Individuals with are more likely to develop leukemia, cancer, Alzheimer's disease, or heart disease at earlier ages than usual (in their 30s and 40s).

The answer for the blank space is "Down syndrome".

Explanation:

Many people with Down syndrome (born with an extra identical duplicate of chromosome 21) have a high probability of developing leukemia, cancer, and Alzheimer’s disease as they become older, usually between between the ages of 30 and 40.