Testing Siblings

Identifying Brothers and Sisters

Who Are My Brothers and Sisters?


  • Generally STR analysis is the best method.
  • We each have a double copy of our DNA one inherited from our mother and the other from our father.
  • Parents have two STR types and randomly pass only one of them to their children.
  • Siblings can have inherited either the same or different STR types from their parents.
  • Probabilities that two people are related as siblings are calculated based on the number of STR types they have in common.
  • When only two siblings are tested the statistical result produced is considerably less powerful than a standard paternity test.
  • It is best to choose a tester that routinely tests 15 or more STR chromosomal locations (known as loci).
  • If samples are available from one or both parents these can also be tested for a much more powerful result.
  • Additional testing using Y-STRs (males only) and/or mtDNA can be purchased especially if STR results are weak or inconclusive.
Read on for a more detailed explanation…

Siblings 2 Many DNA testing companies offer full and half sibling tests. STR testing is the most commonly used method. The best way to establish if two people have both parents in common (full siblings) or just one parent (half siblings) is not just to test the siblings but also to test the supposed parents if they are available to provide samples. Testing both parents tells us everything about the pool of DNA from which two people must derive their DNA if they are siblings. If you are able to test parents as well as siblings and matching results are obtained, then there will be a very strong statistical outcome reported, not unlike a straightforward paternity calculation. If only one supposed parent is available to provide a sample then the statistics wont be as strong but there is still likely to be good statistical strength to support the existence of the relationship if a relationship exists. If no parents are available then STR tests for siblings loose some of their power. Look at this diagram which is explained in more detail in How The Test Works. The numbers in the diagram are the different STR DNA types, or alleles, that a parent couple and their children have. The reason there is always a pair of alleles is that we each have a double copy one that we inherit from our mother and the other from our father. We can only pass on one of our two types to our children.

Paternity Test Diagram

As you can see there are 4 possible sibling combinations for every STR test and it is random chance that determines which of these types a sibling will become. For example both siblings could, by chance, inherit the same DNA allele types as Child 1 and they would have both alleles in common. Alternatively one sibling could inherit Child 1’s types and the other could inherit Child 4’s types and in this case they would have no allele types in common and there would be no DNA matching alleles. As you can see, it is possible that even full siblings might share no STR alleles at all. It follows, therefore, that it is never possible with absolutely certainty, to exclude the possibility that two people are related as siblings even when their DNA results are completely different. With an individual STR test, siblings will either have no alleles in common, one allele in common or both alleles in common. Although this doesn’t sound very helpful, the test may still be worth doing because when a panel of many STR tests is used (often 15 or more) the overall result will often provide some statistical support for the existence of a sibling relationship. Bear in mind though that the statistic is not likely to be very big and occasionally it might be too weak to be considered informative at all. The statistical evaluation relies on the fact that siblings are expected to have more STR alleles in common than unrelated people. When it comes to half sibs, who have only one parent in common and normally share fewer alleles than full sibs, you can imagine that STR tests alone have limited power to demonstrate their relatedness, unless of course parents’ samples are available for testing. The take home message is; a sibling or half sibling DNA test might be worth doing especially if a bit of confirmation is all you need but be aware your results may not help and don’t expect them to be anywhere near conclusive. If you need or want more certainty then read on because it is possible to purchase different types of DNA tests that could help.

It is possible, and sometimes necessary, to use other types of test to provide more evidence that two (or more) people are related as siblings. The first is mtDNA sequencing. Mitochondria are numerous small chambers found within all living cells that produce the energy a cell needs – a bit like a tiny power plant. Mitochondria have their own piece of DNA (denoted mtDNA) that is quite separate from the chromosomal DNA in the nucleus. Mitochondria are present in the mothers egg but none is passed from the father’s sperm when the egg is fertilized. So all of us, male and female, inherit all of our mtDNA from our mothers and none from our fathers. Brothers and sisters therefore will all have the same mtDNA type. In fact everyone who belongs to the same maternal line (mother, grandmother, great grandmother, maternal aunts and uncles, maternal cousins etc, etc.) will have the same mitochondrial DNA type. So if two people have a different mtDNA type you can be certain that they are not full siblings, although they could still have the same father. If there is a mtDNA match then there is a strong possibility they are related through the maternal line and they could be siblings although it doesn’t help determine if they have the same father. The mtDNA test uses DNA sequencing technology and it is generally bit more expensive. It will cost anywhere between $300 to $800 to test a pair of supposed siblings.

The next commonly used method is also a DNA STR method but it targets Y-STR sites found only on the male Y-chromosome. Only males have Y-chromosomes (the male sex chromosomal pair is XY and the females is XX) therefor Y-chromosomes are only passed on from fathers to their sons. This means two or more brothers have the option to test themselves to see if they have the same father by purchasing a Y-chromosome test. If they have the same combination of Y-STR types, i.e. have the same Y-STR haplotype, (a haplotype is just a combination of DNA types that are inherited together) then there is strong evidence that they belong to the same male lineage and have the same father. If their Y-STR haplotype is different they cannot have the same father. It is important to bear in mind that Y-chromosome DNA tests cannot distinguish between males who belong to the same male line and so brothers will not only have the same Y-haplotype as their father but also their paternal grandfather, their paternal uncles and their fraternal nephews and so on. Obviously this option is not available for sisters or comparing a brother and sister. In this situation, assuming her father isn’t available, if the sister could obtain a sample from any male who is known to belong to her father’s male lineage eg a paternal grandfather, uncle or cousin, then this sample could be tested as a proxy for the sister.

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