Which of the following best describes a dominant gene?

A dominant gene is characterized by its ability to express its phenotype in the presence of just one copy of the gene. When a dominant allele is present, it masks the effects of a recessive allele, even when the recessive allele is present in a heterozygous condition. This means that as long as at least one dominant allele exists, its associated trait will be exhibited in the organism. This property is crucial in understanding inheritance patterns and predicting phenotypic outcomes in offspring.

For example, in a gene pair where one allele is dominant (A) and the other is recessive (a), both genotypes AA and Aa will result in the expression of the dominant trait, while only the homozygous recessive (aa) will lead to the expression of the recessive trait. This fundamental principle underpins many aspects of genetics, including the study of inheritance and variation within populations.