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

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

Recessive alleles are various genetic codes that do not produce a phenotype when a dominant allele is present. Recessive allele is only expressed if the individual has two copies of the allele (known as homozygous). For example, the allele for brown hair is recessive, thus, for an individual to have brown hair, he must possess two copies of the ‘brown hair’ allele.

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products