Whenever a pair of alleles has different alleles is there dominance between them?
Not in all cases of a gene includes two different alleles is the dominance complete. There are genes in which heterozygosity take place with incomplete dominance (manifestation of an intermediate phenotype in relation to the homozygous, like in the color of roses, between red and white) and other genes that present codominance (expression of a third different feature, as in the MN blood group system).