An allele is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. "The chromosomal or genomic location of a gene or any other genetic element is called a locus and alternative DNA sequences at a locus are called alleles.
Genes are sections of DNA that contribute to certain traits, characteristics or functions. Genes code for proteins or parts of proteins that influence things like the immune system, skin pigmentation, hormone production, and eye color. Genes are transcribed into RNA molecules, which are then translated into proteins. Segments of DNA demarcated as genes consist of both coding and non-coding regions. Coding regions, also called exons, are the sections transcribed into eventual protein. Non-coding regions, or introns, are not transcribed, but are thought to fulfil numerous other functions such as regulation of transcription. Humans have approximately 20,000 protein-coding genes, which represent less than 2% of the total genome.