An allele is a gene type of a variant. Many genes have a number of different types located at the same location on a chromosome, or genetic locus. In human beings one allele is inherited from each parent as they carry two alleles at each genetic site.
Every pair of alleles is the genotype of a given gene. Genotypes are identified as homozygous when there are two similar alleles at a particular locus and heterozygous when there are two different alleles. Allles contribute to the morphology of the organism which is the organism's external appearance.
Hence, the pair of genes on chromosome that is linked to heredity traits is called allele.
