We can observe a phenotype whether or not we understand the genetics behind it. In real life, there is a gene called HERC2 that works like I described, and most people with blue eyes have the broken allele of this gene.
Nerves, Neuroglia, and Ganglia. A nerve is a bundle of fibres (axons and/or dendrites) outside the CNS. Neuroglia are cells of the nervous system that help protect and support it. Ganglia are groups of nerve cell bodies lying outside the CNS.
Basically, a nerve is a fiber that connects the brain and spinal cord with various parts of the body (aka-receptor organs). Nerves conduct impulses from the brain/spinal cord to these receptor organs as well as conducting impulses from the receptor organs back to the brain/spinal cord.