The ladder rungs are made up of chemicals called bases, with each rung of the ladder consisting of a pair of bases. With the exception of identical twins, the sequence of the bases is different for everyone, which makes each of us unique. For all our outward variation, we are surprisingly alike at the DNA level. Yet DNA base sequence variations influence our physical differences and many of our other characteristics as well. Sequence variation can occur in our genes, and the resulting different forms of the same gene are called alleles .
People can have two identical or two different alleles for a particular gene.
The type of gene a person has at a specific location in their genome is called their genotype . The different physical characteristics of an individual make up their phenotype . Changes within genes can result in proteins that function abnormally or are completely non-functional. Some of these changes can contribute to disease or influence response to a medicine. Mutations can arise spontaneously during normal cell functions, such as when a cell divides, or in response to environmental stresses such as toxins, radiation, hormones and perhaps even diet.