The DNA of a U.S. teenage shooting victim was not found on the grip of the gun of the man accused of killing him, and the accused's DNA wasn't found under the teen's fingernails, a law enforcement expert said Wednesday. Prosecutors hope the testimony will disprove the shooter's claim that he did it in self-defence.

Trayvon Martin was 17 when he was shot and killed by neighbourhood watch volunteer George Zimmerman in Sanford, Fla., in a case that has sparked debate about race and the laws of self-defence. (Associated Press)

The murder trial of George Zimmerman has drawn national attention to issues of race — 17-year-old victim Trayvon Martin was black — and self-defence gun laws.

Zimmerman has pleaded not guilty and says he shot the teen in the chest to protect himself as Martin reached for his gun during a nighttime fight in his Florida gated community.

Florida Department of Law Enforcement DNA expert Anthony Gorgone also testified that Zimmerman's DNA was found among blood on a shirt Martin was wearing under his hooded sweatshirt.

Gorgone's testimony came on the same day that prosecutors presented evidence about Zimmerman's work in a college criminal justice course, which they say shows the neighbourhood watch volunteer knew about Florida's self-defence law and had aspirations of becoming a police officer.

Zimmerman had maintained in an interview with Fox News last year that he did not know about the law, which says a person can invoke self-defence in killing someone if it is necessary to prevent death or great bodily harm.

Prosecutors have sought to portray Zimmerman as a vigilante who profiled Martin as the teen walked home on a rainy night. They said Zimmerman's ability to understand criminal investigations and desire to be a police officer doesn't show wrongdoing, but is relevant to his state of mind on the night Martin was killed.

Defence attorney Mark O'Mara said Tuesday that if prosecutors start bringing up Zimmerman's past, the defence will dig into Martin's past, including fights. The judge had ruled previously that Martin's past fights, drug use and school records couldn't be mentioned in opening statements.