When are parental imprints established?
Parental imprints are established during gametogenesis as homologous DNA passes uniquely through sperm
egg; subsequently during embryogenesis and into adulthood, alleles of imprinted genes are maintained in
"conformational"/epigenetic states: paternal or maternal.
imprints template their own replication, are heritable, can be identified by
molecular analysis, and serve as markers of the parental origin of genomic
Functional consequence: unbalanced gene expression
Beyond merely labeling homologous genetic alleles as descendent from
father or mother, genomic imprints have the significant functional consequence
of stifling gene expression from one of the parental alleles, resulting in
unbalanced gene expression between homologous alleles.
As a result of
imprinting, there is biased allelic expression that favors expression from one
parental locus over the other. Imprinting is oftentimes equated with
parent-of-origin òmonoallelic expression, yet parental allelic exclusion is
seldom 100% efficient; one usually finds various degrees of òleaky expression
from the silenced allele.
Imprinted genes are functionally haploid,
erasing benefits of diploidy at these loci.
It is estimated that
approximately 1-2% of human genes are subject to parental imprinting, but
currently fewer than 100 distinct named genes have been demonstrated to be
Clinical human diseases
and syndromes stemming from the unique vulnerabilities of imprinted loci
include: gestational trophoblastic disease, teratomas, Beckwith-Wiedemann syndrome,
syndrome, Silver-Russell syndrome, transient neonatal diabetes,
/and social-cognitive defects in
and multiple neoplasias associated with loss of imprinting at oncogene
loci. OMIM (On-line Mendelian (!) Inheritance in Man) database of the NCBI
(United States National Center for Biotechnology Information) contains
detailed entries on many imprinted genes and syndromes.
Imprinted genes code for what?
Although a majority of the known imprinted genes code for proteins, others code for
untranslated RNA transcripts.
Another category of parental genomic imprint,
to be contrasted with well characterized examples of monoallelically expressed
genes, are those methylation parental imprints scattered throughout the genome
which are not demonstrated to be functional or associated with specific genes.
Maybe a more pervasive process
Keeping this final category in mind underscores the idea that ògenomic imprinting may be a
more pervasive process than simply a mechanism to monoallelically silence a
handful of genes, as in the broad sense parental genomic imprints are not
required to be associated with transcriptionally active
Moreover, beyond the estimated 500 genes thought to be
imprinted, it is unknown how many parental imprints may be stamped throughout
the genome, or what their pattern and periodicity may be; perhaps foretelling
are imprint gene discovery experiments based on whole genome scanning, which
uncover mostly domains not known to be associated with transcriptional
Mules, hinnies, and Plato
Elucidating the phenomenon of imprinting has provided much insight into molecular
developmental and cancer biology, but also helps explain centuries-old
biological observations. Mule breeders 3 millenia ago observed that a mare crossed with a donkey
yields a mule, whereas a stallion crossed with a donkey produces a hinny,
which has shorter ears, a thicker mane and tail, and stronger legs than the
mule; thus indicating parental sex-dependent influence on phenotype.
Although ancestral donkey crossers would likely have no problem with the
concept and reality of parental genomic imprinting, imprinting more recently
carries an iconoclastic aura, evidence of the powerful influence Gregor
Mendelès writings have exerted;indeed, the phenomenon of imprinting has been classified within
of non-Mendelian genetics, as if Mendels laws represent the Platonic ideal of
genetic behavior.Modern evolutionary biologists welcome the finding of parental
influence on the genome, for it spawns debate over the hypothetical selective
pressures which could be driving and maintaining such a deleterious process.
Atlas of Genetics and Cytogenetics in Oncology and Haematology 2005-03-01
Online version: http://atlasgeneticsoncology.org/teaching/30027/genomic-imprinting