A 75-year-old man genetically fated to develop Alzheimer’s is yet to show any sign of the disease, likely due to his lifelong exposure to high levels of heat while working as a naval mechanic, a study suggests. Doug Whitney inherited a rare genetic mutation that caused many members of his family to develop cognitive decline in their early 50s. But Mr Whitney remains mentally sharp. The likely reason, the study notes, is his exposure to high temperatures while working as a mechanic in the engine rooms of ships. People who inherit Alzheimer’s genes typically experience cognitive decline at an early age , sometimes as soon as in their 30s or 40s. But a tiny number, known as “exceptional resilience mutation carriers”, escape the condition. Scientists now say they may be key to developing treatments for Alzheimer’s . Mr Whitney is only one of the three such people known so far.…