Focus: So who do you think you are? And would you really want to know if you found out?

Genealogy is gripping the nation. Dave Hill can't boast a single murderer, philanderer or can-can dancing great aunt in his family tree. But that doesn't stop him from wondering...

Answering its own ancestral call to inform, educate and entertain, the BBC this week launches not only a series on genealogy but a whole nation-participation package on the theme. The full range of licence fee-justifying, zeitgeist-surfing devices are being deployed. There's a new website and a pull-out guide in the centre of the current Radio Times to help you research your family tree. The validating involvement in Who Do You Think You Are? of a broad church of celebs - Jeremy Clarkson, Meera Syal, Lesley Garrett, Bill Oddie and more - has been secured, and the whole thing culminates in a family history weekend hosted by local BBC radio stations in December.

And, of course, they're on to something. The national broadcaster's embrace confirms the rise of genealogy as a popular pastime. Up and down the land records of wills, births, marriages and deaths are nowadays routinely ploughed through by citizens seeking buried roots, while census-return websites crash under the sheer weight of demand.

The once marginal obsession of mole-ish archive-builders and heraldry freaks, tracking the history of your clan base is now firmly within the orbit of everywoman, everyman.

What, though, are people really looking for? There's no doubt that attitudes to lines of descent have changed with the times. Delving into family pasts once risked rattling skeletons that threatened your respectability: a bigamist here, a bastard there, a besmirching scandal that your forebears had concealed.

By contrast, today's greatest fear is probably of uncovering relentless orthodoxy. Imagine: not one mass murderer, rogue philander, exotic foreigner, even, to be found. Oh, the dullness. Oh, the shame.

Some, though, look for rather more than a store of glamour anecdotes to amuse the aunts and uncles on Christmas afternoon. David Baddiel, one of the Beeb show's star backstory-seekers, describes uncovering clues to the fate and personality of his late great uncle Arno who became "lost somewhere during the Holocaust". He summarises: "I came away from [making] this film with a greater sense of who I am."

The idea that tracing lineage to generations long gone can foster a clearer sense of who you really are has an illustrious history all its own. Its purchase is especially strong among ethnic or religious groups that have been subjected to wholesale persecution and genocide - peoples whose histories have been lost if not deliberately destroyed. In the black diaspora, a longing to reconnect with Africa has been part of a wider political and spiritual project for decades, popularised in the 1970s by Alex Haley's novel Roots and encompassing figures from Malcolm X to Bob Marley. A BBC documentary last year followed two black Britons introduced to small African communities with whom they shared DNA. Emotional scenes were recorded, including talk of "the most amazing day of my life".

Such passions are partly a result of the opening up of history, the valuable consequence of challenges to its traditional narrow focus on monarchs and conquerors in favour of recognising that plainer folk are part of the past too.

Now that we all feel both able and entitled to trace our individual threads as they spread back through humankind's rich tapestry, the genealogy industry seems set to grow and grow.

The data to be dug from conventional public records is already augmented by biotech's capacity not only for tracing distant gene pools but also for exposing deceptions of more adjacent vintage. In the age of commercial DNA testing, how many of us may yet discover that the fellow we've been calling Dad is no such thing?

This fascination with families and their provenance can only intensify as family configurations become more complex in the age of increased re-partnering and separation. So far as I'm aware, my own immediate ancestry could not be more straightforward: skilled West Country artisans on my father's side; domestic service for East Anglian horse-racing households on my mother's; no illegitimate distant cousins or can-can dancing great- aunts; no grand financial collapses; no divorce.

Compare that plain tale with the one my children will pass down: the eldest three born out of wedlock before their mother was born again as a lesbian in 1992 and moved out to pursue this interest with another of like mind who also calls herself my children's "mother"; the three youngest, fruit of my subsequent marriage, growing up alongside half-siblings who move out at intervals to live in their other home under pick'n'mix combinations of surnames.

Do those who draw up family trees have codes and little symbols to denote this sort of thing? Could they help me, never mind my descendants, make sense of it? That seems to be the highest purpose of this form of inquiry: to gain a clearer picture of our place in time's narrative; to know those forces that predated us yet helped make us what we are.

It is a phenomenon that has its dangers, too. It is one thing for genealogy to be enlisted in the cause of uncovering buried truths and re-assembling smashed cultures, another to perceive it as some magic pathway leading to ultimate knowledge of some true, essential self.

In a time of big, new, and often too little challenged claims about the bedrock role of genetic inheritance in guiding human behaviour, it may be tempting to believe that tracking our bloodlines will reveal the deepest secrets of our lives. But our sense of identity - our answer to the Beeb's question, Who Do You Think You Are? - comes from many other places too; places we have visited, experiences we have had, sometimes only inside our heads.

Family trees may be rich sources of wisdom and foster nourishing reflection. But there are other trees, other orchards, and other fruit to pick there. Our histories, family or otherwise, may be a part of us. But they are also things we fashion for ourselves.

'Who Do You Think You Are?' is on BBC2 on Tuesday at 9pm