Diploid organisms inherit genetic material through chromosomes from both parents. Copies of the same gene are known as alleles. In most cases, both alleles are simultaneously expressed and allow various cellular processes to function optimally. If one of the alleles is missing or mutated, the expression of the other allele can compensate; however, this is not true for all genes.

The expression of some genes depends on which parent passed the gene to the offspring, through a phenomenon known as genomic imprinting. In genomic imprinting, the DNA coding for the gene or its regulatory sequence is imprinted with chemical tags such as methyl groups, only in the copy inherited from a particular parent. This tag is resistant to demethylation during fertilization and is passed on to the offspring for selective gene expression.

Sometimes two copies of the gene are inherited from the same parent, and the other parent’s gene is missing. Maternal uniparental disomy is when two copies of the mother’s gene are present; in contrast, paternal uniparental disomy is when two copies are inherited from the father. If there are two copies of the silenced allele and if the typically expressed allele is absent, it often leads to a genetic disorder.

Human Diseases Linked to Genomic Imprinting

Diseases in humans linked to genomic imprinting include Beckwith-Wiedemann syndrome, Angelman syndrome, Prader-Willi syndrome, and some cancers.

Prader-Willi and Angelman syndrome are both associated with chromosome 15. Prader-Willi syndrome occurs when there is maternal uniparental disomy. People with this syndrome can display obesity, sexual under-development, and mental disabilities. Angelman syndrome is associated with paternal uniparental disomy. People with this syndrome may show mental disabilities, developmental deficiencies, sleep disorders and hyperactivity.