- An allele is a variant form of a gene, representing one of two or more possible DNA sequences at a specific genomic location. Each allele may differ by a single nucleotide, multiple nucleotides, or larger structural changes. These variations contribute to the diversity of traits observed within a species.
- Humans and most multicellular organisms inherit two alleles per gene, one from each parent, because they possess paired homologous chromosomes. If both alleles are the same, the individual is homozygous; if they differ, the individual is heterozygous.
- Alleles influence how traits are expressed. Some alleles are dominant, meaning they determine the phenotype even when only one copy is present.
- Others are recessive, expressed only when both alleles are recessive. This relationship explains why certain traits—such as Mendel’s pea flower colours—follow predictable inheritance patterns. In many cases, alleles produce no visible change, but some can lead to distinct phenotypes such as blood types, disease susceptibility, or metabolic differences.
- Modern genetics recognises that traits may be influenced by multiple alleles, codominance, incomplete dominance, or interactions among many genes. Allelic variation is therefore central to understanding heredity, evolution, and population diversity. The concept also underpins fields such as medical genetics, forensic identification, and evolutionary biology.
