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The inner eyelid was a hereditary trait among Vulcans, developed due to the brightness of the planet Vulcan's sun. In 2267, Spock explained this to Doctor Leonard McCoy and James T. Kirk when explaining why his blindness was only temporary. (TOS: "Operation -- Annihilate!")
Humans had the hereditary trait of hesitating briefly before taking action. In 2268, Spock attempted to explain to Ensign Garrovick, that given this fact, he was not to blame for his hesitance to fire at the dikironium cloud creature. (TOS: "Obsession")
Air breathers on the planet Argo were initially transformed into water breathers by surgo-op; only later did their mutations become hereditary. Rila explained this to Kirk and Spock in 2269 when the Vulcan surmised (incorrectly) that they had evolved from their enemies. (TAS: "The Ambergris Element")