Different alleles of the same gene can have different levels of phenotypic expression (outward appearance). Because genes are usually present in two copies (sex chromosomes being the exception in diploid organisms like humans), this difference in expression can be important to the phenotype of the organism. Some alleles mask the effect of all other forms of the gene present. These alleles are said to be dominant. A dominant gene has the same effect when it is present as a single copy (heterozygous) as when it is present as two copies (homozygous). The allele not expressed in the heterozygous form is said to be recessive to the masking allele, the dominant allele.

In the pea plants studied by Austrian monk and botanist Gregor Johann Mendel, the allele for tall plants was dominant to the allele for dwarf plants. When a gene is said to be dominant or...