Consumer DNA testing has become a widely used tool for exploring family connections and understanding personal heritage. Many individuals turn to these tests to confirm or discover relationships, including those with half-siblings. Interpreting half-sibling DNA test results requires understanding the science behind shared DNA and how testing companies present this information.
The Science of Shared DNA
Each person receives half of their DNA from their biological mother and the other half from their biological father. This genetic material, known as autosomal DNA, is organized into 22 pairs of non-sex chromosomes used in relationship testing.
Scientists use a centimorgan (cM) to measure the length of DNA segments and the amount of shared DNA. A centimorgan is not a physical distance; it represents the probability that a DNA section will pass down intact without being broken up by genetic recombination. The more centimorgans two people share, the more closely they are related.
Genetic recombination is a process where DNA segments from a pair of chromosomes are exchanged during the formation of egg and sperm cells. This random shuffling explains why siblings, even full siblings, do not inherit the exact same DNA from their parents. The amount of shared DNA, measured in centimorgans, can vary among close relatives.
Decoding Shared DNA Percentages
DNA testing companies typically provide a total centimorgan (cM) value and a percentage of shared DNA. For half-siblings, who share one biological parent, the expected range is generally 1,160 to 2,436 cM, averaging around 1,759 cM. This usually corresponds to approximately 25% of their DNA.
For comparison, full siblings, who share both biological parents, typically share significantly more DNA, ranging from 1,613 to 3,488 cM, averaging around 2,700 cM (roughly 50%). First cousins, sharing grandparents, usually share less DNA, averaging about 866 cM, with a range between 396 and 1,397 cM (around 12.5%).
The ranges for different relationships can sometimes overlap. For example, a half-sibling match with a high cM count might occasionally overlap with the lower end of a full sibling range. This overlap occurs due to the random nature of DNA inheritance, where individuals can inherit more or less DNA from their shared ancestors. Analyzing these numerical values provides a foundational step in understanding the likely relationship.
Understanding Shared DNA Segments
Beyond the total amount of shared DNA, the way DNA is shared in segments provides additional insights. A shared DNA segment is a continuous stretch of identical DNA between two individuals, indicating inheritance from a common ancestor. The number and length of these shared segments are important for confirming a relationship.
Half-siblings typically share DNA in fewer, but often longer, segments compared to full siblings. Full siblings share DNA from both parents, leading to “fully identical regions” (FIRs) where they inherited the same DNA from both parents. Half-siblings do not have significant FIRs; they primarily share “half identical regions” (HIRs), matching only on DNA inherited from their one common parent.
To be considered significant, a shared DNA segment generally needs to be above a certain centimorgan threshold, commonly around 7 cM, though some companies use 8 cM. Examining segment characteristics, such as overall length and the absence or presence of fully identical regions, offers stronger evidence for a half-sibling relationship than relying solely on total shared cM, especially in ambiguous cases.
Navigating Unexpected Results
Occasionally, DNA test results may not align with initial expectations, presenting shared DNA percentages or segments outside typical half-sibling ranges. Several factors can contribute to these ambiguous outcomes. One common reason involves unknown close relatives, such as a shared aunt, uncle, or even a full sibling previously thought to be a half-sibling.
Genetic inheritance patterns can also be a factor, as the random nature of DNA distribution can sometimes result in higher or lower shared cM values than average for a given relationship. Endogamy, where individuals come from populations with a history of intermarriage, can also cause shared DNA to appear higher due to distant ancestral origins rather than recent close relationships. While rare, testing errors are also a possibility.
When faced with unexpected results, there are practical steps to take for clarification. Contacting the DNA testing company’s support team can provide further data interpretation. Testing additional known relatives, such as a presumed parent or other siblings, can help triangulate shared DNA and narrow possibilities. In complex cases, consulting a professional genetic genealogist offers specialized expertise in analyzing DNA data and constructing family trees.