Though there are not any recordings of the Beatles playing ukulele on recordings, it was in fact one of the first instruments both John Lennon and Paul McCartney learned as children. It was John's first instrument, actually, for his mother played. McCartney was the person who taught Lennon guitar chords, helping him progress from the ukulele to the guitar in this way. He only really knew ukulele chords, apparently, hence why many of his most famous compositions are rather easy to play on ukulele.