Understanding Your Sibling DNA Test Report

Sibling DNA tests can help determine if two people share a biological relationship as full or half siblings. These tests analyze specific parts of your DNA to look for similarities that suggest a shared parent or parents. While the results may seem complex at first, this guide explains them in simple terms. We'll walk you through the report step by step, so you can interpret your findings accurately.

What Is a Sibling DNA Test?

A sibling DNA test compares the genetic material of two or more individuals to assess if they are full siblings (sharing both parents) or half siblings (sharing one parent). It differs from paternity tests, which require matches at every genetic site. Sibling tests allow for some differences because siblings inherit DNA randomly from their parents.

Common reasons for this test include:

• Confirming family ties in adoption or blended families
• Resolving questions about inheritance or medical history
• Exploring genealogy or ancestry

The DNA Science Behind Sibling Testing

To understand the numbers, it helps to know what the lab analyzes. The test targets specific locations in your DNA known as loci (singular: locus).

At every locus, a person has two genetic markers, called alleles. One allele is inherited from the mother, and the other from the father. The lab uses a process called Polymerase Chain Reaction (PCR) to identify which alleles you possess at each location.

Unlike paternity tests, which look for a direct match at every DNA locus, sibling tests analyze shared patterns. True full siblings or half-siblings are expected to share some matching alleles, but not necessarily all of them. If every sibling inherited identical markers, all siblings would be genetic clones.

Interpreting the DNA Profile Chart

On Page 2 of your report, you will find a table that lists the genetic markers tested and compares the alleles for each person. Here is an example of what that data looks like:

What the columns represent:

• DNA Locus: The name of the genetic marker being analyzed (e.g., D8S1179).
• Alleles Columns: These show the genetic values found for each person. If at least one number in these columns is identical for a specific row, the siblings "match" at that marker.
• Sibling Index Columns: These columns display the statistical strength of the match for that specific marker, calculated against standard population data. Generally, more shared alleles support a sibling relationship.

The Combined Sibship Index

At the bottom of your DNA profile chart, you'll find two critical numbers: the Combined Full Sibship Index and the Combined Half Sibship Index.

The Combined Sibship Index is the most significant number on your entire report. It is calculated by multiplying all the individual sibling index values across every DNA locus tested. This final index represents how many times more likely it is that the tested individuals are full or half-siblings than not related at all.

How to Interpret the Combined Sibship Index

Laboratories typically use the following ranges to interpret the strength of the genetic evidence:

Sibship Index Value	Interpretation	Explanation
> 10	Supportive	The genetic evidence supports a relationship. Higher numbers equal stronger evidence. For example, an index of 100 means the individuals are 100 times more likely to be related than unrelated.
0.1 to 10	Inconclusive	The genetic evidence alone cannot determine relationship status with reasonable certainty. This occurs when the DNA patterns could plausibly result from either a sibling relationship or no relationship.
< 0.1	Not Supportive	The genetic evidence strongly contradicts a sibling relationship. For example, a value of 0.01 means individuals are 100 times more likely not to be related.

Are You Full Siblings or Half-Siblings?

If the Combined Sibship Index indicates that a relationship exists, the next step is determining if the siblings share both parents (Full) or just one (Half). The report typically provides two separate values: a Full Sibship Index and a Half Sibship Index.

You can determine the most likely relationship by comparing these two numbers:

• Full Sibling: If the Full Sibship Index is significantly larger, the data points toward the individuals sharing both parents.

• Half Sibling: If the Half Sibship Index is significantly larger, the data points toward the individuals sharing only one parent.

To understand the weight of the evidence, divide the larger index number by the smaller one.

Example Calculation: If a report shows a Full Sibship Index of 100 and a Half Sibship Index of 2, you would calculate 100 ÷ 2 = 50. This means the individuals are 50 times more likely to be full siblings than half siblings.

Ready to Find Answers? 

Confirm your biological relationship with confidence. Order a Sibling DNA Test today to receive accurate, fast, and confidential results.

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