Thirty-five-year-old mother-of-two Hannah Mackay underwent tests when both her father and grandfather were diagnosed with the same form of the disease - frontotemporal dementia - in their mid 50s.
Now it has been confirmed that Hannah, who lives near Haywards Heath, will also develp the disease.
But she is determined to raise vital funds for research into finding a cure because of the heart-breaking concern that both her two children could also develop the condition.
Hannah, a sergeant with Sussex Police, is to take part in an Alzheimer’s Society ‘Memory Walk’ - one of 38 happening nationwide - in Brighton on September 7.
She said: “The day I found out I would develop dementia changed my life, but not necessarily for the worst.
“I remember just feeling at first like I had been kicked in the stomach when they said it was bad news.
“I was crying, but I was crying with relief that the process was over. I knew then that I didn’t need to be afraid of the unknown any more.
“I hope my story can bring people together to raise awareness and vital funds to help scientists find a cure for dementia.
“Dementia is now the UK’s biggest killer but research still falls far behind other health conditions as it’s historically been so poorly funded.”
Knowing her three and six-year-old daughters might develop dementia has strengthened Hannah’s determination.
“I want my children to know that if their mummy does get poorly that she did everything she could to make it better – for them, for me and for other people.
“There’s a 50 per cent chance this will personally affect them too so I’m going to do everything I can to stop that from happening.
“At this stage, they don’t need to know great detail – they just need to know that mummy is doing everything to ensure that she doesn’t get ill by doing something positive.”
She added: “I’m happy to do anything for Alzheimer’s Society because we share the same goals – to beat dementia.”
“I know every penny raised through Memory Walk will help Alzheimer’s Society find a cure, improve care and support people affected by dementia. In the hour-and-a-half it takes to complete your average Memory Walk, 30 people will develop dementia in the UK so I hope people will sign up to Memory Walk now to unite against dementia.
“I promised several people I would fight dementia with whatever I’ve got. I hope I can really make a difference, and if I manage to help and inspire other people then I will die a happy person.”
It was after Hannah learnt she had a 50-50 chance of developing dementia that she decided to remove uncertainty over her future by undergoing genetic tests.
Hannah and her father have both taken part in a research study to help scientists learn more about frontotemporal dementia.
Dr James Pickett, head of research at Alzheimer’s Society, said: “Families like Hannah’s are being cruelly torn apart by dementia, and we urgently need more funding to help us find a cure so others no longer have to face its heart-breaking impact.
“Gene therapy is already bringing hope for people with other previously incurable diseases, which is why signing up to Memory Walk to raise vital funds is so crucial.
“Until the day we beat dementia, our researchers are working tirelessly to find ways to make quality of life better for people living with dementia in the UK and their carers too.”
You can help support the work of the UK Dementia Research Institute by registering for your local Memory Walk now at memorywalk.org.uk