Richard Hamilton: Modern Moral Matters, Serpentine Gallery, LondonDexter Dalwood, Tate St Ives, Cornwall

An old master's political works, long sidelined, are echoed in the less strident imaginings of a distant descendant

It has always seemed ironic that Richard Hamilton should be known as the Father of Pop Art, pipping Warhol to the post with his 1956 collage, Just What Is It That Makes Today's Homes So Different, So Appealing?

As last year's Tate Modern show, Pop Life, suggested, Hamilton's baby was quickly adopted by parents more glint-eyed than he – artists from Andy himself to Damien Hirst, for whom Pop became part of a hellish celebration of money: works churned out by Factories, diamond-covered skulls. While this was going on, Hamilton's own brand of pop became increasingly serious and political and increasingly grim – agitpop, I suppose you could call it. It also became sidelined. One of his few descendants – and at some distance – is Dexter Dalwood, whose work, if less overt, is less strident.

Now a duo of exhibitions by the two men gives us a chance to examine their differences and similarities. The Serpentine Gallery's Hamilton show, subtitled Modern Moral Matters, brings together pictures from the 88-year-old master's long life, starting with a portrait of Hugh Gaitskell from 1964 and ending with a new work, also a portrait, of the Israeli nuclear whistle-blower, Mordechai Vanunu. Tate St Ives's mid-career Dexter Dalwood retrospective is necessarily briefer as to date – Dalwood was three when Hamilton painted Gaitskell – but hardly less impressive in scope.

Like Hamilton, Dalwood starts out by making collages. A look at those in the gallery's curved vitrine reveals an appetite less healthy than cannibalistic. The mountains seen through the window of Nietzsche's Chalet have been clipped from an Ed Ruscha, as has the back-to-front Hollywood sign of White Bronco. Bits of Diane Arbus are, oddly, taken from Francis Bacon, of Manderley, from Gerhard Richter. And, unless I'm wrong, the smeared walls of Rimbaud in Paris have been scissored from a painting of Bobby Sands, called The Citizen, by Hamilton.

So much for the likenesses. The most obvious difference between Dalwood and Hamilton is of presence and absence. For over a decade now, Dalwood has been making meticulously observed but entirely fictive pictures of the homes of the famous and infamous. Typical of these is Burroughs in Tangiers (2005), the picture of a place – Burroughs's Moroccan apartment – the artist has dreamt up but which he depicts with the hyper-reality of a dream. You can see why Burroughs, a literary collagiste, might have appealed. More to the point, and as with the subjects of all Dalwood's anti-portraits – Sharon Tate, Kurt Cobain, Diana Vreeland – the writer is missing from his room.

Where Dalwood might have snipped the shit-daubed walls from Hamilton's picture of Bobby Sands, in other words, he would never snip Sands himself. These are not portraits of people but of an artist's imagining of people. Dalwood's art is shot through with doubt, not simply about whether what we see is right but about the whole rightness of seeing. For all their apparently easy charm, his works ask sophisticated questions about the way we piece images together, how we make visual judgements and come to visual conclusions.

The Death of David Kelly (2008) – an allusion, presumably, to the supposed suicide in 2003 of the former Iraq weapons inspector – offers no clues and takes no sides. Nor does Greenham Common. The titles of the works suggest a political agenda, but other than the fact that Dalwood has hit on them and the fragments of art to go with them, they offer no evidence on which to build a case. That may be the point.

By contrast, Hamilton's pictures of Bobby Sands and a freshly arrested Mick Jagger, of a monstrous Hugh Gaitskell, tell us everything we need to know about their maker's political views: Sands and Jagger are Christ-like martyrs, Gaitskell a masked devil. I know that painting news images questions the news itself, also that doing so in the mid-Sixties was immensely brave and novel. But I always come away from Hamilton feeling as though I've had a finger wagging at me in a pub, and it is not a feeling I enjoy. Without Hamilton there might have been no Dalwood, and for that I am grateful. For the rest, well....

'Richard Hamilton' to 25 Apr (020-7402 6075); 'Dexter Dalwood' to 3 May (01736 796226)

Next Week:

Charles Darwent sees Kingdom of Ife at the British Museum – a collection of West African bronzes made as Europe emerged from the Dark Ages