Sibling relations are almost always the most complex and confusing relationships to decipher. With the recent shift in the nature of family structures, many individuals might be living with half-siblings or step siblings. The easiest way to confirm your sibling relationship would be to take a DNA test, but the test results do bring about a multitude of questions such as “Why are my results different from my sibling?” or “Are my siblings and I even full-siblings?”. These questions are not uncommon, but many just can’t find the right answers to it.
Thus, it is important that we begin to assess, address and understand the fundamentals of full and half-siblings. Let’s begin by talking about the different types of sibling relationships!
Full Siblings vs Half-Siblings
To differentiate between full siblings and half siblings, the key definitive point is the amount of shared genetic information or DNA that two people have. As everyone gets half of our genetic information from each parent, this is the most foolproof way to determine the relationship between you and your sibling.
Full siblings have the same birth mother and the same birth father. This means that full siblings receive all their genetic information from the same parents. The changes in family structure such as the marital status of any parent does not alter the fact that two individuals are full siblings.
Half siblings, or half sisters and half brothers, share only one birth parent and are only related by blood to one parent, either the biological mother or biological father. This means that you and your sibling are considered half siblings if the both of you have the same birth mother but different father, or same birth father and different mother.
Half Siblings vs Step Siblings
To clear all doubts about these two terms, as mentioned above, half siblings are siblings that you share one birth parent with, and are related by blood to one parent. However, a step sibling is someone that you share no blood or genetic relation with at all. They are the child of your step parent, who is not related to you in any way. The main difference is once again, shared genetics.
Alright, so now that we’ve broken down the basic definition of full siblings and half siblings, how do we then tell the difference, in terms of genetics between them?
Genetic Difference between Full and Half Siblings
There are two main ways that you can go about identifying your sibling relationship:
- Calculating the Total Number of Shared centiMorgans (cM)
- Using Fully Identical Regions (FIR) versus Half Identical Regions (HIR)
Calculating the Total Number of Shared centiMorgans (cM)
Before anything, in order to know the number of shared centiMorgans you have with your sibling, you would first have to take a DNA test. It is important to choose the right DNA type that is most suitable for you, depending on what you want to achieve from your test results.
Since each child receives half of their mother’s and father’s DNA, full siblings will share an estimated 50% of genetic information, while half siblings will have approximately 25%.
Estimated number of centiMorgans and shared DNA
To understand your DNA matches in greater depth, and how to find and calculate the total number of shared cMs that you have with your sibling, we have provided a detailed breakdown on identifying DNA matches and shared centiMorgans here.
Using Fully Identical Regions (FIR) versus Half Identical Regions (HIR)
Fully Identical Regions (FIR)
When two people share a fully identical region (FIR), they will have identical alleles on both strands of the same chromosome.
These are recombination processes that can happen when two individuals share FIR:
- Two individuals (full siblings) share identical chromosomes that they have both inherited from their paternal grandparents and maternal grandparents. This means that they have 50% of the same shared DNA.
- When two individuals inherit 25% of shared DNA from their maternal grandfather and from their paternal grandmother, and vice versa.
After you have taken a DNA test, if you and your sibling/s are full siblings, you will probably expect to have the exact same ethnicity results as your sibling/s. However, that is a false assumption. Why so? Well, everyone’s result is based on the inheritance pattern which can differ between siblings. Since you can only inherit at most 50% of your ancestor’s DNA, there are certainly chances that you will inherit different portions of ancestral DNA as your siblings, because DNA inheritance happens randomly. This means that you might not inherit exactly 25% of DNA from each grandparent, and 12.5% from your great grandparents etc.
While full siblings do share significant FIRs, they also share half identical regions. On the other hand, half siblings do not have any FIRs with each other, with the rare anomaly of their parents being in the same population group.
Half Identical Regions (HIR)
Half identical regions (HIR) consist of two chromosomes in a homologous pair, where no less than one of two alleles from an individual is identical to that of the other individual’s pair of chromosomes in the same region. To put it simply, this means that two individuals share a specific DNA segment that is 50% identical.
To find out your sibling relationship by using FIR and HIR, here are some companies that include your FIR and HIR in their DNA test results:
Unexpected Match: What Happens If I Didn’t Know That I Was Not Fully Related To My Sibling?
First of all, being full siblings or half siblings does not dictate your value as a family member. If you received a surprising DNA test result, I’m sure that you will be put in extreme disbelief. Before anything, it would be ideal if you take a retest to double or triple confirm the test results.
In order to find out the exact relationship that you have (or whoever you’re testing for), here are some steps that you can take:
- Always look at all the matches provided to you. Take a look at the closer matches and try to link them together. Have they always been your siblings? Has there been any changes in your family structure? It is always good to have questions!
- If you do not have any known matches, then something is definitely wrong. It would definitely be best if you do a retest using more family members.
If you are having trouble getting your parents to do a DNA test, here are some ways you can approach them:
- Inform them that this is for health purposes. This would definitely entice them to be more participative as health is wealth! There is less reason for them to reject your request if they are doing this for their own health sake.
- Nothing better than just telling the truth. I’m sure your parents will be understanding if you genuinely share with them the real reason you’re doing this DNA test.
- If your parents are really unwilling to help, get your grandparents’ DNA instead. It is always best to get the oldest living family member as it will provide a better overview of your ancestry.
After all these efforts, if your test results still remain the same, talk to your parents. I’m sure they will know some possible reasons that your results turn out that way. In any case, just remember that all siblings are the same, full or not. Love your siblings the same regardless, as they will always be your siblings.
If you have not taken a DNA test and are unsure of which test is best for you, we have curated an in-depth breakdown of different DNA tests offered by 5 different DNA companies!
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