About one in five men over age 80 lose the Y chromosome from their blood cells, and this condition has now been linked to an increased risk of Alzheimer's disease, researchers said.
The condition known a loss of Y, or LOY, is the most common genetic mutation acquired during a man's lifetime.
Previous research has shown LOY can raise the likelihood of cancer and is more frequently found in smokers.
Now, researchers say the condition may serve as a predictive biomarker for a wider range of health problems.
For the study in the American Journal of Human Genetics - led by Lars Forsberg and Jan Dumanski of Uppsala University in Sweden, along with colleagues Britain, France, the United States and Canada - researchers examined cases of LOY in more than 3,200 men with an average age of 73.
Around 17 per cent showed LOY in blood cells.
Those who had been already diagnosed with Alzheimer's had a higher degree of LOY, they found.
Also, those who had not yet been diagnosed with dementia but had LOY were more likely to develop Alzheimer's in subsequent years.
"Having loss of Y is not 100 per cent predictive that you will have either cancer or Alzheimer's," cautioned Forsberg.
Some men with LOY in the study lived with no symptoms well into their 90s.
"But in the future, loss of Y in blood cells can become a new biomarker for disease risk and perhaps evaluation can make a difference in detecting and treating problems early."
According to Chris Lau, professor in the department of medicine at the University of California, San Francisco, the study sheds little light on why Alzheimer's risk may be elevated in these men.
"Although informative, the study is preliminary in nature and only highlights the fact that the Y chromosome could serve important functions beyond male sex determination and sperm production," said Lau, who was not involved in the study.
"What exactly on the Y chromosome that increases the risk of Alzheimer's disease is the key issue."
Since the Y chromosome contains many genes - some unique to men and others shared with women, who do not have a Y chromosome - more research is needed.
"It depends on what is lost to determine what is important for Alzheimer's disease. Without such information, the loss of Y is just an observation," Lau said.