A unit of inheritance, encoded by DNA. If there's a mutation in a gene, this may cause a disease.
A gene is an organized sequence of building blocks for DNA (nucleotide bases or "letters") that "spells out" the information necessary to create a specific messenger called RNA that, in turn, makes a specific protein. Every cell in the body has the same genetic "blueprint"; however, each organ uses a highly specialized combination of genes to produce proteins that are essential to the function and specialty of its particular cells. For example, in the eye, the genes ROM-1 and peripherin are important in forming the outer segment membranes of photoreceptor cells. Other proteins, like rhodopsin, are involved in a rod cell's response to light. Any disruption in the function of the above-mentioned (and other) proteins can cause a retinal degenerative disease.