What is a dominant allele?

A dominant allele is defined as an allele that masks the effect of a recessive allele. This means that when an individual has at least one copy of a dominant allele, its trait will be expressed in the phenotype, regardless of whether the individual has one or two copies of another allele for the same gene.

For instance, if we consider a trait such as flower color in a plant, if one allele (let's say for red petals) is dominant and another allele (let's say for white petals) is recessive, a plant with either one or two copies of the red allele will display red petals. The presence of the recessive allele does not affect the expression of the dominant trait when a dominant allele is present.

This understanding is crucial in genetics, as it provides insight into inheritance patterns and can predict the likelihood of certain traits being passed on to offspring. The other choices describe different scenarios or definitions and do not accurately describe the characteristic of a dominant allele. For example, a dominant allele is not defined solely by its expression needing two copies, as is the case with recessive alleles, nor is it specific to sex chromosomes.