Final answer:
Jesse Gelsinger was not diagnosed with OTC until the age of two due to a late-onset form of the deficiency, which caused milder symptoms and later onset.
Step-by-step explanation:
Jesse Gelsinger was not diagnosed with ornithine transcarbamylase deficiency (OTC) until he was two because he had a late-onset form of OTC, and therefore his symptoms were milder. Typically, severe cases of OTC deficiency are evident shortly after birth due to a critical enzyme not being produced, leading to a build-up of toxic ammonia. However, in late-onset forms, the enzyme may be partially functional or the enzyme deficiency may be less severe, leading to a milder accumulation of ammonia and later onset of symptoms.