What is a sibling DNA test?

Written by Ellen Hinkley
29 January 2019

Sibling tests analyse and compare the DNA of two or more potential siblings to see if they share one, two or neither parents. There are three main conclusions that can result from a sibling DNA test: half-siblings, full-siblings or unrelated.

There are a number of reasons you might want to take a sibling test. You might have doubts whether your father or mother are biologically related to you, but your parent isn’t available for testing. Or you may want to move to a country where your brother or sister lives, and need to prove a biological relationship with them.

If you simply want reassurance about your relationship to your sibling, you can easily purchase a “peace of mind” sibling DNA test online. If you are taking the test for legal purposes and the result needs to be admissible in court, you’d need a “legal” sibling test. If you’re taking the test for immigration purposes, a specialised “sibling test for immigration” is required. It’s worth noting that legal and immigration tests must be carried out by a government-approved lab, and often cost more than peace of mind tests.

If you’re looking for a peace of mind or legal sibling DNA test, you can visit our sibling DNA testing page to read reviews and compare prices. If you’re looking for a sibling test for the purposes of immigration, you may be better off checking out our immigration DNA testing listings.

How sibling DNA testing works

Taking the test

Once you’ve purchased the test online, both you and your potential sibling will receive a testing kit in the post, along with any other family members who have chosen to participate in the process (discussed below). The kits will contain cheek swabs so that each of you can collect “buccal” samples (cheek cells) from the inside of your cheek. Collecting a sample in this way is painless and only takes a few minutes. The kits will include containers for the used cheek swabs, which will prevent contamination when you return them to the lab.

In order to receive the most conclusive result possible, if you know that you definitely share one parent with your alleged sibling, providers recommend that you include this parent in the testing process. This will help the testing company to distinguish between the shared DNA from the known parent, and the DNA that you potentially share from the alleged parent.

For instance, if you are taking a sibling DNA test to confirm that you and your sibling share the same father, and you already know you have the same mother, you should try to include a sample from your mother if you can. If you have a shared parent who is unwilling or unavailable for testing, the test will purely look for shared DNA between you and your potential sibling.

Analysis of samples

Unlike ancestry DNA tests, which analyse your DNA for genetic variations that can be linked to global populations, relationship tests like paternity or sibling tests look for Short Tandem Repeats (STRs). These are segments where your DNA repeats itself in an identifiable pattern, which is passed down from parents to their children. The repeating patterns in your DNA can be compared with those of your alleged sibling to find how likely it is that you’re related – and whether you’re more likely to be full or half siblings.

Siblings usually share less DNA than parents and children do, since not all the DNA they inherit from their parents is the same, which is why siblings (that aren’t identical twins) don’t look exactly alike. Siblings tend to inherit segments of DNA that are identical, some segments that are semi-identical, and some segments that are completely different.

This results in full siblings sharing about 50% of their overall DNA, meaning that about half of their STR markers will match exactly. This is just an average, and the percentage of DNA that two siblings share is different in each specific case. In theory, two siblings could share 0% or 100% of their DNA, though these extremes are highly unlikely in reality.

How conclusive are sibling DNA tests?

An important point to be aware of when taking a DNA sibling test is that they are less conclusive than paternity or maternity tests. This is because the amount of DNA that siblings share varies and can be quite a lot lower than that of a parent and child, so there will be fewer STR markers that match even if the siblings are related. For this reason, DNA sibling tests can’t be used to confirm or disprove that two people are full or half siblings with 100% certainty.

That said, most reports will come with an assessment of the conclusiveness of the results. For example, something called the ‘Combined Relationship Index’ (CRI) will give you a numeric value to indicate how conclusive the results are. The CRI isn’t exclusive to sibling DNA testing, but is used for a range of genetic tests for different types of relationship. The thresholds change depending on the type of relationship, for example, paternity tests generally come out with one of two CRI values: 0.00, which excludes the possibility of paternity, and 100.00, which means the possibility of paternity is highly likely. For the reasons discussed above, sibling tests often produce intermediate CRIs, so the results aren’t as clear cut.

How to get the most conclusive results possible

The conclusiveness of results depends largely on the other members of your family who are available (and willing) to be tested. As previously mentioned, if you are taking a DNA sibling test to determine whether you are full or half siblings, including a known shared parent in the testing process is an effective way of increasing the likelihood of receiving conclusive results.

If your known parent is not available for testing, some testing companies will allow you or your alleged siblings’ biological aunts, uncles or grandparents to submit a sample to improve the conclusiveness of the results. If testing additional family members is of interest, it’s worth asking the testing company which relatives they’re prepared to test, and by how much they expect this to improve the accuracy of the result. The additional kits can then be sent out to the relevant test participants when you order.

You can often combine the types of DNA tested with the available relatives to set up the testing process in a way that delivers the most conclusive result. It’s important to remember that DNA testing companies answer questions about sibling testing all the time, and they will be able to help you set up the best possible test. If you need a sibling DNA test, we advise that you call a few testing companies to explain your situation first, and they’ll help you understand what your options are.

Number of STR markers

One way in which you can maximise your chances of receiving conclusive sibling DNA test results, regardless of how many family members are tested, is to get as many STRs analysed as possible. The number tested ranges from 15 to 68, though not all companies provide information on how many they include in their analysis. The more that are included, the more information there is to base the conclusion on, and therefore the conclusion can be made with more certainty.

Y-DNA tests

One way in which males can prove or disprove that they share a biological father is through Y-DNA testing. This type of DNA takes its name from the Y chromosome, which is exclusive to males and passed directly from father to son. This means that a son will have the same Y-DNA as his biological father, his paternal grandfather, his paternal great-grandfather, and so on.

Although natural changes in Y-DNA do occur, these happen pretty rarely. The fact that males inherit a relatively unchanged copy of this chromosome from their fathers means that any full brothers will share the same Y-DNA, so this can be used to prove or disprove full siblingship.

Mitochondrial DNA tests

Unfortunately, females can’t take Y-DNA tests as females don’t inherit Y chromosomes. However, if you’re looking to prove or disprove that two siblings share a biological mother, then you and your sibling’s mitochondrial DNA (mtDNA) can be tested.

Like Y-DNA, mtDNA also remains largely unchanged across the generations, and because we each inherit our mtDNA from our mothers (regardless of whether we’re male or female), the mtDNA from two alleged siblings can be analysed to determine if it’s been passed to them by the same mother, or two different mothers.

X DNA tests

Both males and females have X chromosomes. Females have two X chromosomes, while males have one X chromosome and one Y chromosome. X DNA can be used in place of STR testing to establish whether two potential half-sisters have the same biological father, providing it is known that they do not share a biological mother.

X DNA can also be used if two potential full-sisters wish to know if they share the same biological father, and their biological mother is available for testing. In this circumstance, the provider can exclude the X DNA inherited from their biological mother and compare the X DNA inherited from each sister’s father.

If two potential full-sisters share the same biological mother but she isn’t available for testing, then the alleged paternal grandmother (the alleged father’s biological mother) can participate in the testing process. As the alleged paternal grandmother will have passed her X DNA to her biological son, and he will have passed this on to his biological daughters; identifying whether both potential full-sisters share this X DNA with their alleged paternal grandmother can prove or disprove that they share a biological father.

The cost of buying a DNA sibling test

DNA sibling tests range quite a lot in price, depending on the provider you choose, the number of markers included, and whether you purchase it for peace of mind or legal reasons. Most tests analyse between 24-35 markers, and cost around £170 to £200. Make sure you check the number of markers tested when you’re researching a test, as a higher price doesn’t necessarily mean more markers.

If you are testing for legal rather than for your own personal reasons, you should expect to pay about £100-£150 more for the test. Note that there are often extra costs that aren’t included in this price, such as hiring a health professional to witness the samples being taken and to certify your identification.

It’s worth remembering that including known or alleged parents in the testing process often comes at no higher cost than testing just the potential siblings. This is because the additional participants cause the results to be easier to interpret and makes a conclusive result more likely. You will usually have to pay extra if you’re testing three or four potential siblings as opposed to just two, but the inclusion of known or alleged parents can actually reduce the price!

If you want to save a bit of money, you may be tempted to order a “free” testing kit, or one that allows you to take your samples without any specialised collection swabs. Although these options may appear to offer better value for money, they often end up costing as much, if not more than buying a kit with everything included.

If you order a “free” kit, you’ll still have to pay for the analysis of your samples, which is the most expensive aspect of the DNA testing process (the kit itself is relatively worthless). Collecting your own samples without specialised swabs may appear to make the process cheaper and easier, but the risk of contaminating the samples is higher, which will potentially make the results less reliable.

On a related note, you should never be tempted to send a “discreet” sample (i.e. a DNA sample taken without the person’s knowledge). These deliver far less reliable results, and in some countries (including the UK) it is illegal to test a person’s DNA without their knowledge.

How long will it take to receive my results?

In general, sibling DNA test results are available pretty quickly – usually under a week. But there are a few things to be aware of when working out how long you’ll have to wait. The length of time varies depending on the company you use, though most will tell you how long results will take on their website.

Unfortunately, the stated processing times don’t usually account for the time it takes for you to receive the testing kit in the post, collect the samples, and send them back to the lab. These are important factors to consider if the testing company is located abroad, as this may add time to each stage of the process.

Once your samples have been received by the lab, you should expect to receive your results between one working day to two weeks later, depending on the length of time stated. Some companies offer “express” testing, meaning you’ll be able to receive your results the day after your samples reach the lab, or even on the same day that they receive your samples. This is useful for those who need results as soon as possible, but this choice often has a much higher price than the standard option.