Born months before Wolfgang Amadeus Mozart died in 1791, Franz Xaver spent his life trying -- and failing -- to step out of his genius father's shadow.
"A child that disappoints their parents... will encounter disgrace and misery. Let these words be a warning to my lovely (son)," his mother Constanze wrote in 1801 to her nine-year-old son.
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Her ominous note is one of many personal letters currently on display at the Mozart Residence in Salzburg, as part of an exhibition organised by the Mozarteum Foundation.
When he passed away in 1844, Franz Xaver -- the last of the Mozart line -- donated hundreds of family documents to the foundation.
"History has sort of forgotten Franz Xaver but he's actually of big importance to us," Mozarteum curator Armin Brinzing told AFP in an interview.
"We owe it to him that so many original manuscripts from the Mozart family including handwritten compositions have survived and are accessible to the public, instead of being destroyed or spread all over the world."
Of the six children born to Mozart and Constanze, only Franz Xaver and his older brother Carl Thomas survived into adulthood.
While Carl Thomas became a government official, Constanze had much bigger plans for her other son.
After her famous husband's death, the widow decided that Franz Xaver "should become the second Mozart", Brinzing said.
"At the age of two, she already made him take piano and music theory lessons," the curator noted.
Constanze hired some of the era's most eminent teachers, including Italian composer Antonio Salieri whose pupils included Franz Schubert and Ludwig van Beethoven.
Even more tellingly, she only addressed her son as Wolfgang Amadeus.
In fact, Franz Xaver himself would sign all his works with "Wolfgang Amadeus Mozart, son".
Letters exchanged between Franz Xaver and his older sibling reveal that from an early age, Franz Xaver felt under "immense pressure" and "not treated very well at home".
Aged barely 13, Franz Xaver gave his highly anticipated first public concert in a packed Vienna hall.
Critics praised his performance -- "he gave a nice if slightly slow rendition of his father's piano concerto," according to one review -- but also warned the boy not to rest on his laurels.
"May he never forget that although the name Mozart currently grants him some indulgence, it will place great demands on him later on," read an editorial in the Allgemeine Musikalische Zeitung, a key 19th-century music magazine also on display at the Residence.
United in death
At 17, Franz Xaver fled the parental nest and took a job as a piano tutor for a wealthy family in the Ukrainian city of Lviv, then part of the Habsburg empire.
He spent the next two decades teaching and performing across Europe as he sought to build up his reputation.
Having inherited his father's excellent ear, he conducted a 400-strong choir and founded Lviv's first music school, now the National Conservatory.
But compared to the original Mozart, Franz Xaver's artistic output was small and generally failed to impress.
"Franz Xaver was a very good pianist especially when he played his father's concertos, but his own compositions enjoyed only mediocre success," said Brinzing, adding that some of them are being rediscovered today.
"That last spark of genius was missing in him. He was considered a gifted musician and composer, but not one of the great ones."
Nowhere was this more apparent than when he was asked to compose a piece for the unveiling of a monument dedicated to his father in Salzburg in 1842.
Riddled with self-doubt, he refused, telling organisers that he was a musician of "little ability" bound to disappoint.
Instead he turned two of his father's unfinished compositions into a cantata, which was greeted with great applause at the inauguration.
Afterwards, Franz Xaver sent a signed copy of his work to Emperor Ferdinand I.
Tradition had it that the ruler paid a small fee in exchange for autographed sheet music.
Having only vaguely heard of Mozart's son, the emperor asked his advisers whether he should reward the composer.
"As everyone knows, the famous father's talent has not been transferred to his son so we should give him some money," an official replied.
Two years later, Franz Xaver died of stomach cancer during a health retreat in the Czech town of Carlsbad, where he was also buried.
Even in death Mozart's spirit still looms large, with Franz Xaver's tomb stone carrying the inscription:
"May his father's name be his epitaph, as his veneration for him was the essence of his life."
By AFP's Nina Lamparski