Half Sibling DNA Percentage: Understanding Genetic Similarities

Many people with half-siblings struggle to understand their place in their family’s lineage. How do they fit into the broader family tree? Which parts of their history do they share, and which might be different?

Uncertainty can cause a disconnect from one’s ancestral roots. People might feel like they’re missing pieces of their family puzzle.

You can bridge gaps in your family tree and discover connections you never knew existed by understanding the genetic bond between half-siblings. Let’s dive into the science of half-sibling DNA and see how it can help you write a richer, more inclusive chapter in your family’s ongoing story.

» Discover the DNA percentage shared with a half-sibling with a home testing kit

Uncovering non-paternal events using triangulation

During genealogical research, you might discover a discrepancy between a supposed father and a child’s genetic relationship. This is known as a non-paternal event. To find it, we can use a method called triangulation.

Here’s how it works:

1. You start by finding genetic matches who are close relatives, such as first, second, or third cousins. These matches’ familial relationships should be well-documented in historical records, ensuring their accuracy.
2. If you and a match share a DNA segment that you don’t share with your supposed father, it could indicate a non-paternal event.
3. To strengthen this finding, you can look for other matches that share the same DNA segment with both you and the original match. If multiple of them confirm this pattern, it’s highly likely that there’s a discrepancy in the paternal lineage.

So what does this mean? You and that person are half-siblings who share 25% of your DNA. The relationship is similar to first cousins who have the same set of grandparents

A deeper look at half-sibling relationships

You can refine your understanding of half-sibling relationships even more by analyzing centimorgans (cM). They are a unit used to measure genetic linkage. I suggest using the Shared cM Project because it gives you valuable data on the expected range of shared cM of different familial relationships. [1]

For example, full and half siblings typically fall within specific cM ranges of 1613–3488 and 1160–2436, respectively. But there can be occasional overlaps, which is why you should consider some additional factors like:

• Fully identical regions (FIR): These regions show where siblings have inherited the same genes from both parents. If there are significant overlapping FIRs, it suggests a close familial relationship like siblings.
• Second-cousin relationships: Platforms like MyHeritage can help you find proven second-cousin relationships, which can further strengthen the connection between half-siblings.
• Cousin analysis: Researching first cousins, aunts, and uncles can also provide valuable insights, as half-siblings share cousins through their common parent.

» Find out more about the most recent common ancestors

Shared cM ranges among familial relationships according to the shared cM project.

An exception in half-sibling shared DNA percentage

In some cases, parents can be closely related, affecting how much DNA half-siblings share. For example, the “three-quarters siblings” are the two children who have one parent in common, but the other pair who don’t are still related to one another.

Let’s take a look at the graph to illustrate the relationship.

a diagram of a tree with a number of dots

Breakdown of the relationship

• In the depicted family, individuals #6 and #7 are three-quarter siblings. They share the same father (#3), but their mothers (#4 and #5) are sisters. These sisters are the children of #1 and #2.
• #6 and #7 share approximately 37.5% of their DNA, which is more than typical half-siblings and less than full siblings. This percentage is halfway between that of full siblings (50%) and half-siblings (25%).
• When half-siblings share a higher percentage of DNA than expected, it may indicate a closer genetic relationship, possibly due to a shared line of common descent.

» Learn the difference between half and full relationships

Methods for identifying genetic relationships

The most common way of finding genetic relationships is through lab or at-home DNA tests. Every company has some nuances when it comes to analyzing genetics, but the main principles are fundamentally the same.

For example, the MyHeritage DNA analysis looks at 700,000 single nucleotide polymorphisms (SNPs), which are DNA variations. It separates your genes from those of every other person alive. Then, it compares your results to those of other test takers in our database to find a match.

Besides SNPs, the human genome has millions of potential variations that could hold valuable information. Whole genome sequencing, which analyzes all of your genetic markup, offers a complete picture but is currently more expensive than the other alternative. We can expect to find additional SNPs for DNA analysis as the cost decreases and more people undergo this testing.

The limitations of DNA testing

Millions of people have taken home DNA tests in recent years. While these methods are valuable for tracing ancestry and genetic relationships, they have limitations such as:

• Inherent size of the human genome: The human genome is so vast — approximately 3 billion base pairs — that it’s impractical to sequence it entirely. Instead, most home DNA tests focus on specific regions of the genome that vary between individuals.
• Human errors: Home DNA tests are highly reliable — over 90% — but they aren’t guaranteed. Accidental laboratory switch-ups, improper sample care, or incorrect harvesting can lead to inaccurate results.
• The closeness of the relationship: The accuracy of the test decreases significantly as the degree of the relationship becomes more distant. For example, third cousins share about 0.78% of their DNA, while fifth cousins have only around 0.098% in common. This means that the probability of accurately detecting a third cousin is 90%, but for fifth cousins, it drops all the way down to 15%.
• Data noise: Genetic recombination, mutations, and other factors can introduce noise into the analysis, making it more difficult to distinguish between different degrees of relationship accurately. Nevertheless, half-siblings will be easy to detect in the noise because of their shared 25% of DNA.

» Find out how this only child found three siblings using a DNA test

The power of half-sibling DNA testing

Understanding the genetic bond between half-siblings can provide invaluable insights into your family history. By unraveling the complexities of your family tree and discovering connections you may have never known existed, you can gain a deeper sense of belonging and a richer understanding of your ancestral roots.

Through DNA testing and careful analysis of genetic data, you can bridge the gaps in your family history and write a more inclusive and complete chapter in your family’s ongoing story.

» Start looking into half-sibling DNA percentages with a home testing kit