Both dramatic irony and situational irony come into play in Chapter 21, when it is revealed that Judge Pyncheon's son has been dead for some unknown amount of time:
Unknowingly, he was a childless man, while striving to add more wealth to his only child’s inheritance. Hardly a week after his decease, one of the Cunard steamers brought intelligence of the death, by cholera, of Judge Pyncheon’s son, just at the point of embarkation for his native land. By this misfortune, Clifford became rich; so did Hepzibah; so did our little village maiden, and, through her, that sworn foe of wealth and all manner of conservatism, the wild reformer—Holgrave!
Clifford, Hepzibah, Phoebe, and Holgrave have assumed during the week after Judge Pyncheon's death that their fate, and the fate of the House of the Seven Gables, is in the hands of Cousin Jaffrey's son. Cousin Jaffrey, for his part, has spent his life trying to direct the Pyncheon wealth through his bloodline. Clifford was initially supposed to inherit the House of the Seven Gables. Although Cousin Jaffrey was able to take the house for himself (at least temporarily) by pinning Uncle Jaffrey's death on Clifford, Clifford has remained a loose end that threatens Cousin Jaffrey's ability to pass the house down to his progeny. Cousin Jaffrey's villainy in the book has all been due to his greed, not only on his own behalf but on behalf of his son. His final act was to threaten Clifford; the stress seems to have brought on the hereditary end that many Pyncheon men face, choking on their own blood.
As it turns out, Cousin Jaffrey had no need to hoard his wealth for future generations or to threaten Clifford. He died for his son without ever knowing that his son, in fact, died first. There is no one left who could have inherited the house except for Clifford and Hepzibah (and, after them, Phoebe and Holgrave). This twist constitutes situational irony because Cousin Jaffrey's antagonism against his relatives should have at least gained his son something. Instead, it results in his death and speeds the house into the hands of the very people he is trying to keep it from. Dramatic irony is also at play because the living characters get the satisfaction of knowing about the blunder Cousin Jaffrey never knew he committed. This villain ends up a laughingstock who accidentally solves everyone's problems but his own.
