This will definitely help you out, especially the last two paragraphs, read carefully.
In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. Sexual differentiation is generally started by a main gene, a sex locus, then a multitude of other genes follow in a domino effect. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). Environmental sex determination occurred before genetic; it is thought that a temperature-dependent reptile was the common ancestor to sex chromosomes.
In some species of reptiles, including alligators, some turtles, the tuatara, and a few birds, sex is determined by the temperature at which the egg is incubated during a temperature sensitive period. For some, this is achieved by hotter temperatures being one sex and cooler temperatures being the other. For others, the extreme temperatures are one sex and the middle temperature is the other. These temperature thresholds are known as Pattern I and Pattern II, respectively. The temperatures required for the specific sexes are known as the female promoting temperature and the male promoting temperature. When the temperature stays near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes. Some species set their temperature standards based on when an enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature. Some species such as the Nile Tilapia, Australian skink lizard, and Australian dragon lizard have sex determined by chromosomes, but this can later be switched by the temperature of incubation. These species seem to be in a transitional state of evolution.
It is unknown how exactly temperature sex determination evolved. It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season. However, at this stage it's mostly hypotheses.
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