A sibling DNA test determines the likelihood that two individuals share one or both biological parents when parental samples are unavailable. The test compares the genetic profiles of the two people to calculate a statistical probability of a full or half-sibling relationship. Because DNA inheritance between siblings is a random process, this test cannot offer the absolute certainty of a parent-child test. Instead, it provides a strong statistical measure. Understanding the report requires examining the core science, the raw data, and the final statistical indices that summarize the findings.
The Genetic Basis of Sibling DNA Testing
The foundation of a sibling DNA test lies in the principle of shared inheritance, quantified by examining specific genetic markers known as loci. At each locus, every person inherits two versions of DNA, called alleles—one from each biological parent. Siblings naturally share a proportion of these alleles because they draw from the same parental gene pools.
Full siblings are expected to share approximately 50% of their DNA, while half-siblings are expected to share about 25%. The test compares the number of shared alleles across the genome to determine if the observed sharing pattern aligns with the expected rate for full siblings, half-siblings, or unrelated individuals.
The test looks for Short Tandem Repeats (STRs), which are repeating sequences of DNA that vary greatly between individuals. Full siblings have a higher chance of sharing two alleles at a given locus compared to half-siblings, who share at least one allele from the common parent. The greater the overlap in these markers, the higher the calculated probability of a close biological relationship.
Decoding the Raw Data: Allele Matches and Loci
The raw data section provides a detailed, locus-by-locus comparison of the two individuals’ genetic profiles. This is typically presented as a table listing the name of the genetic marker (locus) and the two alleles inherited by Subject A and Subject B at that location. Loci names often include identifiers like D8S1179 or TH01.
Each locus entry lists the two alleles the person possesses, represented by numerical values. For example, if Subject A is “12, 14” and Subject B is “14, 15,” they share one allele (14). If they are both “12, 14,” they share both alleles at that locus.
If two individuals are full siblings, the report will show a significantly higher number of shared alleles across all tested loci than if they were unrelated. This tabular data serves as the direct genetic evidence that feeds into the final statistical metrics reported in the summary.
Interpreting the Sibling Index and Probability
The statistical summary includes the Combined Sibling Index (CSI), also known as the Likelihood Ratio (LR), and the final Probability of Relationship percentage. The Sibling Index is a ratio comparing two hypotheses: the likelihood of observing the genetic profiles if the individuals are siblings versus the likelihood if they are unrelated.
A CSI greater than 1.0 supports the relationship. For example, a value of 100 means the tested individuals are 100 times more likely to be siblings than not. A CSI greater than 100 provides strong evidence supporting a full sibling relationship. Conversely, a CSI less than 1.0 suggests the genetic evidence favors the hypothesis that the individuals are unrelated.
For half-sibling analysis, a lower threshold is used, with a CSI greater than 10 indicating a supportive result. The Probability of Relationship percentage is derived from the CSI and represents the likelihood that the tested individuals are related, given the genetic evidence. A result of 90% or greater is considered conclusive evidence of a biological relationship, while percentages between 10% and 90% are deemed inconclusive.
To determine the likelihood of full versus half-sibling status, laboratories calculate both a Full Sibling Index and a Half Sibling Index. If the Full Sibling Index is the larger number, the genetic evidence favors a full sibling relationship.
When Results Are Complex or Inconclusive
An inconclusive result, expressed as a Probability of Relationship between 10% and 90%, means the genetic evidence is neither strong enough to confirm nor deny the relationship. This outcome is more common in sibling tests because the amount of shared DNA among siblings is inherently variable. Random inheritance can lead to full siblings sharing less DNA than average, or unrelated individuals sharing more by chance.
A primary reason for ambiguity is the absence of parental DNA. Including a known parent, especially the mother, significantly increases the test’s accuracy by allowing the lab to isolate which alleles each sibling inherited from the unknown parent. Without this reference, the statistical analysis must account for more possibilities.
Another complication arises if the alleged parents are closely related, a condition known as consanguinity. If the parents are related, the statistical model’s assumption about the general population is incorrect, which can artificially inflate the number of shared markers. If an inconclusive result is returned, the most effective step to gain clarity is to test a known, shared parent or another close relative.