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Every cell in the body carries a complete set of genetic instructions, written in a long molecule called DNA, that runs to roughly three billion individual chemical letters. Only a small slice of this, called the exome, is actually translated into the proteins that build and run the body; the rest, sometimes loosely called “non-coding” DNA, was once assumed to be filler but is now known to contain the switches, signals, and structural elements that control when and how genes are turned on. Whole genome sequencing is a laboratory test that reads essentially this entire three-billion-letter sequence at once, rather than reading only the coding exome, as whole exome sequencing does, or a handful of chosen genes, as a gene panel does.
A doctor recommends whole genome sequencing in a few specific situations. It is increasingly used as a first-tier test for critically ill infants and children, particularly in intensive care, where a fast, comprehensive answer can directly change urgent treatment decisions. It is also recommended when a person’s symptoms strongly suggest a genetic cause but whole exome sequencing has already been performed and did not provide an answer, since a meaningful share of genetic conditions are caused by changes that lie outside the coding exome altogether. In some centers, particularly where cost and turnaround time allow, it is offered as a first-line broad test in place of whole exome sequencing, since it can be reanalyzed for a wider range of causes later without needing to draw a new sample.
It is worth understanding from the outset that whole genome sequencing, like other forms of genetic testing, is a diagnostic tool rather than a guarantee of an answer. A clear genetic explanation is found in a meaningful proportion of cases, discussed further below, but many families will not receive a definitive result on a first attempt, and the sheer volume of information this test produces means that interpretation depends heavily on the experience of the genetics team reviewing it. A geneticist or genetic counselor is generally involved both before testing, to set realistic expectations, and after, to explain what the result actually means.
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