1.nounA theoretical unit of heredity of living organisms; a gene may take several values and in principle predetermines a precise trait of an organism's form (phenotype), such as hair color.
2.nounA segment of DNA or RNA from a cell's or an organism's genome, that may take several forms and thus parameterizes a phenomenon, in general the structure of a protein; locus.
"A change in a gene is reflected in the protein or RNA molecule that it codes for."