A weird little corner of genetics (or more specifically, epigenetics).
Genomic imprinting is when identical nucleotide sequences produce different phenotypes depending on whether they come from the male or female parent. No one knows why this happens, but it most likely has something to do with the processes of spermatogenesis or oogenesis.
Some of the epigenetic items that influence imprinting are genomic methylation, replication-timing, and differences in chromatin structure.