A hair found on evidence linked to one of the men accused of murdering teenager Stephen Lawrence had only a one in 1,000 chance of not coming from the victim, the Old Bailey has heard.
It was recovered from an evidence bag containing jeans owned by David Norris.
Terry Melton said mitochondrial DNA analysis of the hair showed it was from Mr Lawrence or a maternal relative.
Mr Norris, 35, and Gary Dobson, 36, deny murdering 18-year-old Mr Lawrence in Eltham, south-east London, in 1993.
The geneticist gave evidence to the Old Bailey trial via a video link from the US.
She was asked to establish by statistical means the likely owner of the hair in question, which was found in the debris of a bag containing jeans owned by Mr Norris.
The geneticist said she was given sample DNA profiles from the two defendants and Stephen's best friend, Duwayne Brooks, among others.
The profile of the two millimetre hair "excluded everyone with the exception of Stephen Lawrence," she told the court.
The one in 1,000 probability cited by the geneticist related to a European database.
When compared to a US database, the probability of it being someone other than Stephen rose to one in 400.
Dr Melton told the court that in calculating the probabilities, the scientists were "extremely conservative".
When questioned about the possible limitations of the databases she used, she told the court that her European database of 7,830 individuals had no UK samples.
Stephen Batten QC, representing Mr Norris, suggested that neither the US or European database was representative of south-east London, where Mr Lawrence was killed.
But Dr Melton denied that assertion.
She also told the court that her laboratory had lost a hair sent for analysis by the investigation into the teenager's death. However, it was not the one under consideration.