The fundamental assumption of Mendelian genetics is that behavior of an allele is identical whether it arrives to a zygote through paternal or maternal germline pathway. Gametic imprinting phenomena discovered and studied in mammals show limitations of the classical view in special cases. Two sources of evidence were essential to describe gametic imprinting. The first approach based on genetic evidence demonstrated that some maternally and paternally derived regions of certain chromosomes were not equivalent. Paternal or maternal disomy of the regions containing particular genes caused significant effects on viability and development of progeny. The second set of data was obtained by nuclear transplantations and parthenogenetic activation of mammalian oocytes. These data suggested that the contribution of parental genomes was not equivalent and differential imprinting of nuclear genes during gametogenesis was very likely. The number of loci found in mice, which show gametic imprinting, is 34 and continues to grow. It is generally accepted that gametic imprinting is a mammalian invention and there are differences in imprinting pattern between species. Most hypotheses propose involvement of imprinted genes in the control of fetal growth and fetal-maternal interactions, thus keeping a balance between contradictory fetal and maternal requirements. Molecular mechanisms responsible for gametic imprinting still remain to be studied, but for several genes it was shown that imprinting marks are imposed by a parent-specific methylation process during gametogenesis. These marks are resistant to global demethylation during cleavage and to global de novo methylation after implantation and maintain different methylation patterns in paternal and maternal alleles of imprinted genes. About 20 to 25% of all transgene loci studied demonstrate similarities with imprinted genes. For instance, methylation of some transgenes is dependent on parental gametic pathway and reversible in the next generation. There are data indicating that selection of modifier genes may change the effects of gametic imprinting. It is possible that future selection and crossbreeding programs may take gametic imprinting into consideration.