The inventor of DNA fingerprinting has offered to act as an expert witness in the Madeleine McCann case.
Sir Alec Jeffreys said DNA matches alone did not establish guilt and all Madeleine's genetic characters would be found in at least one family member.
Gerry and Kate McCann, suspects in their daughter's disappearance, are considering commissioning independent tests on a Portuguese hire car.
But the Madeleine's Fund board will not allow money to go on legal expenses.
It has been reported that DNA evidence with a 100% match to Madeleine was found in the car, but this has been played down by Portuguese police.
In an exclusive interview with the BBC's Newsnight programme, Sir Alec said there could be a potential problem in assigning a profile to Madeleine given that all other members of her family would have been in the car.
"DNA testing seeks to establish whether DNA sample A from a crime scene, came or did not come from individual B," he said.
"So if you get a match there's very strong evidence that it did come from B.
"It is then up to investigators, the courts and all the rest of it to work out whether that connection is relevant or not.
"DNA doesn't have the words innocence or guilt in it - that is a legal concept. What it seeks to establish is connections and identifications."
Earlier John McCann said his brother Gerry thought the Portuguese police had "gone up a cul-de-sac".
He told BBC's The One Show that Gerry had said they had "lost track" of what they were doing and urged the police to be straight with them.
"If they have got something that suggests Madeleine really is dead then for goodness sake tell the family who have the strongest feeling for this."
He added that had the police done a better job early on in the investigation then Gerry and Kate would not be suspects now.
Madeleine McCann has been missing since 3 May
Asked if he doubted their innocence at all, he said: "If you know the people, and you look at the whole situation, you look at it and think this is ridiculous."
Both parents deny vehemently any involvement in their four-year-old daughter's disappearance.
The McCann family were on holiday at a resort in Praia da Luz in the Algarve when they reported Madeleine missing from their apartment on 3 May.
They returned to their home in Rothley in Leicestershire at the weekend and are currently waiting for a judge to decide if they face charges.
It is understood the couple are planning their own forensic tests amid suspicion about claims made by detectives in interviews with Kate McCann.
The McCanns appointed lawyers in Portugal and Britain after being named as formal suspects - but they will not be paid out of money donated by well-wishers.
A statement made by Esther McVey from the board of Madeleine's Fund said payment of legal defence costs would be legally permissible so long as they were repaid in the event of a guilty conviction.
However, she said the fund's directors recognised the spirit in which the generous donations were given and decided not to pay for the McCanns' legal costs.
"We stress that Gerry and Kate have not asked for these costs to be paid," she said.
So far donations totalling £1,036,104.17 have been received for Madeleine's Fund: Leaving No Stone Unturned, according to the official campaign website.