Can a DNA Test Prove Half-Sibling Status

Finding family members has never been easier.

At-home DNA tests excel at making confident connections between relatives – even distant and ‘long lost’. This feat is accomplished by simply comparing your DNA to that of others in a database, highlighting those that share a certain number of identical genes. By analyzing the extent of genetic similarity between you, the service can make an educated guess on your relationship.

This power can be used in the other direction, helping to answer questions about close family members. Some of you may be in a situation with a complicated family, or maybe you got lucky by finding relatives with a genetic test. But, if you’re asking can a DNA test prove half-siblings, the answer is – unfortunately – not really. Read on to find out why you can’t make this distinction with certainty.

The genetics of family

99.9% of your DNA sequence is shared with every other human on earth. Thankfully, the human genome has over 6 billion A’s, T’s, G’s and C’s; the 0.01% difference that makes you unique still amounts to over 6.4 million variants and mutations. The well-known services – like 23andMe and Ancestry – investigate at least 600,000 of these to build their reports and find your family.

You received your DNA – in roughly equal proportions – from your parents: 50/50. Each half is a mixed bag from mom or dad, a relatively random mixture of their own DNA sequences. Because of this process, only about 50% of your DNA would be matched by a sibling – barring identical twins or rare cases. So, with immediate family – parents, siblings, children – half of their DNA is matched with you in each case.

Moving a step away cuts this number in half; grandparents, grandchildren, aunts, uncles, nieces, and nephews will share about 25% of your DNA. But, biology and genetics are never an exact equation.  Dividing DNA has an element of randomness that can make these numbers fuzzy. The exact percentage can vary significantly, anywhere from 12% (remember this number) all the way up to 35%. 

The same applies to half-siblings – roughly 25% matched – effectively a step removed from immediate family.

Half-siblings or cousins

By taking another step away – namely cousins – the 25% is cut in half again, to 12.5%. This value alone falls within the range mentioned above and can always be higher or lower given the variability of inheritance. Again, the exact percentage can range anywhere from 5% to 20%.

Most DNA testing companies will use a wide range of matched DNA to best capture potential immediate family members. This means that individuals who share as little as 15% of your genetic material may be labeled as half-siblings. The same range equally applies to aunts and uncles, nieces and nephews. But, a person with at least 15% shared genetics can also just be a cousin.

These ‘closer than normal’ cousins are simply a quirk of biological variability. However, there are some situations that increase the chance. Some ethnic groups have a cultural pressure to marry within their societal boundaries. This decreases the gene pool and increasing genetic similarity.

Further, some insular communities may have a history of forming relationships with distant family – 2nd or 3rd cousins for instance – that can have impacts on genetic distribution for generations.

Can a DNA test prove half-siblings?

The overlapping genetic ranges of cousins and half-siblings make them difficult to tell apart with certainty, from the percentage of matching DNA alone. Even worse, a half-brother would present – genetically – in a highly similar manner to a nephew, uncle or even grandfather! Without any other contextual or genealogical clues, they’re impossible to tell apart.

This is where a good DNA test can be worth the money. At-home DNA tests for finding relatives, like Ancestry and MyHeritage, have massive databases – both genetic and historic – that help assist the weaknesses of genetic genealogy. By prompting known relatives to get tested and confirming the relation, modern algorithms can update, clarify or change your family suggestions.

At-home DNA testing to prove half-siblings is not reliable on its own. By adding known family members to the mix and some research, genetic testing is the perfect starting point for building your family tree.