Use single-gene inheritance to investigate a gene"s function so have to know the molecular nature of wild-type and mutant alleles at both the structural & the functional level. Structural differences between alleles at the molecular level. Alleles (i. e a & a) are identical in most of their sequences & different only at one or several nucleotides go the 1000s that make up the gene. Allele changes because of rare chemical accident -> new allele is created = Mutation -> can occur anywhere along the nucleotide sequence of a gene. Mutation: the process that produces a gene or a chromosome set differing from that of the wild type; the gene or chromosome set that results from such a process. Mutation could be change in the identity of a single nucleotide or the deletion of one or more nucleotides or even the addition of one or more nucleotides. Many ways a gene can be changed by a mutation.