John Beaufort, 1st Marquess of Somerset & 1st Marquess of Dorset, later 1st Earl of Somerset,(1373 – 1410) was the 1st of 4 illegitimate (later legitimized) children of John of Gaunt, 1st Duke of Lancaster, & his mistress Katherine Swynford, later his wife. John Beaufort & his wife Margaret, the daughter of Thomas Holland, 2nd Earl of Kent & Alice FitzAlan, had 6 children. He was an ancestor of the Tudor dynasty.