In simple dominance, when a dominant allele pairs up with a recessive allele in a heterozygous individual, the dominant trait will be expressed. This means that the individual will show the phenotype associated with the dominant allele.

For example, consider a gene that determines eye colour, where brown eyes are dominant over blue eyes. If an individual has one copy of the brown eye allele (B) and one copy of the blue eye allele (b), they will have brown eyes. This is because the brown eye allele is dominant, and its presence in the genotype will result in the brown eye phenotype.