A gene is a DNA sequence that provides the genetic information to produce a protein. Genes are located on your chromosomes and occupy very small segments. There are two copies of each gene in humans since mammals have two copies of each chromosome (with the exception of sex chromosomes).
In higher mammals and humans genes could have complicated structures such as promoters or control regions, introns, and exons. Promoters are DNA sequences adjacent to the beginning of the genes and control gene activation. Introns are sequences inside the gene although they do not code for protein sequence they are necessary for correct coding. Exons are the regions inside the gene between introns that contain the coding DNA sequences. Most of the harmful mutations occur in exons and as a consequence change the structure of the coded protein.