What distinguishes dominant alleles from recessive alleles?

Dominant alleles are defined by their ability to express their traits in the phenotype even when present in just one copy, which is referred to as being heterozygous. In contrast, recessive alleles only display their traits in the phenotype when an individual is homozygous for that allele, meaning that both alleles are recessive. This distinction is fundamental to understanding patterns of inheritance, as dominant alleles can 'mask' the presence of recessive ones in a heterozygous genotype. For instance, if a dominant allele codes for brown eyes and a recessive allele codes for blue eyes, an individual with one copy of the brown eye allele (heterozygous) will have brown eyes, while an individual with two copies of the blue eye allele (homozygous) will have blue eyes. This dynamic explains why the assertion regarding the manifestation of dominant versus recessive alleles accurately identifies their key difference in terms of phenotypic expression.