The structure of an organism -- its shape, appearance, and function -- are all determined by its genes. The collection of all genes in a type of organism is called a genome. A location of a specific gene within a genome is called a locus, and the set of different forms of the gene are alleles.
To make an analogy from computer programming, a genome is a string; the number of loci in the genome corresponds to the length of the string, and the number of alleles at a given locus corresponds to the number of different characters that can occur at that position in the string. In the simulator described elsewhere in this chapter, a genome is represented by a string of bits, so there are two alleles -- represented by 0 and 1 -- at each locus.