your correct. when a platty breeds a normal you get lessers and platty sibs

as randy said the lessers can not contain the gene as they would already be plattys if they did.

the normal sibs are the ones that carry the gene.

a lesser to a platty sib breeding can create a platty

Ralph also made a platty from a lesser or platty ( cant remember) to a sib sib ( result of a platty sib x platty sib breeding)