Dominant allele

A dominant allele is one copy of the gene that will always be expressed whether homozygous or heterozygous for the gene. The phenotype of the organism will show the dominant trait whenever this allele is present.

Dominant alleles are often designated a capital letter for example (Ww) - here the W represents the dominant wildtype allele whilst w is the recessive allele.

(WW) - Homozygous dominant (W phenotype expressed)

(Ww) - Heterozygous (W phenotype expressed - due to the dominant allele masking the expression of the recessive allele). 

(ww) - Homozygous recessive (w phenotype expressed)