Last Night's Viewing: Who Do You Think You Are? BBC1, Rab C Nesbitt BBC2

The basics of Who Do You Think You Are? are now very well established. You get the scene in which the subject sits with an elderly relative, cooing over sepia photographs of Victorian coves. You get the sequence in which the subject meets a star-struck chap in a cardigan on the steps of the local archives. You get people running a white-gloved finger over copper-plate writing in an old registry book. All of them featured last night, but Tracey Emin did manage to introduce one thing that I'd never seen before on the show. She briefly swivelled our attention 180 degrees, away from the past and towards the future: "The fact that I'm never going to have any children means I'm the end of my line. After me, I stop. I'm the last of my kind. There is no more."

It was an odd way of thinking about this show, a recognition that although the generational chain that made us recedes without a break into the historic past there's absolutely no guarantee that it will continue into the future. Unless you've already got children, it might stop with you. And if Who Do You Think You Are? is running in 150 years' time, it's unlikely that anyone will be going "Oh! My! God!" when they discover that there was once an Emin who was a bit of a name in the art world. Not entirely impossible, though: Tracey didn't seem to have taken into account her brother's children or the other family her Turkish-Cypriot father had, which suggested she was thinking of "my kind" in a rather narrow sense.

She didn't seem that interested in her father at all, as it happens, even though his grandfather had been a Sudanese slave in the Ottoman empire, which is the kind of exotica that WDYTYA? usually finds irresistible. Instead, she was solely concerned with following her maternal line into the past. And she seemed conflicted about what she wanted to discover. "If I find out that I come from the most loving, simple ordinary... suburban family... I think I'll just go and slit my wrist," she said at the beginning of the show. But then she seemed to suggest just the opposite: "My fear is that I'm opening up a can of worms that shouldn't be touched." Far better, I would have thought, than opening the can and finding it had no worms in it at all, but she seemed genuinely dismayed, rather than gratified, to discover that her great-grandfather had ended up in a Victorian reformatory at the age of 13 after stealing a set of bathroom taps.

A strain of naive wishfulness soon became apparent. Emin expressed the forlorn hope that the reformatory would turn out to be a charitable institution set up to rescue talented young boys from the East End and give them an education. It wasn't, and the reform doesn't seem to have worked. After his release, Harry got three months' hard labour for a burglary in which he stole £4, a violin and a pair of accordions (it can't have been a very surreptitious getaway). "Maybe he wanted the violin to play?" Emin said tentatively, anxious to find some evidence of creative aspiration in her familial past. She only really cheered up when it emerged that Harry's dad had been part of a family of Warwickshire travellers, living in tents and making besoms, or witches' brooms, to sell door to door. "The fact that I've come from this amazing family makes me feel a better person," said Tracey, a near-perfect illustration of the fantasy of inherited virtues that is one of WDYTYA?'s most unvarying components.

Susan Boyle took a brief cameo in Rab C Nesbitt, appearing from beyond a copy of Variety in the Giblets pub, where she was notionally judging Govan's Got Talent, in which Ella and Mary were appearing as a pair of singing char-ladies called The Scrubbers. Neither The Scrubbers nor Boyle would be advised to give up the day job.