Last month, the actor Chris Hemsworth announced he’s taking a break from acting after learning through genetic testing that he carries two copies of the APOE4 gene variant, sometimes called the “Alzheimer’s gene.” Alzheimer’s disease is the most common type of dementia, a general term for the loss of cognitive function marked by a severe decline in memory, thinking, and reasoning. Dementia is caused by damage to brain cells; however, it is not always clear what triggers this damage. Hereditary...