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.
A sympathetic nerve block can be used to diagnose and/or treat pain disorders involving the sympathetic nervous system. The injection consists of a local anesthetic. The sympathetic nervous system is a network of nerves throughout your body. The nerves branch from your spine and control some body functions, such as the closing of blood vessels. A problem with these nerves can affect blood flow and cause symptoms in the hands or feet. They may hurt, burn, feel cold, or be tender to touch. The sympathetic nervous system is controlled by bunches of nerves called ganglions. One large ganglion, called the stellate ganglion, helps control nerves in the upper body.