Define briefly about alleles

Alleles are a pair of genes that occupy a specific location on a particular chromosome and control the same trait. Alleles may appear in pairs or in multiple forms of alleles, which affect a specific trait of the offspring. Locus is the location where a gene is found on a chromosome. A pair of alleles determine the same trait, for example, eye colour; one allele codes for black eyes, and another allele codes for brown eyes. All the alleles found in an organism make up the genotype. If a pair of alleles are similar, the organism's genotype is called homozygous. If the pair of alleles are different, the organism's genotype is called heterozygous. A dominant allele will overrule the characteristics of a submissive allele in a heterozygous genotype. But in some traits, an allele is neither dominant nor recessive. For example, the human blood group ABO.