What is a recessive allele?

A recessive allele is characterized by its expression in an organism's phenotype only when two copies of that allele are present, meaning that both inherited alleles from each parent must be recessive for the trait to manifest. This is due to the nature of alleles, where one may dominate the effect of the other; thus, when a dominant allele is present alongside a recessive one, the dominant trait will be expressed.

The understanding of recessive alleles is foundational in genetics, particularly in predicting inheritance patterns in offspring. When both alleles at a locus are recessive, their combined effects allow the specific trait associated with the recessive allele to be visible in the phenotype, which might not be the case when a dominant allele is present.