Expert testimony is not limited to oral answers. In the appropriate case, actual or demonstrative exhibits may be used.
An actual exhibit in a forensics case might be a PowerPoint or overhead illustration of the DNA report, showing the genetic types found in the evidence and the suspect's sample, and showing whether they are the same or different.
When actual evidence is displayed or used during the expert's testimony, chain-of-custody procedures must be addressed. The expert will have to show that the evidence has been properly preserved and is the same evidence that was obtained at the crime scene or other pertinent location. Often, opposing counsel will stipulate (concede) that proof of the chain of custody is established.
A demonstrative exhibit might be a display showing the DNA double helix. When a demonstrative exhibit is to be used, the attorney will often check first with opposing counsel and/or the trial judge to ensure that there will be no admissibility issue.
The expert must be clear that the demonstrative exhibit is not evidence from this case and is only an illustrative tool. It may be important to clarify who created the exhibit: the expert, the attorney or a third party.
When using an exhibit (either demonstrative or actual), the expert may need to stand to one side or otherwise move to make full use of the exhibit's potential. Permission to do so must be requested from the judge, as witnesses are ordinarily restricted to the witness stand/chair. The request may come from counsel or from the expert himself.