Generalize: If either allele is the allele for a trait, the organism will exhibit that trait.
An organism will express the recessive trait only
when
recessive

1. Dominant allele

2. Homozygous recessive i.e. two recessive alleles

Explanation:

An allele is an alternate form of a gene. According to Mendel, an organism receives two forms of this gene called ALLELE. Each allele codes for a particular trait. In his experiments, he discovered that one allele is capable of masking the expression of another in a gene. Hence, the organisms will exhibit the trait encoded by that allele. He called this allele that masks; DOMINANT allele while the allele that is masked; RECESSIVE allele.

Based on this, a homozygous dominant (two dominant alleles) and heterozygous (both dominant and recessive alleles) will exhibit the dominant trait while the recessive trait will only be expressed when the genotype of the organism contains two recessive alleles i.e. homozygous recessive.