Why Your First Cousin’s DNA Results Might Be Misinterpreted as a Half-Brother

Imagine receiving a DNA result that surprises and confounds you: your first cousin is identified as your half-brother. Don't panic—understanding what these discrepancies might mean can help clarify the situation.

Misinterpretation of Results

One of the most common reasons for misinterpretation of DNA results is the misclassification. Different DNA testing companies use various algorithms to estimate familial relationships. Sometimes these algorithms may mistakenly categorize your cousin as a half-brother instead of a first cousin.

Shared Parentage

Perhaps the most straightforward explanation is shared parentage. If you and your cousin share one parent, you would indeed be half-siblings. For instance, if your parent is also a sibling of your cousin's parent, you are in a half-sibling relationship through that shared parent.

Non-Paternity Events

Non-paternity events can lead to unexpectedly close genetic relationships. If one of your parents is not the biological parent, it can result in a more genetically close match than expected, causing your cousin to be identified as a half-brother.

Genetic Overlap

In rare cases, the amount of shared DNA between first cousins can be similar to that of half-siblings, especially with a large amount of shared ancestry. This genetic overlap is what causes your cousin to appear as a half-brother in the DNA results.

Testing Errors

Errors in DNA testing or contamination of the sample can also lead to inaccurate results. While these instances are less common, it's crucial to consider all possibilities to ensure the accuracy of the findings.

Mathematical and Biological Limits of DNA Testing

It's important to understand the limits of DNA testing from a genetic and mathematical perspective. Mathematically, people can't contribute 50% DNA from each parent and 25% from each grandparent, as each generation halves the amount of unique DNA inherited. Therefore, the percentage of DNA shared decreases significantly over generations. The notion of 50% DNA from siblings or 25% from cousins is an oversimplification.

Each person, regardless of their family history, begins life with a single set of two sets of 23 chromosomes (100%) from their parents. At fertilization, these are combined to form a set of 46 chromosomes, doubling the genetic material to 200%. From there, genetic overlap occurs through shared ancestry, leading to the complex patterns seen in DNA testing results.

Genetic Counselors and Further Testing

If the discrepancy between your DNA results and reality is significant and concerning, it may be beneficial to consult with a genetic counselor. They can offer expert insight into the results and guide you through further testing with a different service for clarification.

Additionally, retesting with another DNA service can provide a more accurate picture. The reliability of the results heavily depends on the quality of the sample and the testing method used, so a different round of testing can help mitigate any inconsistencies.