Genomic imprinting: The phenomenon of parent-of-origin gene expression. The expression of a gene depends upon the parent who passed on the gene.
For instance, two different disorders - Prader-Willi syndrome and Angelman syndrome -- are due to deletion of the same part of chromosome 15. When the deletion involves the chromosome 15 that came from the father, the child has Prader-Willi syndrome, but when the deletion involves the chromosome 15 that came from the mother, the child has Angelman syndrome.
Genomic imprinting plays a critical role in fetal growth and development. Imprinting is regulated by DNA methylation and chromatin structure.