An allele is one of two or more versions of DNA sequence at a given genomic location. It is a variation of the same sequence of nucleotides at the same place on a long DNA molecule. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous. Alleles can be as simple as a single nucleotide difference or an insertion or deletion of hundreds of bases. They include all the variant forms of a particular gene. A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles present or the proportion of heterozygotes in the population. The combination of alleles that an organism carries constitutes its genotype. Alleles do much more than just decide physical traits. They also influence our risks for developing certain diseases, how we react to medications, and even if we develop allergies to certain things.