What does the term 'dominant' refer to in genetics?

The term 'dominant' in genetics refers to an allele that expresses its effect even when paired with a different allele (the recessive allele) in a heterozygous condition. This means that if an organism has one dominant allele and one recessive allele for a particular trait, the dominant allele will determine the organism’s phenotype, while the recessive allele has no observable effect.

In many cases, dominant alleles can mask the expression of recessive alleles. For example, if an organism has the genotype Aa (where "A" is the dominant allele and "a" is the recessive allele), the phenotype will reflect the presence of the dominant allele "A." This characteristic is crucial for understanding inheritance patterns and gene expression.

The other options describe scenarios that do not align with the definition of 'dominant.' For instance, referring to a gene that only expresses in homozygous conditions contradicts the nature of dominant alleles, which express in heterozygous conditions as well. Similarly, while it is true that dominant and recessive alleles can show contrasting effects, describing dominance as having a "completely different effect" does not encompass the full definition, as the dominant allele typically expresses the trait more prevalently rather than a