In the first Jurassic Park novels and movies, all dinosaurs in Jurassic Park are female, because for safety reasons the dinosaurs shouldn't be able to reproduce if they escaped into the wild.
Sex determination in dinosaurs
In the movie and novel, the geneticists turned off the genes that allowed the dinosaurs to produce the hormones to become a male. All vertebrate embryos start off with female genitalia, and given the right hormones, the offspring will become male. The geneticists, in the movie and novel, turn off these genes for safety purposes, but in The Lost World on Isla Sorna, both males and females exist for breeding. The product of these breedings are eggs, and the eggs are possibly taken back to Site A, Isla Nublar, where the embryos are denied the ability to become male. However, in the movie and novel, the genes of frogs were used to fill in the gaps in the dinosaur genetic code. They mutate, and the dinosaurs become sequential hermaphrodites, like the frogs, and can switch genders from female to male or female to hermaprodite so that they can fertilize their own eggs with their own sperm.
In the '80s it was observed that female frogs could become fully functional males.
In the Jurassic Park novel and the movie it is stated that when genes from the dinosaur genome were lost, the frog-version of those genes were built into the new dinosaurs. In the story, the presence of these frog-genes gave the dinosaurs the ability to switch sexes and reproduce.
In reality, this is very unlikely. The ability to switch sexes is a very complex mechanism that demands the interplay of many genes. If the Jurassic Park scientists deliberately wanted to build this ability into their dinosaurs, it would have been a very hard job. Therefore, it is unlikely they gave their dinosaurs this ability by chance.
- Grafe T.U., Linsenmair K.E. (1989). Protogynous Sex Change in the Reed Frog Hyperolius viridiflavus. Copeia, 1024-1029. Retrieved from http://www.jstor.org/stable/1445989