Genomic imprinting is a process that causes genes to be expressed in a parental-origin-specific manner. In contrast to expression of genes from both chromosome homologues.
In other words, one of the parent's alleles was preferred and expressed over the other in different regions. Research shows that cells in multiple brain regions specifically express a maternal allele for a gene that encodes an enzyme involved in making several neurotransmitters. The paternal copy of this gene was preferentially expressed in the adrenal gland which produces hormones.
The mechanism that would explain this phenomenon remains unclear, but is assumed to be quite complex. It is interesting to believe genomic imprinting may occur in other species, and why one parental allele may be favored or suppressed over another in different regions in recognizable patterns.