Genomic imprinting is an epigenetic chromosomal modification that describes the preferential expression of a specific parental allele. Genomic imprinting is a complex genetic phenomenon, that is difficult to define, and as of June, 2001, no adequate explanation has been found for why genomic imprinting exists.

Genomic imprinting is normal genetic process where only one allele of a gene is expressed while the other allele remains genetically silent. Which allele is expressed and which remains silent depends on from which parent the genes are inherited.

Genomic imprinting would appear to contradict Mendel's observation that both parents make an equal genetic contribution to the offspring. While this is true for the inheritance of most genes, there exists a small subset of approximately 50 genes, where only one parent contributes a functioning gene to the offspring.

Following fertilization of a mammalian embryo most of the genes contributed by each parent begin...