What is the main difference between dominant and recessive alleles?

The key distinction between dominant and recessive alleles lies in how many copies of each are needed for their traits to be expressed in an organism. Dominant alleles require just one copy to manifest their associated traits. This means that if an organism has at least one dominant allele for a particular gene, the trait that allele represents will be visible in the phenotype. In contrast, recessive alleles necessitate two copies for their traits to be exhibited, which means that an organism must inherit one recessive allele from each parent for the recessive trait to be expressed. This relationship between the alleles explains why a dominant trait can mask the presence of a recessive trait when both are present in the genotype.

The other options misrepresent these genetic principles, reinforcing the importance of understanding how alleles interact in inheritance.