1. Introduction

In most genes, an individual normally possesses two copies or alleles:

• One inherited from the mother
• One inherited from the father

Usually, both copies can contribute to gene expression.

However, a special group of genes behaves differently. For these genes, expression depends on the parental origin of the allele.

This phenomenon is called genomic imprinting or parent-of-origin-specific gene expression.

For an imprinted gene:

Maternal allele may be active + paternal allele may be inactive

or

Paternal allele may be active + maternal allele may be inactive

Therefore, the organism can effectively use only one parental copy of a particular imprinted gene.

Genomic imprinting is an important example of epigenetic regulation because it changes gene activity without changing the DNA sequence itself.

2. Definition of Genomic Imprinting

Genomic imprinting is an epigenetic phenomenon in which the expression of certain genes depends on whether the gene was inherited from the mother or the father.

The important point is:

The DNA sequence of the maternal and paternal copies may be essentially the same, but their epigenetic marks can cause them to have different expression patterns.

3. Basic Concept

Consider an imprinted gene called Gene X.

Maternal expression

Maternal allele → Active

Paternal allele → Silenced

Therefore:

Gene X expression → Mainly from maternal allele

Paternal expression

Maternal allele → Silenced

Paternal allele → Active

Therefore:

Gene X expression → Mainly from paternal allele

This creates parent-of-origin-specific expression.

4. Why Is Imprinting Called Epigenetic?

Epigenetics refers to heritable or relatively stable changes in gene activity that occur without changing the underlying DNA sequence.

Major mechanisms include:

• DNA methylation
• Histone modifications
• Chromatin remodeling
• Regulatory non-coding RNAs

Genomic imprinting uses these mechanisms to establish different expression states on maternal and paternal alleles.

Therefore:

Same DNA sequence

↓

Different epigenetic marks

↓

Different gene activity

↓

Different developmental effects

5. Imprinted Genes Are Usually Monoallelically Expressed

Many imprinted genes show monoallelic expression.

This means that mainly one allele is expressed.

For example:

Maternal allele	Paternal allele	Expression
Active	Inactive	Maternal
Inactive	Active	Paternal

This is different from a typical non-imprinted gene, where both alleles may be expressed.

6. Parent-of-Origin Effect

The most important characteristic of genomic imprinting is the parent-of-origin effect.

The biological effect of an allele can depend on whether it came from:

• Mother
• Father

For example, a mutation in an imprinted gene may produce a disease only when inherited from one particular parent.

Thus:

Same gene + different parental origin → potentially different phenotype

This is why inheritance patterns of imprinting disorders can be unusual.

7. How Imprinting Is Established

Imprints are established during gametogenesis, when sperm and eggs are being produced.

Male and female germ cells establish different epigenetic marks.

General process

Primordial germ cells

↓

Erasure of previous imprinting marks

↓

Development of germ cells

↓

Establishment of sex-specific imprinting marks

↓

Mature sperm or egg

↓

Fertilization

↓

Embryo inherits maternal and paternal imprints

The specific imprint depends on whether the allele passed through the maternal or paternal germline.

8. Erasure and Re-establishment of Imprints

An important feature of imprinting is that existing parental imprints are erased in developing germ cells and then new imprints are established according to the sex of the individual producing the gamete.

For example:

A person inherited an allele from their mother.

When that person’s germ cells develop, the previous parental imprint is erased and replaced with an imprint appropriate for that person’s germline.

Thus:

Old imprint → Erased

↓

New germline-specific imprint established

This is essential for imprinting to function correctly across generations.

9. DNA Methylation

DNA methylation is one of the major mechanisms involved in genomic imprinting.

A methyl group is added to cytosine residues, commonly at CpG sites.

This can alter the accessibility and activity of nearby regulatory regions.

Simplified mechanism

DNA methylation

↓

Changes chromatin/regulatory environment

↓

Transcription reduced or prevented

↓

Gene silencing

Therefore, methylation can contribute to the inactive state of an imprinted allele.

However, imprinting is not simply “methylation everywhere”; it involves specific regulatory regions and additional chromatin mechanisms.

10. Imprinting Control Regions

Many imprinted gene clusters contain specialized regulatory regions called imprinting control regions (ICRs).

These regions carry parent-specific epigenetic states and help control expression of nearby genes.

General mechanism

Maternal ICR state

↓

Specific gene expression pattern

or

Paternal ICR state

↓

Different gene expression pattern

Thus, an ICR can coordinate the expression of multiple genes within an imprinted genomic region.

11. Differentially Methylated Regions

Differentially methylated regions (DMRs) are genomic regions that show different DNA methylation patterns depending on parental origin.

For example:

Maternal chromosome → methylated at a particular DMR

Paternal chromosome → unmethylated at that DMR

This difference can determine which genes are expressed.

12. Histone Modifications

Histones are proteins around which DNA is organized.

Chemical modifications of histones can influence chromatin structure and gene expression.

Important modifications can include:

• Histone acetylation
• Histone methylation
• Other histone modifications

Some combinations promote transcription, while others promote transcriptional repression.

Therefore:

DNA methylation + histone modifications + chromatin organization

can work together to maintain an imprinted state.

13. Chromatin Structure

Gene expression depends partly on whether DNA is accessible to transcription machinery.

Open chromatin

DNA is relatively accessible.

↓

Gene expression can occur.

Condensed/repressive chromatin

DNA is less accessible.

↓

Gene expression is reduced or silenced.

Imprinting can involve establishment of different chromatin states on maternal and paternal chromosomes.

14. Non-Coding RNAs

Some imprinted genomic regions produce non-coding RNAs that participate in regulation of nearby genes.

These RNAs may influence:

• Chromatin organization
• Transcription
• RNA stability
• Gene silencing

Therefore, imprinting can involve multiple layers of epigenetic regulation.

15. Imprinting and Fertilization

During fertilization, the embryo receives:

Maternal genome + paternal genome

The two genomes carry different imprinting patterns.

After fertilization, many epigenetic marks undergo extensive remodeling, but important imprinting marks are protected or appropriately maintained so that parental-origin information is retained.

Simplified sequence

Egg

Sperm

↓

Zygote

↓

Maintenance of critical parental imprints

↓

Parent-specific gene expression

↓

Normal development

16. Imprinting and Embryonic Development

Genomic imprinting is particularly important during development because imprinted genes regulate processes such as:

• Embryonic growth
• Placental development
• Nutrient allocation
• Cell proliferation
• Differentiation
• Tissue development
• Metabolism

Incorrect imprinting can therefore cause developmental abnormalities.

17. Imprinting and Placental Development

The placenta is an important tissue in which many imprinted genes function.

Imprinted genes can regulate:

• Placental growth
• Nutrient transfer
• Fetal growth
• Maternal-fetal interactions

This has contributed to the idea that parental genomes can have different developmental interests regarding fetal growth and resource allocation.

18. Genomic Imprinting and the Conflict Theory

A well-known evolutionary explanation is the parental conflict hypothesis or kinship theory of genomic imprinting.

According to this evolutionary framework, maternally and paternally expressed genes may sometimes have different effects on resource allocation between mother and offspring.

In simplified terms:

• Some paternal alleles may favor greater fetal growth or resource acquisition.
• Some maternal alleles may favor regulation of fetal growth and resource use.

This is a model used to explain the evolution of imprinting, although the biological functions of individual imprinted genes are more complex.

19. Important Examples of Imprinted Genes

Several well-studied genes demonstrate genomic imprinting.

IGF2

IGF2 (Insulin-like Growth Factor 2) is a major example.

In many human tissues, the paternal allele is preferentially expressed while the maternal allele is generally silenced.

IGF2 promotes growth-related processes.

H19

H19 is a long non-coding RNA gene located in the same genomic region as IGF2.

Its expression pattern is complementary to IGF2 in the classic imprinting model.

The IGF2/H19 region is therefore an important example of coordinated imprinting control.

20. IGF2/H19 Imprinting Mechanism

The IGF2/H19 region demonstrates how an imprinting control region can regulate neighboring genes.

A simplified model:

Paternal chromosome

ICR methylated

↓

Insulator activity altered

↓

IGF2 expression promoted

↓

H19 expression suppressed

Maternal chromosome

ICR unmethylated

↓

Insulator function maintained

↓

IGF2 restricted

↓

H19 expressed

This is a simplified representation of a more detailed regulatory system involving CTCF and chromatin organization.

21. Imprinting and Growth Regulation

Imprinted genes often have important roles in controlling growth.

For example:

IGF2 → growth-promoting activity

Therefore, inappropriate activation or loss of imprinting can disturb normal growth regulation.

This demonstrates why imprinting is important during embryonic development.

22. Prader-Willi Syndrome

Prader-Willi syndrome (PWS) is a well-known disorder involving abnormal expression of imprinted genes in the chromosome 15q11-q13 region.

In the typical molecular mechanism, there is loss of expression of paternally expressed genes in this region.

Possible mechanisms include:

• Deletion of the paternal region
• Maternal uniparental disomy
• Imprinting defects

Common features can include:

• Hypotonia during infancy
• Feeding difficulties in early life followed by characteristic hyperphagia
• Developmental and learning difficulties
• Short stature or growth abnormalities
• Hypogonadism

The exact clinical presentation varies.

23. Angelman Syndrome

Angelman syndrome also involves the chromosome 15q11-q13 imprinted region but results from loss of function of the maternally expressed UBE3A in relevant neurons.

Mechanisms can include:

• Maternal deletion
• Paternal uniparental disomy
• Imprinting defects
• Pathogenic variants affecting UBE3A

Common features include:

• Developmental delay
• Severe speech impairment
• Movement or balance problems
• Characteristic behavioral features
• Seizures in many affected individuals

Thus, Prader-Willi syndrome and Angelman syndrome demonstrate how parental origin matters.

24. Prader-Willi vs Angelman Syndrome

Feature	Prader-Willi syndrome	Angelman syndrome
Region	15q11-q13	15q11-q13
Typical affected parental contribution	Loss of paternal gene expression	Loss of maternal UBE3A expression in neurons
Important mechanism	Imprinting defect/deletion/UPD	Imprinting defect/deletion/UPD/UBE3A alteration
Major developmental effects	Growth, feeding and endocrine abnormalities	Neurological and developmental abnormalities

The important concept is:

Same chromosomal region + different parental origin → different disorder

25. Uniparental Disomy

Uniparental disomy (UPD) occurs when both copies of a chromosome or chromosomal region are inherited from one parent and none from the other parent.

Normal

Mother → one copy

Father → one copy

Uniparental disomy

Mother → two copies

Father → zero copies

or the reverse.

UPD can cause disease when the affected region contains imprinted genes because normal parent-specific expression is disrupted.

26. Loss of Imprinting

Loss of imprinting (LOI) occurs when an allele that should normally be silenced becomes active or when normal parent-specific regulation is otherwise disrupted.

For example:

Normally

Maternal allele → ON

Paternal allele → OFF

Loss of imprinting

Maternal allele → ON

Paternal allele → ON

This can produce excessive gene dosage.

Loss of imprinting has also been studied in cancer biology.

27. Imprinting and Cancer

Abnormal imprinting can contribute to cancer through altered expression of growth-regulating genes.

Mechanisms can include:

• Loss of imprinting
• Abnormal DNA methylation
• Epigenetic silencing
• Abnormal activation of growth-promoting genes

For example, abnormal regulation of the IGF2/H19 region has been observed in several cancers.

Therefore:

Normal imprinting → controlled gene expression

Abnormal imprinting → altered growth regulation → possible disease

28. Imprinting and Dosage

Because only one parental allele may be active, imprinted genes are particularly sensitive to changes in gene dosage.

Consider:

Maternal allele = active

Paternal allele = inactive

If the active maternal allele is deleted:

No normal expression

Even though another copy exists, it may be epigenetically silenced.

This explains why deletions involving imprinted regions can produce unusual inheritance patterns.

29. Imprinting and Developmental Timing

Imprinting is especially important during:

• Gametogenesis
• Fertilization
• Early embryonic development
• Placental development
• Growth
• Organ development

The timing of establishment, maintenance and erasure of epigenetic marks is highly regulated.

30. Imprinting and Germ Cells

Germ cells are particularly important because imprinting information must be reset between generations.

General cycle

Parental imprint

↓

Transmission to offspring

↓

Imprint maintained during development

↓

Erasure in developing germ cells

↓

New sex-specific imprint established

↓

Transmission to next generation

This allows the same genomic region to carry different imprinting states depending on whether it passes through sperm or eggs.

31. Imprinting and Epigenetic Reprogramming

Early embryonic development involves extensive epigenetic reprogramming.

Many epigenetic marks are erased or remodeled.

However, imprinting marks must be appropriately protected or re-established to preserve parent-of-origin information.

Therefore:

Global epigenetic reprogramming

Maintenance of essential imprints

↓

Normal embryonic development

32. Imprinting and Stem Cells

Imprinting is important in stem-cell biology because abnormal imprinting can affect:

• Growth
• Differentiation
• Developmental potential
• Placental-like behavior
• Gene dosage

When stem cells are cultured or experimentally reprogrammed, imprinting patterns may sometimes become altered.

Therefore, imprinting status is an important consideration in developmental and regenerative biology.

33. Imprinting and Assisted Reproductive Technologies

Because imprinting is established and maintained through precise epigenetic mechanisms, disturbances in epigenetic regulation have been investigated in relation to assisted reproductive technologies.

However, imprinting disorders remain rare, and the relationship between assisted reproduction and imprinting is complex.

The key biological principle is that normal development requires accurate establishment and maintenance of parental epigenetic information.

34. Imprinting vs Ordinary Gene Regulation

Feature	Ordinary gene regulation	Genomic imprinting
Alleles	Often both can be expressed	One parental allele may be preferentially expressed
Parent of origin	Usually not decisive	Critical
Main mechanisms	Transcription factors, signaling, chromatin	Epigenetic marks linked to parental origin
Expression	Can vary by tissue and condition	Often parent-specific
Inheritance pattern	Usually conventional	Can show unusual parent-of-origin effects

35. Genomic Imprinting vs Mutation

These are fundamentally different.

Mutation

A change in the DNA sequence.

Example:

A → G

Imprinting

A change in gene activity caused by epigenetic regulation without changing the underlying DNA sequence.

Example:

DNA sequence unchanged

↓

DNA methylation/chromatin changes

↓

Gene silenced

Thus:

Mutation = DNA sequence change

Imprinting = parent-specific epigenetic regulation

36. Imprinting vs X-Chromosome Inactivation

These are also different phenomena.

Genomic imprinting	X-chromosome inactivation
Depends on parental origin	Primarily associated with dosage compensation in XX cells
Can affect autosomal and X-linked genes	Involves an entire X chromosome in typical female somatic cells
Usually affects specific genes/regions	Broadly silences one X chromosome
Maintains parent-specific expression	Balances X-linked gene dosage