Genes and traits
Read the difference between a gene a gecko carries and a trait you can see.
A gene is part of what a gecko can pass to its hatchlings. A trait is something that gene makes visible. A gecko can carry a gene without showing it, so looks alone cannot tell you everything it might pass on.
Open a gecko in your Collection and choose its Genetics tab. The line there names genes that show, genes known to be carried, and strength traits. A question mark means a gene is possible, not confirmed.
See the Axanthic library sheet for one gene that can travel unseen, then follow how genes pass down.