What are alleles?

An allele is a different version of a gene. It refers to one of several possible variations in a DNA sequence, which could be as small as a single base or a segment of multiple bases. Humans inherit two alleles for each gene—one from each parent since they are diploid organisms. Alleles can come in pairs, and sometimes there are multiple alleles that influence how a trait is expressed. An organism's genotype is determined by the combination of alleles it carries. If an organism has two identical alleles for a trait, it's called homozygous. If it has two different alleles, it's called heterozygous. Certain alleles exert dominance over others, determining the phenotype of an organism. When an organism carries both a dominant and a recessive allele at a particular locus, it typically displays the dominant phenotype.