Allele - GCSE Biology Definition

What is an allele?

In GCSE biology, an allele is a version of a gene. Each individual has two alleles of each gene, one on each member of a homologous pair of chromosomes. The genotype of an individual is determined by the alleles present for each gene.

An individual with two identical alleles of a gene is said to be homozygous and an individual with two different alleles of a gene is referred to as heterozygous. Alleles that are dominant will always be expressed in the phenotype of an organism, while recessive alleles are only expressed in the phenotype if the dominant allele is absent.