The amendment bars anyone who receives "a title of nobility bestowed from a foreign power" from holding the office.
The British monarch had to pull out of the Festival of Remembrance after she sprained her back.
The couple snuck into the last two seats at the front of an airliner for a five-hour flight from New York to Los Angeles.
Thomas Markle Sr. said the Duchess of Sussex should be stripped of her title for embarrassing the British royals.
"She already knew the best thing to do was to go and have a rest and have a real medically framed treatment," Prince Albert of Monaco said.
The Duke and the Duchess of Cambridge donned their best outfits as they attended the Royal Variety Performance on Thursday.
The Duchess also approached a stall selling hot sauces, telling the vendor, "Let mommy have a taste. My boo loves hot sauce."
The Duchess of Sussex talked about her children, Prince Harry, and her pre-royal days on "The Ellen DeGeneres Show."
The British monarch could be experiencing low oxygenated blood following her back injury.
The royal joined Harry and Meghan for their last night out in Toronto before they went public with their relationship.
The Duchess of Sussex said Archie loves being a big brother to his baby sister Lilibet Diana.
Two women and one man following the couple were trying to hide camera equipment under their coats and bags.
The Queen was pictured in Windsor's Oak Room - a sitting room that doubles as a private office, sharing a brief conversation with General Sir Nick Carter.
The Duchess of Sussex was a series regular in the legal drama "Suits" before she married Prince Harry.
The Duke and Duchess of Sussex were expected to join the queen since this is her first Christmas without Prince Philip.
Thomas Markle Jr. is hoping for "the smallest of any type of response" to the letter he wrote to his sister on the show.
The British monarch pulled out of the Festival of Remembrance at the last minute because of a back injury.
Royal babies are always, by tradition, welcomed into the church with water from Jordan river.
Elizabeth Debicki shares an uncanny resemblance to the late Princess of Wales in Season 5 of "The Crown."
Thomas Markle Jr. said the Duke and Duchess of Sussex "keep very much to themselves, and don't socialise much."