What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Genetic Imprinting: How Parental Genes Shape Expression

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Genetic Imprinting

Genetic imprinting is an epigenetic phenomenon in which a subset of genes is expressed in a parent-of-origin-specific manner. For an imprinted gene, the allele inherited from the mother and the allele inherited from the father are not functionally equivalent: one copy is transcriptionally silenced, while the other remains active. This violates the classical Mendelian expectation that maternal and paternal alleles contribute equally to the phenotype.

In diploid organisms, most autosomal genes are expressed from both alleles. Imprinted genes are the exception—roughly 100–200 are known in humans, clustered in about 20 genomic regions. The choice of which allele is silenced is determined not by the DNA sequence itself but by epigenetic marks—chemical modifications to DNA and chromatin that are heritable through cell division but do not alter the nucleotide sequence. This distinction places imprinting squarely within the domain of epigenetics, where the genetic expression program is modulated by reversible, environmentally responsive marks.

Imprinting is essential for normal mammalian development. It regulates fetal growth, placental function, neurodevelopment, and postnatal metabolism. Disruption of imprinting—through mutation, uniparental disomy, or epigenetic errors—causes well-characterized human syndromes, including Prader-Willi, Angelman, and Beckwith-Wiedemann syndromes. Understanding imprinting therefore matters not only as a fundamental biological curiosity but as a clinically relevant axis of gene regulation.

The Mechanism of Genetic Imprinting

Imprinting is achieved through a coordinated set of epigenetic modifications that establish differential activity between the two parental alleles. The central players are DNA methylation, histone post-translational modifications, and higher-order chromatin architecture. These marks are not distributed uniformly across the genome but are concentrated at specific regulatory elements known as imprinting control regions (ICRs).

DNA Methylation at Imprinting Control Regions

DNA methylation is the best-characterized imprinting mark. It involves the covalent addition of a methyl group to the fifth carbon of cytosine residues, almost exclusively in the context of CpG dinucleotides. This reaction is catalyzed by DNA methyltransferases (DNMTs): DNMT3A and DNMT3B establish new methylation patterns during gametogenesis, while DNMT1 maintains existing methylation during DNA replication by recognizing hemimethylated CpG sites and methylating the newly synthesized strand.

In imprinted domains, ICRs are differentially methylated regions (DMRs): one parental allele is hypermethylated, the other hypomethylated. The methylation status of an ICR determines whether it can bind insulator proteins or act as a promoter for a long non-coding RNA, thereby controlling the expression of downstream genes.

The methylation mark is read by proteins containing a methyl-CpG-binding domain (MBD), such as MeCP2 and Kaiso. These proteins recruit histone deacetylases and chromatin remodelers, leading to a compact, transcriptionally repressive chromatin state. Conversely, the unmethylated allele remains accessible to transcription factors and RNA polymerase II.

Histone Modifications and Chromatin Remodeling

DNA methylation does not act alone. Imprinted loci are marked by allele-specific histone modifications that reinforce the transcriptional state. The silenced allele typically carries histone H3 lysine 9 dimethylation (H3K9me2) and H3 lysine 27 trimethylation (H3K27me3)—marks associated with heterochromatin and facultative silencing. The active allele, by contrast, is enriched for H3 lysine 4 methylation (H3K4me3) and histone acetylation, which promote an open chromatin conformation.

These histone marks are deposited by specific enzymes. Polycomb repressive complex 2 (PRC2) catalyzes H3K27me3, while SETDB1 and SUV39H enzymes deposit H3K9me. The maintenance of these marks through cell division requires the Polycomb and trithorax systems, which recognize existing marks and re-establish them on daughter chromatids.

Chromatin architecture also contributes. Imprinted clusters often adopt allele-specific topologically associating domains (TADs). The CCCTC-binding factor (CTCF), an insulator protein, binds to unmethylated ICRs and creates chromatin loops that physically separate enhancers from promoters. When the ICR is methylated, CTCF cannot bind, and the enhancer gains access to a different set of promoters. This architectural switch is the mechanistic basis for the reciprocal expression patterns seen in imprinted clusters.

How Imprinting Is Established and Maintained

Imprinting is not a static mark. It undergoes a carefully choreographed cycle of erasure, establishment, and maintenance across the lifespan of an organism. This cycle ensures that imprints are reset according to the sex of the parent transmitting the allele.

Erasure and Establishment in Germ Cells

Imprints are erased in the primordial germ cells (PGCs) of the developing embryo. Around embryonic day 10.5 in the mouse (approximately week 4–6 in human development), PGCs undergo genome-wide demethylation. This erasure is both passive—through failure to maintain methylation during replication—and active, involving the TET (ten-eleven translocation) enzymes, which oxidize 5-methylcytosine to 5-hydroxymethylcytosine and further derivatives, ultimately leading to base excision repair and replacement with unmethylated cytosine.

The result is that germ cells entering gametogenesis carry no parental imprints. This is a critical checkpoint: any imprinting mark that escapes erasure would be inherited inappropriately and could cause developmental abnormalities.

Following erasure, new imprints are established during gametogenesis. In the male germline, this occurs in prospermatogonia before birth; in the female germline, it occurs postnatally during oocyte growth. The establishment phase is mediated by DNMT3A and its cofactor DNMT3L, which lacks catalytic activity but is required for targeting DNMT3A to imprinted loci. The timing differs between the sexes: paternal imprints are established before meiosis, while maternal imprints are established during the growth phase of oocytes in the antral follicle.

Maintenance and Reprogramming in Early Embryo

After fertilization, the zygote undergoes another wave of demethylation. The paternal genome is actively demethylated within hours of fertilization, while the maternal genome is passively demethylated over subsequent cleavage divisions. Imprinted genes are protected from this global reprogramming. The protection is mediated by the protein ZFP57, a zinc-finger protein that binds to a specific DNA motif within ICRs and recruits the maintenance methyltransferase machinery, including DNMT1 and its cofactor UHRF1.

This protection is essential. Without ZFP57, imprints are lost in the early embryo, leading to aberrant biallelic expression or silencing. The maintenance phase continues throughout somatic development: every time a cell divides, DNMT1 must recognize hemimethylated CpG sites at ICRs and methylate the daughter strand. Failure of maintenance—due to DNMT1 deficiency or disruption of UHRF1—results in passive demethylation and loss of imprinting.

Later in development, imprints are maintained in somatic tissues but are again erased in PGCs, completing the cycle. This resetting ensures that the imprinting pattern reflects the sex of the current parent, not the grandparent.

Examples of Imprinted Genes

Imprinted genes are not randomly distributed; they tend to cluster in chromosomal regions that contain shared regulatory elements. The study of these clusters has revealed the logic of imprinting and its connection to human disease.

IGF2/H19: A Classic Imprinted Cluster

The insulin-like growth factor 2 (IGF2) and H19 genes are located adjacent to each other on human chromosome 11p15.5 and are reciprocally imprinted. IGF2 is expressed only from the paternal allele, while H19, a long non-coding RNA, is expressed only from the maternal allele. This cluster is the paradigm for understanding how an ICR controls expression through chromatin looping.

The ICR lies between the two genes and contains binding sites for CTCF. On the maternal chromosome, the ICR is unmethylated, so CTCF binds and creates a chromatin loop that isolates IGF2 from a shared enhancer located downstream of H19. The enhancer instead activates H19. On the paternal chromosome, the ICR is methylated, CTCF cannot bind, the loop is not formed, and the enhancer activates IGF2 while H19 is silenced.

Loss of imprinting at this locus—for example, biallelic expression of IGF2—is associated with Beckwith-Wiedemann syndrome, characterized by overgrowth, macroglossia, and predisposition to embryonal tumors such as Wilms tumor. This example illustrates how a single methylation mark at an ICR can coordinate the expression of two genes with opposite imprinting patterns.

UBE3A and Angelman Syndrome

The ubiquitin-protein ligase E3A (UBE3A) gene on chromosome 15q11-q13 is imprinted in a tissue-specific manner. In most tissues, both alleles are expressed. In neurons, however, the paternal allele is silenced, and only the maternal allele is active. The silencing is mediated by a paternally expressed antisense transcript, UBE3A-ATS, which overlaps UBE3A and triggers transcriptional repression through the recruitment of histone methyltransferases and the formation of repressive chromatin.

Angelman syndrome results from loss of function of the maternal UBE3A allele. This can occur through deletion of the maternal 15q11-q13 region, mutation of the maternal allele, paternal uniparental disomy (inheritance of two paternal copies), or an imprinting defect that silences the maternal allele. Patients present with severe intellectual disability, absent speech, ataxia, seizures, and a characteristic happy demeanor.

The neighboring gene SNRPN (small nuclear ribonucleoprotein polypeptide N) is also imprinted, but in the opposite direction: it is expressed from the paternal allele. SNRPN lies within the Prader-Willi syndrome critical region, and its promoter functions as the ICR for the entire cluster. Deletion of the paternal copy, maternal uniparental disomy, or an imprinting defect that silences the paternal allele causes Prader-Willi syndrome, characterized by neonatal hypotonia, hyperphagia, obesity, and hypogonadism.

Why Does Genetic Imprinting Occur?

The existence of imprinting raises an evolutionary question: why would an organism silence one of its two alleles, thereby losing the protective effect of diploidy? Several hypotheses have been proposed, with the parental conflict hypothesis being the most influential.

The Parental Conflict Hypothesis

The parental conflict hypothesis, also called the kinship theory, was proposed by David Haig in the 1980s. It argues that imprinting arises from a conflict between the interests of the maternal and paternal genomes. In mammals, the mother invests resources in offspring during gestation and lactation. The father's genes, however, are not present in the mother and do not directly experience the cost of resource allocation.

From the father's perspective, it is advantageous to extract more resources from the mother, even at the expense of the mother's future offspring. From the mother's perspective, it is advantageous to distribute resources evenly among all her offspring, current and future. This conflict predicts that paternally expressed genes will promote growth and resource extraction, while maternally expressed genes will restrain growth.

This prediction is borne out by the data. IGF2, a paternally expressed growth factor, promotes placental and fetal growth. H19, which is maternally expressed, produces a non-coding RNA that limits IGF2 expression. The paternally expressed gene PEG3 also promotes placental growth, while the maternally expressed gene GRB10 inhibits growth. The correlation between parental origin and growth effects is remarkably consistent across imprinted loci.

Other Evolutionary Explanations

The parental conflict hypothesis is not the only explanation. The "ovarian time bomb" hypothesis, proposed by John Varmuza and Mellissa Mann, suggests that imprinting evolved to prevent parthenogenesis—the development of an embryo from an unfertilized egg. In mammals, parthenogenetic embryos fail because they lack paternally expressed genes required for placental development. Imprinting may have evolved as a byproduct of a mechanism that prevents uniparental reproduction.

Another hypothesis, the "sex-specific selection" model, proposes that imprinting allows different alleles to be optimized for male and female reproductive success. A gene that is beneficial when inherited from the father but harmful when inherited from the mother could be silenced on the maternal allele, allowing the paternal allele to be expressed without the cost of maternal expression.

These hypotheses are not mutually exclusive, and the true evolutionary history of imprinting likely involves contributions from multiple selective pressures. What is clear is that imprinting is an ancient mechanism, present in marsupials and eutherian mammals, and that it has been repeatedly co-opted for different developmental functions.

Methods Used to Study Imprinting

Identifying imprinted genes and understanding their regulation requires a combination of genetic, epigenetic, and transcriptomic approaches. Each method has strengths and limitations, and converging evidence from multiple techniques is often necessary.

Allele-Specific Expression Assays

The gold standard for identifying imprinted genes is allele-specific expression analysis. This requires distinguishing transcripts from the maternal and paternal alleles. The most common approach uses single-nucleotide polymorphisms (SNPs) in the transcribed region. RNA is extracted from a tissue, converted to cDNA, and sequenced. The relative abundance of each SNP allele indicates whether one parental allele is preferentially expressed.

For example, in RNA-seq data from a heterozygous individual, an imprinted gene would show a 100:0 ratio of reads from one allele, whereas a non-imprinted gene would show approximately 50:50. This approach can be applied genome-wide using RNA-seq, but it requires the tissue to be heterozygous at informative SNPs. Crosses between genetically distinct mouse strains (e.g., CAST/EiJ × C57BL/6J) are commonly used to maximize the number of informative SNPs.

Epigenetic Profiling and Mouse Models

Epigenetic profiling complements expression analysis. Bisulfite sequencing is the standard method for detecting DNA methylation. Treatment of DNA with sodium bisulfite converts unmethylated cytosines to uracil, while methylated cytosines remain unchanged. After PCR amplification and sequencing, the methylation status of each CpG can be determined. This technique can be applied to ICRs to determine which allele is methylated.

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is used to map histone modifications and transcription factor binding. Antibodies against H3K4me3, H3K27me3, CTCF, or other proteins are used to enrich for specific chromatin regions, which are then sequenced. This reveals the allele-specific chromatin state at imprinted loci.

Mouse models are essential for functional studies. Targeted deletion of ICRs, DNMTs, or ZFP57 in mice recapitulates human imprinting disorders and reveals the regulatory logic of imprinted clusters. Conditional knockouts allow tissue-specific analysis, which is important because imprinting is often tissue-specific. For example, UBE3A is imprinted only in neurons, and this tissue specificity would be missed in a whole-embryo analysis.

Clinical Significance and Imprinting Disorders

Imprinting disorders are caused by disruption of the normal parent-of-origin-specific expression pattern. They can arise through several mechanisms: deletion of a chromosomal region, uniparental disomy, point mutations in an imprinted gene or its regulatory elements, or epigenetic errors that alter methylation at an ICR.

Prader-Willi and Angelman Syndromes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are the best-known imprinting disorders. Both involve chromosome 15q11-q13, but they affect opposite parental alleles. PWS results from loss of function of paternally expressed genes in this region, while AS results from loss of function of the maternally expressed UBE3A gene.

The genetic causes are instructive. Approximately 70% of PWS cases are due to a de novo deletion of the paternal 15q11-q13 region. About 25% are due to maternal uniparental disomy—the child inherits two maternal copies of chromosome 15 and no paternal copy. The remaining cases are due to imprinting defects that silence the paternal allele. The distribution is similar for AS, but with the parental origins reversed: deletion of the maternal region, paternal uniparental disomy, or an imprinting defect that silences the maternal allele.

The clinical presentations are distinct. PWS is characterized by hypotonia and feeding difficulties in infancy, followed by hyperphagia and obesity in childhood, along with intellectual disability and behavioral problems. AS is characterized by severe developmental delay, absent speech, ataxia, and a happy, excitable demeanor with frequent laughing and smiling.

Imprinting and Assisted Reproduction

Assisted reproductive technologies (ART), including in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), have been associated with an increased risk of imprinting disorders. The absolute risk is low—on the order of 1 in 5,000 to 1 in 10,000 for Beckwith-Wiedemann syndrome—but it represents a several-fold increase over the general population.

The mechanisms are not fully understood, but several factors may contribute. Hormonal stimulation of the ovaries may disrupt the establishment of maternal imprints during oocyte growth. In vitro culture of embryos may affect the maintenance of imprints during the preimplantation period. The timing of embryo transfer relative to the maternal-to-zygotic transition may also matter.

These observations have practical implications. Patients undergoing ART should be counseled about the small increased risk of imprinting disorders. Clinicians should consider the possibility of an imprinting disorder in children conceived through ART who present with growth abnormalities or developmental delay. Ongoing research aims to identify which specific aspects of ART protocols—ovarian stimulation, embryo culture media, or cryopreservation—contribute to the risk.

Common Pitfalls and Misconceptions

Students frequently encounter several misconceptions when learning about imprinting. Clarifying these will help you avoid common errors in exams and in the laboratory.

Imprinting is not the same as X-inactivation. X-inactivation silences one entire X chromosome in female mammals to achieve dosage compensation between XX females and XY males. The choice of which X is inactivated is random in most cells, and the inactive state is maintained clonally. Imprinting, by contrast, silences specific genes based on parental origin, and the choice is deterministic, not random. While both involve epigenetic silencing, they operate through different mechanisms and serve different purposes.

Imprinting is not permanent. Imprints are erased in the germline and re-established according to the sex of the parent. This means that an imprinted gene that is silenced in a somatic cell can be reactivated in the germline and then silenced on the opposite allele in the next generation. The imprinting pattern is reset every generation.

Not all genes are imprinted. Only a small fraction of the genome—roughly 1% of genes in humans—is imprinted. The vast majority of genes are expressed from both alleles. Imprinting is the exception, not the rule.

Imprinting is not a mutation. Imprinting is an epigenetic modification that does not alter the DNA sequence. A gene can be silenced by imprinting yet have a perfectly normal sequence. This is why imprinting disorders can occur even when the DNA sequence is intact.

Imprinting is not always tissue-specific in the same way. Some imprinted genes are imprinted in all tissues, while others are imprinted only in specific tissues or at specific developmental stages. UBE3A, for example, is biallelically expressed in most tissues but imprinted in neurons. This tissue specificity complicates both diagnosis and research.

Loss of imprinting is not the same as loss of methylation. While loss of methylation at an ICR often leads to loss of imprinting, the relationship is not always direct. Some imprinted genes are regulated by histone modifications or chromatin architecture independently of DNA methylation. Conversely, methylation at an ICR can be present but not functional if the chromatin context is disrupted.

Summary and Key Takeaways

Genetic imprinting is a fundamental epigenetic mechanism that breaks Mendelian rules by silencing one parental allele. It is established through DNA methylation at ICRs, reinforced by histone modifications, and maintained by proteins such as ZFP57 and DNMT1. The imprinting cycle—erasure in germ cells, establishment during gametogenesis, maintenance after fertilization—ensures that imprints reflect the sex of the transmitting parent.

Imprinted genes regulate growth, development, and behavior, and their disruption causes well-characterized human syndromes. The parental conflict hypothesis provides a compelling evolutionary explanation, and modern genomic techniques have enabled the identification and study of imprinted genes at scale.

Frequently Asked Questions

What is an example of genetic imprinting?

The IGF2/H19 cluster on chromosome 11p15.5 is the classic example. IGF2 is expressed only from the paternal allele, while H19 is expressed only from the maternal allele. The reciprocal expression is controlled by a single ICR that is methylated on the paternal allele and unmethylated on the maternal allele.

How does genetic imprinting work?

Genetic imprinting works through epigenetic modifications, primarily DNA methylation at ICRs. The methylation status of an ICR determines whether insulator proteins like CTCF can bind, which in turn controls whether enhancers can access promoters. Methylated ICRs also recruit repressive histone modifications that silence the allele.

How does genetic imprinting occur?

Imprinting occurs through a cycle of erasure and re-establishment. Imprints are erased in primordial germ cells, then re-established during gametogenesis in a sex-specific manner. After fertilization, imprints are maintained in somatic cells through the action of DNMT1 and ZFP57, which protect ICRs from the global demethylation that occurs in the early embryo.

Why does genetic imprinting occur?

The most widely accepted explanation is the parental conflict hypothesis, which proposes that imprinting reflects a conflict between maternal and paternal genomes over resource allocation. Paternally expressed genes promote growth, while maternally expressed genes restrain it. Other hypotheses include the prevention of parthenogenesis and sex-specific selection.

What happens in genetic imprinting?

In genetic imprinting, one parental allele of a gene is transcriptionally silenced while the other remains active. The silenced allele carries DNA methylation and repressive histone marks at its ICR, while the active allele is unmethylated and associated with open chromatin. The result is monoallelic, parent-of-origin-specific expression.

Is genetic imprinting permanent?

No. Imprints are erased in the germline and re-established during gametogenesis. This resetting ensures that the imprinting pattern in the offspring reflects the sex of the parent transmitting the allele. In somatic cells, however, imprints are stably maintained through cell division.

Can genetic imprinting be inherited?

Imprinting patterns are inherited in the sense that they are transmitted from parent to offspring through the gametes. However, the imprint is reset each generation. What is inherited is not a specific methylation pattern but the capacity to establish the appropriate imprint based on the sex of the parent.

Key Takeaways

Genetic imprinting is an epigenetic phenomenon where one parental allele is silenced, resulting in monoallelic, parent-of-origin-specific expression.
The molecular basis of imprinting is differential DNA methylation at imprinting control regions, reinforced by histone modifications and chromatin architecture.
Imprints are erased in primordial germ cells, re-established during gametogenesis, and maintained after fertilization through the action of DNMT1 and ZFP57.
Classic examples include IGF2/H19, UBE3A, and SNRPN, with disruptions causing Beckwith-Wiedemann, Angelman, and Prader-Willi syndromes.
The parental conflict hypothesis explains imprinting as a consequence of evolutionary conflict between maternal and paternal genomes over resource allocation.
Imprinting is not permanent, not the same as X-inactivation, and affects only a small fraction of genes.
Assisted reproductive technologies are associated with a small but significant increase in the risk of imprinting disorders.