What is the primary difference between dominant and recessive alleles?

The primary difference between dominant and recessive alleles lies in how they express their traits in an organism's phenotype. Dominant alleles are characterized by their ability to manifest their traits even when only one copy is present in a pair of alleles. This means that if an organism inherits a dominant allele from one parent, that trait will be expressed regardless of the allele contributed by the other parent. This is significant in understanding inheritance patterns, as dominant traits can overshadow recessive traits when both are present.

In contrast, recessive alleles require two copies—one from each parent—to be expressed in the phenotype. This means that a trait governed by a recessive allele will only be visible if the organism has two recessive alleles (homozygous recessive). Therefore, option C correctly highlights the defining characteristic of dominant alleles and emphasizes their role in genetic expression.