Developing colon cancer under the age of 35 is rare, but when it does occur, one-third of the cases are due to heredity, a new study suggests.

Colon cancer is the third most common cancer and the fourth-leading cause of cancer deaths worldwide.

Generally, people have about a 5 percent risk of developing it, but those who inherit certain genetic syndromes tend to be much younger when the disease is diagnosed.

Researchers at the University of Texas MD Anderson Cancer Center reviewed data from 193 people diagnosed with the disease under age 35.

While they expected to find that many of the patients had genetics that predisposed them to the disease, but they were surprised to find genes accounted for fully one-third of the group, the researchers said.

"Based on our findings, patients under the age of 35 need to be evaluated by a genetic counselor. Period," said Eduardo Vilar-Sanchez, M.D., the lead researcher. "The translation of that information extends well beyond the patient, as there are tremendous benefits from being able to share genetic risk with their parents, siblings, and many other family members.”