How Accurate Is DNA Testing?

A practical look at why modern relationship testing can provide powerful evidence, and what influences the strength of a result.

The renewed Thomas Jefferson and Sally Hemings story illustrates just how much information modern genetics can recover. Researchers analysing DNA from preserved hair and DNA from living descendants reported evidence favouring direct Thomas Jefferson paternity over proposed alternative Jefferson-family scenarios.

For someone considering a DNA test today, however, the more immediate question is simple: how accurate is relationship testing?

Paternity testing compares inherited DNA

In a paternity test, laboratories compare genetic markers from the child with those of the tested man and, where applicable, the mother. The purpose is to determine whether the genetic evidence is consistent with the tested man being the biological father or whether paternity can be excluded.

Results are interpreted statistically. Rather than treating every DNA test as identical, it is important to look at the specific laboratory report and the probability or relationship statistic it provides.

Why DNA is such useful evidence

DNA is inherited biologically. That makes genetic testing fundamentally different from relying on names, photographs, memories or family accounts. Those other sources can be valuable, but they do not provide the same direct biological evidence.

The Jefferson investigation is a particularly unusual example because the researchers were working with centuries-old material. Routine relationship testing today generally starts with a much more straightforward sample, commonly a cheek swab, which is collected for laboratory analysis.

What affects the quality of a DNA result?

Reliable relationship testing depends on the whole testing process, not simply the existence of a DNA sample. Important considerations include:

• The type of relationship being investigated
• The quality and suitability of the DNA samples
• The genetic markers analysed
• Laboratory quality procedures and contamination controls
• Appropriate statistical interpretation of the genetic evidence

Different relationships require different tests

Paternity testing is only one form of relationship testing. Sibling, grandparent and other kinship questions may require different comparisons and can produce different levels of statistical evidence. Choosing a test designed for the relationship you want to investigate is therefore important.

Evidence instead of assumption

One of the clearest lessons from historical DNA investigations is that genetics gives researchers a way to test biological claims against measurable evidence. For families today, that same principle can provide valuable clarity when the biological relationship itself is uncertain.

Choosing your DNA test

If you are unsure which relationship test fits your circumstances, That DNA Company can help you identify the appropriate testing route. Start with the relationship you want to establish and choose a test designed to answer that question.

References

1. Smithsonian Magazine: Jefferson and Hemings DNA investigation
2. Green et al., The DNA Legacy of Thomas Jefferson, bioRxiv
3. PBS NewsHour: Lock of hair settles questions about Thomas Jefferson and Sally Hemings