An allele is one of two or more alternative forms of a gene or genetic locus found at the same position (locus) on a chromosome. Each individual inherits two alleles for each gene, one from each parent, which may be identical (homozygous) or different (heterozygous). Alleles can influence traits, such as eye color or disease susceptibility, and play a key role in genetic diversity within populations. In the context of VCF files, an allele typically refers to the reference (REF) or alternate (ALT) sequences at a specific genomic position.