechanism that introduces the most significant variation between siblings is chromosomal recombination, a process that occurs during the formation of reproductive cells. Before a parent passes a set of 23 chromosomes to a child, the two homologous chromosomes pair up and physically exchange segments of DNA in an event known as “crossing over.” This shuffling essentially breaks and rejoins the strands of DNA, creating hybrid chromosomes that blend the parent’s two original copies.
This process ensures that the chromosome a child receives is not simply an exact copy of the one the parent inherited, but a patchwork of both. For example, a parent might have a chromosome linked to both Western Europe and Scandinavia. Due to recombination, one child could inherit a version containing a larger block of Scandinavian markers, while a sibling might receive a version with fewer. This creates a novel combination of the parent’s genome, making each sibling’s complete set of 46 chromosomes genetically unique.
Why Ancestry Percentages are Estimates
Ancestry percentages reported by testing companies are statistical estimates derived from comparing an individual’s DNA to various reference populations. These populations are collections of DNA samples from people historically known to have lived in a specific region, serving as the template for matching a user’s segments. The company’s algorithm scans a user’s DNA for single nucleotide polymorphisms (SNPs)—specific genetic markers—to determine the likelihood that a particular segment originated from a given reference population.
Because of the unique segments inherited through random selection and recombination, siblings will have different sets of SNPs to compare against these databases. A minor difference in inherited segments can shift the statistical probability for a regional match. This shift might cause one sibling’s result to cross an algorithm’s threshold for a particular ethnicity while the other sibling’s result falls just below it. The accuracy of these results is also limited by the comprehensiveness and diversity of the company’s reference populations.
How Much Variation to Expect Between Siblings
Differences in ancestry percentages are a normal outcome of genetic inheritance, and variations of 5 to 15 percentage points in specific regions are common between full siblings. To provide a metric for the relationship, genetic genealogists measure shared DNA in centimorgans (cM). Full siblings typically share between 2,209 and 3,384 centimorgans of DNA, with an average of approximately 2,629 cM.
If the shared centimorgan count falls significantly outside this range, particularly below the lower threshold, it may indicate a half-sibling relationship, which typically shares an average of 1,700 cM. Testing companies also look for “fully identical regions” (FIRs), where the DNA on both copies of a chromosome pair matches exactly. This characteristic is present in full siblings but generally absent in half-siblings.