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Sibling Testing

Sibling DNA Testing

The TRUTH about Siblingship DNA Testing and its Accuracy:

While sibling testing can provide very accurate and excellent results it is not guaranteed to do so. Many labs or companies will not be up front about the difficulties and issues with this kind of a test. They are in the sales business and will just sell a test of this nature to you if you ask for it. However, our company takes a different approach to these matters. We believe that education is key and that it is important for you to understand how DNA testing works before making this important decision. (The below information applies to any sibling test at any lab.)

Please read this entire page! After we explain how sibling DNA testing works we will explain other types of DNA tests that can be performed in order to give you similar answers. If you still have questions, please do not hesitate to contact us as we are happy to answer any questions you might have.

General Information About Siblings: Due to the fact that true siblings have the same parent(s), they have more genes in common than two unrelated individuals. When matches between two potential siblings are found, calculations are made to determine the probability that a sibling relationship exists. However, there are millions of genetic combinations when a child is created, and it’s quite possible, merely based on the “genetic lottery”, that two siblings will not show having similar DNA even if they look alike.

The only siblings that do share an identical genetic makeup are identical twins. This is so because after the mother’s egg is fertilized with the father’s sperm, it splits in half essentially creating a clone of its self.  Fraternal twins are two separate eggs and two separate sperm groaning together in the womb and therefore will have different genetic makeups, just like normal siblings.

Unfortunately, sibling DNA testing is not as simple as parentage test (mother/father DNA testing). With any relationship DNA test, results based on a match or no match at each genetic location, along with a statistical calculation on how strong that match is. Parents will match their child at each genetic location since that child received half of their DNA from each parent from the egg and sperm. However, when comparing siblings in a DNA test, there are multiple genetic possibilities. Therefore, while we can guarantee a 0% OR 99.9+% result for a paternity or maternity test, we cannot guarantee that same outcome for a sibling test. No lab can.


It is best explained like this: When the egg is fertilized by the sperm the chromosomes group together in pairs. Therefore, each person has two numbers at every genetic location. The numbers represent the frequency that the genetic material replicates and therefore expresses its self. When we perform almost any type of DNA test we must first create a DNA profile for each person. Once that profile is created we can run a series of different genetic relationship tests.  When a profile is created each person should have two sets of numbers (one from mom and one from dad)  at every genetic location (loci). We test a minimum of 16 loci to insure accuracy.These two numbers will be listed under columns titled Allele A and Allele B(Pronounced A-leel).

Again, one number is given by the mother and one number is given by the father to each child.  Either number can be located on the A or B side. For instance, the A side does not belong to the mother and the B to the father. It is a random placement. Therefore, it is not necessary to worry about the A or B, but rather, just focus on the numbers themselves.

When we look at a person’s two numbers at a genetic location we do not know which number comes from the mother or father until we profile the parents’ DNA and compare it against the child’s. If we tested a child with their biological mother and father then we would see something like this:

Mother:     Father:     Child:
12 & 14     2 &  6       14  & 2


If you look at the child’s DNA, they received the 14 from the mother and 2 from the father. Match! Easy, right? So why isn’t sibling DNA testing easy?

Well, you see how each parent has TWO numbers? In the case of the child above, they received a 14  and a 2 from the parents. However, let’s say this family has multiple children, there will be multiple possible genetic outcomes since not every child will receive a 14 and a 2. We like to call this the “genetic lottery”.

Below are examples of how just a mother can have multiple children and each will have a different number from her.

Mother: Loci has 12 and 14 (see below for random possibilities)

First child: 12.  Second child: 14. Third child:14 -or-
First child: 12.  Second child: 12. Third child:12 -or-
First child: 14.  Second child: 14. Third child:12 -or-
First child: 14.  Second child: 14. Third child:14


…and so on. Now add a father to the mix and you could have full siblings with multiple outcomes. Here are five examples:


Mother: 12 & 14     Father: 2 & 6
(See below for random genetic outcomes with one number from each parent.)

First Child:       14 & 2
Second Child:   6 & 12
Third: Child:    12 & 6
Fourth Child:    6 & 14
Fifth Child:       2 & 14


Example 1: Let’s say that the First Child and the Third Child wanted to perform a DNA sibling test to see if they were likely to be siblings. Although they have the same two parents and are full siblings they do not match each other at this particular loci which means that we cannot prove that they are related at this loci even though they are in fact full siblings.

Example 2: Let’s say that the Second Child wanted to have a DNA test with the Fourth Child. The result would show they they have one Allele in common, a 6, but it’s not a full match. Also not very strong, but stronger than no match at all.

Examples 3 & 4: Now, if the First Child and Fifth Child tested with each other, or the Second and Third Child tested with each other, they would match each other at both genetic locations and it would be a very good result.


However, it is important to keep in mind that in the example above the numbers represent only one specific genetic location or loci. We test a minimum of 16 loci to insure accuracy and that means the the random genetic lottery for each set of siblings is practically endless. This is why sibling DNA Testing can be so difficult and the result cannot be guaranteed in the same way as a parentage test. In most cases, it helps to add as many siblings or family members to a non-parentage DNA test as possible so that we can obtain as much genetic information about the family as possible.  Below, under the “Half Sibling” section, we explain why it is best to include the mother in a DNA sibling test. For other types of tests in place of sibling DNA tests, skip the “Half Sibling” section and go to “Alternative DNA Testing Options”, below.


Let’s say that there are two individuals with different mothers who want to test to see if they share the same father. Not only do they have to eliminate all of the maternal (mother) DNA, which they do not share, but then if they do have the same father they still have a 50% chance of having received the same DNA at each genetic location. This can be a challenging test.

This is why, although it does cost more money, we always recommend including one or both mothers, if possible, in any test of this nature. If the mothers are included then we can eliminate half the child’s DNA from the test and by eliminating half of each person’s DNA we increases the accuracy of the test. What we are left with is the paternal (father’s) DNA only, then we can run a statistic that analyzes the DNA and asks “are these two people more likely to be related as half siblings or not at all?”.

We understand that for various, unfortunate reasons, a mother or mothers cannot be included in a sibling DNA test. We can still run the test without her, but the the truth is that the results will not be as accurate. However, some people do not have any other options and so they must hope for the best outcome when buying a test of this nature.


Although we have explained above why is is best to include the mother in a sibling DNA test, it is also best to include as many siblings as possible or other relatives, such as Aunts or Uncles to get the best result. Because multiple siblings from the same parents will have different sets of DNA, we can more easily reconstruct family relationships in this way.


MtDNA or Mitochodrial DNA Testing: Both men and women have an X sex chromosome. This X is from the mother and is called Mitochondrial DNA,  also known as Mito or MTDNA. The Mitochondrial DNA is a vast amount of genetic information and is passed along from generation to generation. Any person from the same maternal line will have approximately the same Mitochondrial DNA. This test can go back for generations as far back thousands of years and is also one method for tracing ancestry. Therefore, two individuals who think they might have the same mother can test their MTDNA to see if they match.

Y-Chromosome DNA Testing: Only men have a Y sex chromosome. The Y-chromosome test is useful for males who need to find out whether they are related to another male along the same male lineage, such as their father. For example, a brother who shares the same father, an uncle or cousin who shares your grandfather, or a grandson and his grandfather will have the same Y-chromosome. This test can go back for generations as far back thousands of years and is also one method for tracing ancestry. Thus, this test is very powerful for determining male progenitor relationships, and it helps to solve the question of whether two males are related along the male line.

Grandparent DNA Testing is used to establish a maternal or paternal relationship in the place of the mother or father. For instance, this test can provide the same result as a paternity test if both paternal (father’s)  grandparents and the mother of the child are included in testing. This is because both paternal grandparents will together make up half of their child’s DNA. However, when you eliminate the mother and just test one Grandparent, the effectiveness of the test decreases. Along with not being as as conclusive, this test is more expensive than a Paternity and should only be purchased if a paternity test is just not an option. For more information, click Grandparent DNA Testing.

Avuncular DNA Testing: Avuncular means an Aunt or Uncle Relationship.  Avuncular DNA testing means that we will test the Aunt or Uncle of a child to see how well they match. In this case, the Aunt or Uncle is a sibling of the possible parent in question and the same rules as sibling testing apply.  This type of test is best to do if the mother of the child c Aunt or Uncle is a full sibling to the possible parent as they will share more DNA. Since each child receives one number from the mother and one from the father the same goes for when they have a child. They either pass their mother or father’s number along to their child. There is a 50/50% chance that two children will get the same number. In the case of an Avuncular test, we hope that the two siblings in question share a good amount of DNA. If they do, the niece or nephew will match them better. Just like a single grandparent DNA test, this test is not as conclusive and is more expensive than a Paternity and should only be purchased if a paternity test is just not an option. When I spoke to the lab tech, she recommended this test over the single grandparent just by a margin.

If you still have any questions, need additional assistance or would like to purchase a DNA Test, please contact us by Web Form or call (888) 362-4339 to speak to live person during business hours. If you select us as your DNA service provider, we will give you the manager’s cell phone number so that you may contact us after hours or on the weekend. For legal advice, please consult a family attorney.

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