What is a recessive allele?

A recessive allele is defined as an allele that is only expressed phenotypically when two copies are present. This means that for an individual to express the trait associated with a recessive allele, they must inherit that allele from both parents, leading to a homozygous condition (having two identical alleles for a trait).

In contrast, if an individual has only one copy of a recessive allele paired with a dominant allele, the dominant allele will mask the expression of the recessive allele, resulting in the dominant phenotype being expressed instead. This is why a recessive allele must be present in two copies (as opposed to just one) for its associated trait to be displayed in the phenotype of the organism.

The other descriptions provided do not accurately capture the essence of a recessive allele; therefore, they do not apply in this context. This understanding of recessive alleles is fundamental in genetics, particularly when studying inheritance patterns and predicting phenotypes in offspring based on parental genotypes.