Genomic Imprinted genes are expressed in a parent-of-origin-specific manner. Thus, their expression is determined by the parent that contributed them. In such their inheritance is independent of the classical Mendelian inheritance. This means that Imprinted genes violate the usual rule of inheritance that both alleles in a heterozygote are equally expressed [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Imprinting.html].
Imprinting involves a sex-specific process of chemical modification to the imprinted genes, so that they are expressed unequally, depending on the sex of the part of origin.