Why can DNA be tested from tissues where a particular gene is not active?

DNA can be tested from tissues where a particular gene is not active because the human genome—the complete set of genetic instructions—is present in almost every cell of the body.1

While individual genes are frequently switched on or off depending on the cell type or environmental needs, the underlying DNA sequence remains constant across these tissues. Because the genetic code is maintained in duplicate copies within the chromosomes of nearly every cell, scientists do not need a gene to be actively expressing a protein to analyze its structure.12

Techniques such as polymerase chain reaction (PCR) allow researchers to isolate and copy specific DNA fragments from a sample, enabling the detection of mutations or variations regardless of whether that specific gene is currently being transcribed or translated into a protein.23