In the first Jurassic Park novels and movies all dinosaurs in Jurassic Park are female, because for savety reasons the dinosaurs shouldn't reproduce. In reality it would be more likely that all dinosaurs were males.
In the "80s it was observed that female fogs could become fully functional males:
"...seven adult female (forgs) -- including six previously observed to lay eggs -- developed functioning testicular nodules and aggressive behavior typical of male frogs... Four of the seven "secondary males" copulated with females, fertilizing up to 70 percent of the eggs and generating normal offspring, the investigators say." (Jumping gender: frogs change from she to he
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 build 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.