Sea turtles do not have sex chromosomes. In most species, gender is determined during fertilization. However, the sex of most turtle, alligators, and crocodiles is determined after fertilization. The temperature of the developing eggs is what decides whether the offspring will be male or female. This is called temperature-dependent sex determination, or TSD.
Temperature both influences sexual differentiation and rate of development of Caretta caretta. As it does with many other chelonian species, within the limits of embryonic tolerance cooler temperatures produce male hatchlings (26 and 28°C, C. caretta; 26°C, C. mydas), and warmer temperatures produce female hatchlings (32°C, C. caretta; 29 and 32°C, C. mydas) and the temperatures between these produce both sexes. The phenomenon of temperature dependent sex determination has important consequences in conservation practices. The middle one-third of the incubation period is the sex determining period for C. caretta. Prior to stage 21, the embryonic kidney is not differentiated into a functional mesonephros and there is no visible gonad. Thus, within this critical period, incubation temperature has a particular influence on the growth of urogenital tissue. The rate of embryonic development is faster at the warmer temperatures. A variety of factors (maternal behaviour in choosing a nest site, the zygote's response to temperature in becoming male or female, environmental effects (temporal or spatial temperature of the nesting area) interact to determine the sex ratio.