Expressing doubt over the methods used to ensure that the dinosaurs were all female, Ian Malcolm asked Henry Wu how exactly one determined that. Wu answered that sex organs varied with the species, and depending on which species it was it was easier to determine. He explained though that the reason they were confident all the animals were female was that they were created that way. He mentioned how they had manipulated the animals chromosomes during inter-egg development to remove hormones that would turn a female embryo into a male.
Jurassic Park by Michael Crichton page 109 (First Edition, Thirty Fourth printing)