A recessive allele is only expressed as a phenotype in an organism if:

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Genes are forms of DNA that are passed on from parent to child and are an important part of what decides how children look and act. Genes affect the way our bodies work, including how we look. Our eye, hair and skin color are decided by genes.
The different forms of a gene are called alleles. A simplified example would be dark or light eye color in humans.
Some alleles are dominant, which means they overshadow the other allele, which is recessive. Building off our previous example, the allele for dark eye color is the dominant allele and the allele for light eye color is the recessive allele. This means that if a human were to have both the dominant allele (dark eyes) and recessive allele (light eyes) for eye color, the dominant allele (dark eyes) would show.
A genotype is the combination of alleles present in an organism. For a person's eye color, the genotype may consist of two alleles for dark eyes (homozygous dominant), two alleles for light eyes (homozygous recessive), or one of each alleles (heterozygous).
The phenotype is the expression of the genes. An example of a phenotype is simply "dark eyes" or "light eyes".
The genotype determines the phenotype. The only way that a recessive allele can be expressed as a phenotype is if it is homozygous recessive. The genotypes and phenotypes for dark eyes (D) and light eyes (d):
DD = homozygous dominant = dark eyes
Dd = heterozygous = dark eyes
dd = homozygous recessive = light eyes

A Recessive Allele Is Only Expressed as a Phenotype in an Organism