What are dominant and recessive alleles?

Alleles are different versions of the same gene, which give different characteristics (or phenotypes) when expressed. Each gene has a pair of two alleles, and these pairs can be homozygous, meaning both alleles are the same, or heterozygous, where the alleles are different. In a heterozygous pair, the dominant allele will be the one which is expressed, as it dominates over the recessive allele, masking its characteristics. Recessive alleles can only be expressed in homozygous pairs, where both alleles are recessive.