How is the gender of a baby sea turtle determined?

The developing hatchlings do not have sex chromosomes so their gender is determined by the temperature within the nest.

There is a "pivotal" temperature which varies slightly among species, ranging between approximately 83-85 degrees fahrenheit (28-29 degress celsius), at which embryos within a nest develop into a mix of males and females. Temperatures above this range produce females and colder temperatures produce males.

This answer is a direct copy paste from seeturtles.org