John Adams' grandfather, President John Adams, had married his third cousin, Abigail Smith. Intermarriage skipped a generation with John Quincy Adams, who married a non-relative. But, at 25 years old, John Quincy's second-eldest son, John, married his first cousin on his mother's side, 22-year-old Mary Catherine Hellen, in a private ceremony at the White House.
Exactly nine months and seven days after the wedding, Mary Catherine gave birth to the couple's first child, a daughter named Mary Louisa, in the White House family quarters. Mary and John gave her the name Mary, after her mother, and the middle name Louisa after her paternal grandmother Louisa Catherine Adams, the wife of John Quincy Adams.
In 1853, Mary Louisa Adams also married a family member--her second cousin, William Clarkson Johnson, the son of her first cousin, Abigail Louisa Smith Adams, and President John Adams' great-grandson. Both bride and groom descended from President John Adams--the wedding constituted the first marriage between descendants of two presidents. While both Mary Louisa and her new husband were descendants of President John Adams, only Mary Louisa was directly related to President John Quincy Adams.
The Adams' were not the only presidential family to intermarry. In 1905, Franklin Delano Roosevelt married Eleanor, his fifth cousin once removed. Eleanor did not have to change her name upon marrying, since her maiden name was also Roosevelt. Her father, Elliot, was the brother of former President Theodore Roosevelt.