DNA, or deoxyribonucleic acid, contains the genetic code that makes each of us unique. DNA is passed down from parents to their children, so siblings who share the same parents also share some of the same DNA. So can DNA be used to tell if two people are siblings? The short answer is yes, DNA can often provide strong evidence of a sibling relationship.
How DNA is inherited
To understand how DNA can reveal sibling relationships, it helps to review some basic genetics. Humans have 23 pairs of chromosomes, with one chromosome in each pair inherited from the mother and one from the father. The DNA in our chromosomes determines traits like eye color, height, and so on.
When a child is conceived, each parent passes along 23 chromosomes. The child ends up with 23 pairs, getting one chromosome from each parent in each pair. Siblings receive DNA from the same parents, so their chromosome pairs contain some of the same DNA.
Full siblings (who share the same parents) inherited one chromosome from the same father in each pair, and one chromosome from the same mother in each pair. Half siblings (who share only one parent) only share on average 50% of their DNA, while full siblings share around 50-100% of their DNA, depending on which DNA regions are examined.
Analyzing sibling DNA
To determine if two individuals are siblings, scientists examine specific locations on their DNA. These locations contain short tandem repeats (STRs) – repeated sequences of DNA base pairs. The number of repeats at a specific STR location can vary between individuals. By looking at multiple STR locations, scientists can generate a genetic profile for each individual.
Siblings will have more matching STRs across their genetic profiles compared to two unrelated individuals. Researchers use biostatistics to calculate the probability that the STR profile match between two individuals would occur if they are actually siblings versus unrelated.
| STR Location | Sibling 1 Repeats | Sibling 2 Repeats |
|---|---|---|
| D8S1179 | 13 | 13 |
| D21S11 | 28 | 30 |
| D7S820 | 11 | 11 |
As this example shows, sibling 1 and 2 have matching repeats at 2 of the 3 STR locations examined. This indicates they likely inherited some DNA from the same parents. The more STR locations that match between two individuals, the higher the probability they are siblings.
Limits of sibling DNA testing
While DNA can provide very strong evidence that two individuals are siblings, it doesn’t provide absolute proof. STR testing only looks at certain locations in DNA, so it is possible siblings may differ at the examined STRs by chance and appear unrelated.
The accuracy of sibling DNA testing can be reduced if only one parent is shared between siblings. Half siblings share only around 50% of their DNA, so they have a greater chance of appearing unrelated.
Additionally, identical twins have 100% matching DNA. So sibling DNA testing cannot differentiate between identical twins, or confirm if twins are fraternal or identical.
Uses of sibling DNA testing
Sibling DNA testing has some important uses:
Immigration
DNA can help confirm family relationships during immigration procedures to prevent fraudulent visa applications based on fake family connections.
Determining biological parents
DNA can help adopted children or children conceived using sperm donors find unknown siblings that share biological parents. This can aid in tracking down biological parents.
Inheritance disputes
Sibling DNA testing may be used in inheritance disputes to confirm or refute relationships to a deceased parent. This helps ensure rightful heirs receive inheritance.
Finding relatives
DNA databases like 23andMe allow users to connect with unknown genetic relatives, including possible siblings. Users can opt-in to search the database for sibling-level matches.
Criminal investigations
Law enforcement may use sibling DNA to identify remains, track siblings involved in a crime, or establish connections in other investigations.
Conclusion
DNA contains unique information that provides strong evidence of family relationships like siblingship. Comparing DNA at specific genetic locations can reveal if two individuals likely share a mother and father. While not 100% absolute proof, sibling DNA testing gives high accuracy and helps power important applications like immigration, genetic genealogy, and forensics. The expanding use of DNA databases and growth of at-home genetic testing means sibling DNA testing will likely become even more accessible and widespread in the future. Though limitations exist, DNA remains one of the strongest tools for providing biological evidence that people are siblings.