Guide to Sibling DNA Testing
Sibling DNA Testing is a way to determine whether two (or more) individuals share one or both biological parents.
Full-Sibling Study
This sibling test is for people who want to determine if they have both parents in common. In other words, share the same mother and the same father.
Half-Sibling Study
This sibling test is for people who want to determine if they have one parent in common. In other words, share the same mother or the same father).
How does Sibling DNA Testing work?
As in a paternity test, we generate a DNA profile for each person. The test analyses different genetic locations on the DNA strand. Next, we identify the allele (alternating form of DNA) at each location and give it a number. Then, we compare the DNA profiles of each test participant. The test works on the basis that related individuals should have more DNA in common than unrelated individuals.
On average the alleles (numbers) of full siblings will match at more DNA locations than half siblings. Likewise, half siblings will match at more than unrelated people.
Test Limitations
Sibling analysis is not as conclusive as paternity testing. For example, in a paternity test, exactly half of the child’s DNA will match the father and the other half will match the mother. Whereas the DNA of siblings (except identical twins) will not match each other. This is because the inheritance of alleles (numbers) from biological parents is random. At each genetic location (part of the DNA) a person possesses two alleles (numbers). A person will only pass one of these alleles to their offspring. Which allele gets passed on is completely random. Thus, one child could inherit one allele from his father, while the child’s brother or sister could inherit the other. As a result, the sibling analysis uses statistical calculations based on probability. In turn indicating whether two individuals are true biological relatives.
The statistical analysis uses factors such as:
- The number of allelic matches within the DNA profiles.
- How likely it is that we find the same allele at that same DNA location, if testing a random, unrelated individual from the same ethnic background.
These calculations determine the most likely relationship between the individuals tested.
How are Sibling DNA Test results interpreted?
We use the values from the calculations to generate a percentage probability value. Then, based on the circumstances provided, we will generate a set of hypotheses. The calculation compares each hypothesis to determine which scenario is more likely.
The percentage probability will usually fall into one of the three following categories:
0-10%
When we get a percentage probability in this range, the evidence suggests that this relationship DOES NOT exist. Or, it could be that the hypothesis suggested is incorrect.
90 – 100%
When we get a percentage probability in this range, the evidence suggests that this relationship DOES exist. In other words that the hypothesis suggested is correct.
10 – 90%
If we get a percentage probability within this range, it can yield an INCONCLUSIVE result. This means that the laboratory does not have enough evidence to provide a result. Thus, we cannot determine which relationship/hypothesis is more likely.
An INCONCLUSIVE result is rare. But we cannot issue conclusive results unless they provide very strong evidence that a relationship does or does not exist.
Causes of inconclusive sibling test results include:
• The alleles shared between potential relatives are common within the relevant general population. In other words, there is insufficient genetic evidence to confirm which relationship exists. • There is a low level of allelic (number) matches in the DNA profile. This is due to the chance nature of inheriting alleles within families (as explained above). • Mutation causing mismatches at further locations within the DNA profile.
If we provide an INCONCLUSIVE result, there may be a need for further analysis. In turn, this could incur extra charges.