The broadcaster, 28, says she bought the genetic test to explore her family's history but recalls ' bursting into tears' when she was told she had a 60% genetic chance of developing the disease.
The controversial tests, which can be ordered online, have been criticised in the past as they give a high degree of false positive results.
Flora, speaking to The Sunday Times, said: "When I saw my gene results, I burst into tears. The duplicitous gene gifted from my parents was hidden somewhere inside me. I could feel it travelling through my blood, under my skin.
"Now I have my results, I'm particularly aware of any mention of Alzheimer's, whether a character portrayed on television or a story recalled by a friend — every tale feels like a glimpse of my potential future."
The 28-year-old said her father AA Gill, who died in 2016 after a battle of cancer, was an ambassador for The Alzheimer's Society after watching his own father struggle with the disease.
James Pickett, head of research at The Alzheimer's Society, said: "Anyone considering getting a genetic test should do so with their eyes open.
"It's a highly personal decision, but could cause great anxiety without the correct support and understanding. For anyone concerned about dementia, the first port of call should be a GP, not a genetic test."