Allele is an alternate form of a gene, which is the basic unit of inheritance. A gene is located at a particular site on the chromosome and can have several alleles for that locus. For example, A, B, and O are different alleles for the ABO blood-type marker locus of a gene. Alleles greatly influence the expression of physical and behavioral phenotypes or traits such as eye color. For instance, the apolipoprotein E (APoE) gene is a well-known risk factor for developing Alzheimer’s disease. The APoE gene has three common alleles: epsilon 2, epsilon 3, and epsilon 4. There is some evidence that carriers of the APoE epsilon 4 allele are at a greater risk for the development of Alzheimer’s disease. In contrast, the APoE epsilon 3 allele has been suggested as a “protective” factor in the development of Alzheimer’s disease (Plomin et al. 2003).