How are alleles and genes related?
An allele is a form of a gene. For example, in humans, the ABO gene for the ABO blood group is located on chromosome 9, (http://en.wikipedia.org/wiki/ABO_blood_group_system) and has three alleles (forms), A, B, and O. A person can inherit only two alleles for the ABO blood group, one from each parent. The two alleles that a person inherits determines that persons ABO blood group.