A gene is a unit of hereditary information. Except in some viruses, genes are made up of DNA, a complex molecule that codes genetic information for the transmission of inherited traits. Alleles are also genetic sequences, and they too code for the transmission of traits. So, what it is the difference between a gene and an allele?
The short answer is that an allele is a variant form of a gene. Explained in greater detail, each gene resides at a specific locus (location on a chromosome) in two copies, one copy of the gene inherited from each parent. The copies, however, are not necessarily the same. When the copies of a gene differ from each other, they are known as alleles. A given gene may have multiple different alleles, though only two alleles are present at the gene’s locus in any individual.
Alleles can sometimes result in different phenotypes (observable traits), with certain alleles being dominant (overriding the traits of other alleles) or, in some cases, multiple alleles acting in a codominant fashion. An example of the latter is the human ABO blood group system, in which persons with type AB blood have one allele for A and one for B (persons with neither allele are type O). An example of dominant allele expression is flower color in pea plants. A plant with purple flowers actually has a genotype (genetic makeup) consisting of a gene with a dominant P and a recessive p allele.