Genetics is one of the most fascinating parts of medicine and, with the right tools, you can find a series of interesting correlations, as well as answers to questions such as ‘how do half siblings show up on Ancestry DNA?’. 

However, the process is not simple and is not necessarily relevant when it comes to pinpointing the exact relationship between members of the same family. It rather measures how much DNA you share with another person from your family and, based on that value, you are placed in a certain family category such as close family. 

If you’re looking to find more about the fascinating process, here is what you need to know. 

 

Relationships and autosomal DNA 

Autosomal DNA is the most common type of DNA used in genealogy and it indicates how close two people are related. The principle behind it is simple – the more DNA you share, the closer you are or the stronger the family bond. 

However, things can get complicated as each category of family relationships has a range of possible shared DNA, and this is why, in some cases, some results might overlap. 

Depending on various genetic factors, some 2nd cousins can share more DNA than the average, which means the DNA test might interpret the results differently and may not give you an accurate description of the exact family connection between them. 

 

How do half-siblings show up on Ancestry DNA?

The unit measurement for DNA is centimorgans (cMs). Generally speaking, half-siblings will show up in the “Close family” Ancestry DNA but it is possible for them to be also placed in the “First cousin” category since the categorization of the matches is based on the amount of shared DNA, as we previously mentioned. 

Most often, half siblings share between 1300-2300 cMs of DNA, while full siblings share between 2300-3300 cMs. 

Keep in mind that the process of assigning DNA to a certain family category is called “predicted relationship” and is rarely an exact description of the relationship you share with another person. 

 

The main family groups based on the number of centimorgans

The amount of shared DNA with a member of the family ranges from 40 cMs to 3300 cMs, depending on how strong the link between two members is. 

For instance, the strongest link you’ll get is the bond between a parent and a child. The max-probability prediction says that two people are related by the strongest family bound, parent-child when their amount of shared DNA is above 3,300 cMs. 

Group A or the Full siblings usually share a common DNA of between 2,300 cMs and 3,300 cMs but, as we previously mentioned, half-siblings can also share a DNA value of 2,300 cMs, which is when the Group A and Group B family links overlap (rare cases) 

Group B is dedicated to half-siblings, grandparent-grandchild relationships, as well as aunt/uncle-niece/nephew relationship. People in this group share anywhere between 1,300-2,200 cMs of common DNA. 

Group C is designed for first cousins (1C), great grandparent/grandchild, half aunt/uncle-niece/nephew, and great aunt/uncle-niece/nephew relationships. Similarly, first cousins and half-siblings can share a similar amount of DNA, which is why test results in this category are not very accurate. 

Group D family members only share a DNA of 340-650 cMs, and it refers to half 1C (half first cousin once removed 1C1R), and half great aunt/uncle-niece/nephew. 

Finally, there is also a Group E, where only 200-340 cMs of DNA is shared, and it involves weak family links, as with second cousins (2C), first cousin twice removed (1C2R), and half 1C1R. 

Genealogy also talks about the other two family categories, Group F, and Group G, with a common DNA of 75-90 cMs, respectively, 40-75 cMs. These family categories include third cousin once removed (3C1R), second cousin thrice removed (2C3R), and second cousin once removed (2C1R), as well as half 2C, half 1C2R. 

The lower the amount of shared DNA, the more overlap there is amongst relationships, and this is why a DNA test cannot tell exactly the degree to which one person is related to another, except when they share the strongest DNA connection, meaning one of them is the parent, and the other one is the child. 

Therefore, half-siblings with strong genes can either appear in the “Immediate family” category (similarly as full siblings) or the First cousin category. But, as we previously saw, there is more than one type of relationship shared within each family category. 

 

Can sibling ethnicity help you determine the difference between full and half-siblings? 

As we mentioned before, establishing which two people are full or half-siblings is a tricky and complex process that doesn’t always have accurate results. Half-siblings can have a strong DNA connection, which means their results can also be interpreted as being full siblings, especially if they share a common ethnical background as well. 

Ethnicity estimates can offer clues to your relationship degree but cannot be used to determine the exact nature of a relationship. 

For instance, you can take two half-siblings who share the same DNA with their mother and have different fathers. If both fathers are from the same ethnical group (i.e. they are both from Eastern or Western Europe), chances are they will share a higher rate of common DNA. If you’re using the ethnicity estimate as the only guide, you might think that they are full siblings, based on the results. 

 

How to distinguish full from half-siblings on Ancestry DNA? 

If two siblings share an amount of DNA that falls into the overlap between the full and half-sibling range, there are other methods to discover the truth. Ancestry DNA changed its calculating algorithm back in 2016 and now can offer you a more accurate result. 

Shared Matches is one of the features recently introduced on Ancestry DNA that allows you to compare various results and determine, with higher accuracy, whether two people are half or full siblings. 

Generally speaking, full siblings will share close relatives (other siblings, grandparents, uncles, aunts), in common, as well as first cousins and verified second cousins. This means that only one aunt or uncle from one side of the family that is not shared can say a lot about the sibling relationship between two people and help them identify themselves as half-siblings instead of full siblings. 

Again, the result also depends on the amount of shared DNA and whether or not the amount falls within the range of full or half-siblings. 

On the same note, third cousins don’t always share DNA (although most of them do, in a small amount), so you cannot only depend on sharing a third cousin match to verify if there is a sibling relationship between you and a different person. 

 

Conclusion

DNA tests are not always accurate when it comes to determining which relatives are your cousins and which ones are your half-siblings. Since both categories can share a similar DNA range, the overlap between these two categories will let you wonder. 

As we previously mentioned, the weaker the DNA bond, the more likely it is for Ancestry DNA tests to fail at identifying the exact relationship between two people. They can be either in the first cousin group (which also included the relationship between uncles and aunts and nieces and nephews) or in the Immediate family group, same as half-siblings. 

It is almost impossible for half-siblings to share the right amount of DNA as to be mistaken for full siblings, given that, generally speaking, full siblings share about 50% of their DNA. 

This means that they took around 25% of their father’s DNA, and 25% of their mother’s. When comparing half-siblings, only 25% of their DNA will be shared, either on their mother’s part or on their father’s. Therefore, the probability of two half-siblings sharing the same amount of DNA to be considered full siblings is almost zero. 

On the other hand, there is a higher chance for DNA tests to show that two half-siblings could also be first cousins, given that they share about only 25% of their DNA. 

In this case, even when other criteria are included, for instance, ethnicity, results can still be deceiving, as both half-siblings can share a similar ethnical background. This is mainly common in people who remained faithful to their original land for more than two-three generations, i.e., people who were born and raised in Eastern Europe, Ireland, and Scotland, or Western Europe. 

Thus, properly identifying the family relationship between two people is a long and complicated process. Generally, there are about three different patterns and result-calculating algorithms implemented to distinguish between first cousins and half-siblings when it comes to Ancestry DNA. 

FIRs (Fully Identical Regions) on Ancestry DNA tell you exactly how to interpret the results and how to even tell identical twins apart from a parent or child. 

 

 

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