There are two main reasons. First, there are introns and exons existed in eukaryotes. Introns do not contain the genetic information but are in large amount in chromosome. So, if the mutation occurs in the introns, it will be recessive. Second, one amino acid will corresponding to several base sequences. For example, UUU and UUC all represent Phe. This is called degeneracy. So if the mutation did not change the amino acid, it is also recessive.
