Now for a few additional definitions on top of the previous concepts.
A lethal gene is a gene that results in the death of its carrier. Genes do exist which cause diseases (such as Huntington’s disease, for example) instead of harmless changes such as blue eyes. Typically these genes kill their “host” before that host can have children, and so the gene dies with them.
Genes which kill their host but only later in life (again, Huntington’s is a good example) are called “time bomb” genes, because they wait a substantial amount of time before causing problems.