Any one of a series of two or more different genes that occupy the same position (locus) on a chromosome. Since autosomal chromosomes are paired, each autosomal locus is represented twice. If both chromosomes have the same allele, occupying the same locus, the condition is referred to as homozygous for this allele. If the alleles at the two loci are different, the individual or cell is referred to as heterozygous for both allele's. ABO blood type is determined by three alleles: A, B and O (technically the 'null allele').