An allele is a variant form of a gene, which is a segment of DNA that codes for a specific trait. Alleles are located at the same position, or genetic locus, on a chromosome, and humans are diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. If the two alleles are the same, the individual is homozygous for that allele, and if the alleles are different, the individual is heterozygous. Alleles contribute to the organisms phenotype, which is the outward appearance of the organism. Some alleles are dominant or recessive, and when an organism is heterozygous at a specific locus and carries one dominant and one recessive allele, the organism will express the dominant phenotype.

In summary, an allele is a version of a gene that codes for a specific trait, and humans inherit two alleles, one from each parent, for any given genomic location where such variation exists.