which term describes a type of trait that is usually expressed only when an organism has two identical alleles for the trait

The answer is; homozygous

An organism can be homozygous dominant or homozygous recessive. The former means the organism has two of the dominant alleles for the trait while the latter means the organisms have two of the recessive alleles for the trait. When the organism has a dominant and recessive allele, it is referred to as heterozygous.

Describes a trait or an allele that is expressed only when two recessive alleles for the same characteristic are inherited.