BENEDICT CUMBERBATCH is just a bit too similar to Sherlock Holmes — he is so brainy that he puts off girls.
"I think that is why possibly he has trouble with girlfriends."
Tracy, a picture frame restorer, added: "He is such a lot like Sherlock — he is quick-thinking like him, but not harsh.
"He's incredibly well read. In comparison to some he is quite an intellectual."
But she says that unlike his chilly-hearted, loner alter-ego on the series that is winning more than eight million viewers every week, Benedict is pining for a partner to share his life.
She says the actor, 35, has now split from fashion designer girlfriend Anna Jones.
The couple got together after the actor's 12-year relationship with The Thick Of It actress Olivia Poulet broke up in March last year.
Tracy said: "One of his regrets is that he hasn't found someone to settle down with.
"I think they would have to be someone not in the acting profession. Someone who was happy to hold the fort while he went off and pursued his career."
And that career is currently on fire — with white-hot flames.
As well as his role in Sherlock — whose last episode of series two screens tomorrow night — he stars in Steven Spielberg's movie War Horse which opened yesterday.
He will also voice both dragon Smaug and villain Necromancer in blockbuster The Hobbit, alongside Sherlock co-star Martin Freeman, who has the title role.
And on top of all that, he is to play the baddie in the next Star Trek movie as well.
But it is no surprise to Tracy that he is such a great actor — it's the family business.
Their mum is Wanda Ventham, 76 — an actress best known for Seventies telly sci-fi series UFO and for playing Cassandra's mother Pamela in Only Fools And Horses.
His dad is Timothy Carlton, 72 — who dropped his real surname for one that was less of a mouthful.
His television appearances have included comedies Executive Stress and Next Of Kin.
The couple married in 1976 after Wanda had divorced Tracy's father, businessman James Tabernacle.
Tracy, who lives in Wiltshire, said: "I look like my mother but Ben definitely has his father's features."
Not that he is all that happy with the striking, high-cheekboned looks that have won so many female fans on Sherlock.
His sister laughed and said: "He says he looks like Shergar. I think it is in self-defence, to get in the criticism about himself first."
But when it comes to star quality, Tracy reckons it's their mum that Benedict takes after. Wanda was a huge star in her heyday — even though young Ben was too little at the time to really understand her level of fame.
Tracy said: "She still gets fans coming up to her on the red carpet and Ben will ask her, 'What are you doing?'
"She will say, 'I was once famous as well, you know'."
Benedict was brought up in posh Kensington, west London, and was sent to board at Brambletye Preparatory School in West Sussex at the age of eight.
He then went on to £29,000-a-year Harrow — whose former pupils include eight PMs.
Generations ago the family had made a mint as owners of a sugar plantation in Barbados.
Tracy says it was a struggle for dad Timothy to send Benedict to private schools — a privilege about which the actor has often seemed uncomfortable. Tracy said: "His father had to scrimp and save to pay for Benedict to go to private school.
"He set up a trust fund even before he had the idea of having Benedict.
"Timothy gets a little annoyed when Ben decries the benefits of independent schooling, because he struggled like crazy to pay for it.
"He shouldn't forget the benefits of Harrow because they did a lot to encourage drama and the arts."
Even as a youngster at his previous prep school, Benedict had already caught the acting bug.
As a 12-year-old he stole the show in A Midsummer Night's Dream in Brambletye's 300-seat theatre — opened the previous year by none other than Judi Dench. He played clownish Bottom the weaver, the over-ambitious wannabe actor who has his head magically replaced with one from a donkey.
The Shakespearian role gave the actor his very first review, in the school magazine.
It declared: "Benedict Cumberbatch's Bottom will be long remembered."
Brambletye's deputy headmaster Andrew Callender, who taught the youngster and was also his housemaster, is not surprised by his former pupil's career success.
He remembered: "Benedict was a remarkable young pupil.
"He was not only very talented at sport — particularly his rugby and cricket — but his acting came to the fore very early on. He had boundless energy."
Tracy recalls that some of that energy was also taken up with spot-on impersonations of some his teachers as well as family members.
She said: "He's such a good mimic — he can imitate someone in a minute. He would mimic me, mum and dad — and quite a few teachers.
"He would take the mick out of the headmaster."
Benedict has previously said that, after Harrow, he was determined not to hop aboard the traditional public-school conveyor belt and head to Oxford or Cambridge.
Instead he went to the University of Manchester to study drama — after taking a gap year to teach English at a monastery in Tibet.
Set designer Michael Holt, who supervised Benedict's dissertation at Manchester, told The Sun that it was obvious from the start that he was a sensation-in-the-making. Mr Holt recalled: "He certainly came to us as a very accomplished actor.
"I remember one of his Masters from Harrow calling me to tell me what a great talent was arriving. And indeed he was."
He adds that unlike cold fish Sherlock, Benedict made plenty of friends — even if he stood out from his mates in his seriousness about acting.
He said: "He had a lot of friends, he was a very pleasant, thoughtful chap and could be quiet.
"He had a professional attitude, he wasn't starry-eyed.
"He knew it was something you had to work at."
THE last episode in this series of Sherlock airs tomorrow night on BBC1 at 9pm.
From Bottom to War HorseHE became an overnight celebrity when the first series of Sherlock hit screens in 2010 – but Benedict has been acting since he was a schoolboy.
Aged 12 he starred as Bottom in his school production of A Midsummer Night's Dream in 1989.
He also impressed his teachers with his acting skills as a pupil at posh Harrow. He has since been a regular on stage and screen. Highlights include:
2003: Playing son of Hugh Laurie's character in TV's Fortysomething.
2004: Bafta nomination for portraying the young Stephen Hawking in BBC film.
2005: Olivier award nomination for best supporting stage actor in Hedda Gabler.
2007: Roll in movie Atonement.
2011: Alternated roles of the monster and the scientist in the National Theatre's play Frankenstein with Jonny Lee Miller (and went starkers as the monster).
2011: Spy Peter Guillam in movie Tinker, Tailor, Soldier, Spy.
2012: Major Jamie Stewart in War Horse.
2013: Smaug and Necromancer in The Hobbit.