Final answer:
A gene that only influences the expression of a trait when paired with another less active gene is called recessive. Recessive traits are only expressed when an individual is homozygous for the recessive allele. Dominant alleles mask the recessive ones and can express the trait in both homozygous and heterozygous states.
Step-by-step explanation:
A gene that only influences the expression of a trait when paired with another less active gene is called recessive. A recessive trait will be observed in individuals that are homozygous for that trait. Dominant alleles, on the other hand, can mask the presence of recessive alleles and are observed in both the homozygous ('AA') and heterozygous ('Aa') genotypes. For example, in the case of blood types, the allele for blood type A is dominant over the allele for blood type O. If a person has one A allele and one O allele, the dominant A allele will determine the blood type. Conversely, for a person to express the recessive O blood type, both alleles must be O. Therefore, in situations where pairs of alleles are considered, and one allele is dominant while the other is recessive, the presence of the dominant allele will be the determining factor in the phenotype of the organism.