Dominant allele is the allele that determines phenotypical features that manifest in homozygous or heterozygous genotypes. In Genetics the dominant allele is represented in uppercase, e.g., “A”, and its recessive allele is written in lowercase, “a”. In molecular terms generally the recessive allele has a nucleotide sequence previously identical to the corresponding sequence in the dominant allele but that during evolution was inactivated by mutation. This fact explains the expression of the dominant phenotype in heterozygosity (since one functional allele is still present).