With the exception of the genera Platemys (XY, Chelidae), Staurotypus (XY, Kinosternidae), and Siebenrockiella (XY) and Kachuga smithii (ZW) (Bataguridae), turtles lack heteromorphic sex chromosomes (either XY male heterogamety, or ZW female heterogamety); their mechanism for sex determination is different than in humans. Of the other turtles, only a few do not practice temperature (environmental-) dependent sex determination (TSD) (Bull and Vogt, 1979; Bull, 1980; Bull et al., 1982a, 1982b; Paukstis and Janzen, 1990; Ewert and Nelson, 1991; Janzen and Paukstis, 1991). The incubation temperature of the eggs at a sensitive period of development in the middle trimester triggers the gonadal development leading to the sex of the hatchling (Bull and Vogt, 1981).

Two TSD patterns have been discovered in turtles (Bull, 1980; Ewert and Nelson, 1991; Janzen and Paukstis, 1991). Pattern I, found in some Bataguridae, and the Carettochelyidae, Cheloniidae, Dermochelyidae, Emydidae, and Testudinidae, has a single transition zone of temperature below which incubation yields nearly or totally 100% males and above which only females are produced. Pattern II, known from the Pelomedusidae, Kinosternidae, Macroclemys temminckii (Chelydridae), and a few Bataguridae has two transition zones, with males predominating at intermediate temperatures and females at both extremes; in most species no constant incubation temperatures within the transition zone yield 100% males (however, exceptionally, constant temperatures do yield 100% males in some Sternotherus and in Chelydra). Pattern I occurs chiefly in turtles in which the adult females are larger than adult males; Pattern II is found mainly in turtles with females smaller than males or in which body size is not dimorphic (Ewert and Nelson, 1991). The smaller sex is typically produced at the coolest incubation temperatures.

Ewert and Nelson (1991) proposed four possible explanations for the various patterns of sex determination in turtles: phylogenetic inertia, temperature-dependent differential fitness, sib-avoidance, and group-structured adaptation in sex ratios, and we refer the reader to their paper for details. A few other turtles, some softshells (Trionychidae) and the wood turtle, Clemmys insculpta (Emydidae), have homomorphic sex chromosomes and genetic sex determination (GSD) (Ewert and Nelson, 1991; Janzen and Paukstis, 1991).

Incubation temperature seems to have no great influence on sex ratios in turtles of the family Chelidae (Bull et al., 1985).