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.

Difference Between Epigenetic and Genetic: Key Concepts

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

Introduction to Genetics and Epigenetics

Core Definitions

Genetics is the study of heredity at the level of the DNA sequence itself. The genetic information of an organism is encoded in the linear order of nucleotide bases—adenine, thymine, guanine, and cytosine—along the DNA molecule. This sequence constitutes the blueprint for every protein, every regulatory RNA, and every functional element in the cell. Genetic variation arises when that sequence is altered: a base is substituted, deleted, or inserted, or when larger chromosomal segments are rearranged. These alterations are permanent, irreversible changes to the primary structure of the genome.

Epigenetics, by contrast, refers to heritable changes in gene expression that do not involve any alteration to the underlying DNA sequence. The term, coined by Conrad Waddington in 1942, originally described the processes by which a genotype gives rise to a phenotype during development. Modern usage is more precise: an epigenetic trait is a stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence. The key word is heritable—the change must be passed on to daughter cells during mitosis or, in some cases, to offspring during meiosis.

The distinction is not merely semantic. Two cells in the same organism—a neuron and a hepatocyte, for example—contain identical DNA sequences, yet they express entirely different sets of genes. The difference is epigenetic. The DNA in both cells is wrapped around histone proteins, chemically modified by methyl groups, and organized into higher-order chromatin structures, but the specific pattern of these modifications differs between cell types. That pattern is what makes a neuron a neuron and a hepatocyte a hepatocyte.

Why the Distinction Matters

Understanding the difference between genetic and epigenetic changes is fundamental to grasping how organisms develop, how diseases arise, and how traits are inherited. Genetic mutations are the substrate of evolution—they provide the raw variation upon which natural selection acts. Epigenetic modifications, on the other hand, provide a mechanism for phenotypic plasticity without altering the genome. They allow a single genome to produce multiple cell types, to respond to environmental cues, and to maintain stable gene expression programs over the lifetime of an organism.

The distinction also has profound clinical implications. Genetic mutations in cancer are often irreversible and difficult to target therapeutically; epigenetic alterations, by contrast, are frequently reversible by pharmacological agents that inhibit the enzymes responsible for writing or erasing epigenetic marks. This has led to a new class of cancer drugs, the epigenetic therapies, which are now in routine clinical use.

The Nature of Genetic Changes

Genetic changes are alterations in the nucleotide sequence of DNA. They can be as small as a single base pair or as large as the gain or loss of an entire chromosome. The fundamental property of a genetic change is that it changes the information content of the genome.

Types of Mutations

Point mutations are single-nucleotide substitutions. A transition replaces a purine with a purine (A↔G) or a pyrimidine with a pyrimidine (C↔T); a transversion replaces a purine with a pyrimidine or vice versa. The functional consequence depends on the context. A synonymous mutation changes a codon to another codon that specifies the same amino acid—the protein sequence is unchanged. A missense mutation changes the codon to specify a different amino acid, which can alter protein structure and function. A nonsense mutation introduces a premature stop codon, leading to a truncated protein that is often nonfunctional.

Insertions and deletions (indels) add or remove nucleotides. If the number of nucleotides inserted or deleted is not a multiple of three, the reading frame is shifted, producing a frameshift mutation. Every amino acid downstream of the mutation is changed, and a premature stop codon is frequently encountered. Frameshift mutations are almost always deleterious.

Chromosomal rearrangements include deletions, duplications, inversions, and translocations of large genomic segments. The Philadelphia chromosome, for example, results from a reciprocal translocation between chromosomes 9 and 22, t(9;22)(q34;q11), which fuses the BCR gene on chromosome 22 with the ABL1 gene on chromosome 9. The resulting BCR-ABL fusion protein has constitutive tyrosine kinase activity and drives chronic myeloid leukemia.

Copy number variations (CNVs) are duplications or deletions of DNA segments larger than 1 kilobase. They can alter gene dosage, disrupt coding sequences, or create novel fusion genes.

Inheritance Patterns

Genetic changes are inherited according to Mendelian principles when they occur in the germline. A dominant mutation is expressed when present in a single copy; a recessive mutation requires both alleles to be affected. Mutations in mitochondrial DNA are inherited maternally, since the oocyte contributes the vast majority of mitochondria to the zygote.

Somatic mutations, by contrast, occur in non-germline cells and are not passed to offspring. They accumulate throughout life and are the driving force behind cancer. The distinction between germline and somatic mutations is clinically critical: germline mutations can be detected by genetic testing of blood or saliva, while somatic mutations require analysis of the affected tissue.

The Nature of Epigenetic Changes

Epigenetic changes are chemical modifications to DNA or to the proteins that package DNA. They do not alter the sequence of nucleotides, but they alter how that sequence is read by the transcriptional machinery. The three principal mechanisms are DNA methylation, histone modification, and non-coding RNA-mediated regulation.

DNA Methylation

DNA methylation is the covalent addition of a methyl group to the C5 position of cytosine, producing 5-methylcytosine (5mC). In mammals, this occurs almost exclusively at CpG dinucleotides—cytosine followed by guanine. The reaction is catalyzed by DNA methyltransferases (DNMTs): DNMT3A and DNMT3B establish new methylation patterns during development (de novo methylation), while DNMT1 maintains existing patterns during DNA replication by methylating the newly synthesized daughter strand opposite a methylated CpG on the parental strand.

CpG dinucleotides are unevenly distributed across the genome. They are concentrated in CpG islands—regions of 300–3000 base pairs with a high G+C content and a high frequency of CpG sites. Approximately 60–70% of human gene promoters are associated with CpG islands. In normal cells, promoter-associated CpG islands are almost always unmethylated, regardless of whether the gene is active or silent. Methylation of a promoter CpG island is strongly associated with transcriptional repression and is a hallmark of silenced genes, particularly on the inactive X chromosome and at imprinted loci.

Gene bodies, in contrast, are frequently methylated in actively transcribed genes. This gene-body methylation is evolutionarily conserved and may suppress spurious transcription from cryptic promoters within the gene.

Histone Modifications

Histones are the protein components of chromatin. The core histones—H2A, H2B, H3, and H4—form an octamer around which 147 base pairs of DNA are wrapped to form the nucleosome. Each histone has an N-terminal tail that protrudes from the nucleosome and is subject to a wide array of post-translational modifications, including acetylation, methylation, phosphorylation, ubiquitination, and sumoylation.

Histone acetylation is the addition of an acetyl group to lysine residues, catalyzed by histone acetyltransferases (HATs) and removed by histone deacetylases (HDACs). Acetylation neutralizes the positive charge of lysine, weakening the electrostatic interaction between the histone tail and the negatively charged DNA backbone. This loosens chromatin structure and generally promotes transcription. Histone acetylation is highly dynamic and is associated with active regulatory elements.

Histone methylation occurs on lysine and arginine residues and is catalyzed by histone methyltransferases (HMTs) and removed by histone demethylases (KDMs). Unlike acetylation, methylation does not alter the charge of the histone. Its effect depends on which residue is methylated and to what degree (mono-, di-, or trimethylation). Trimethylation of lysine 4 on histone H3 (H3K4me3) marks active promoters; trimethylation of lysine 36 on histone H3 (H3K36me3) marks the body of actively transcribed genes; trimethylation of lysine 27 on histone H3 (H3K27me3) marks silenced developmental genes; and trimethylation of lysine 9 on histone H3 (H3K9me3) marks constitutive heterochromatin, such as centromeres and telomeres.

Non-coding RNAs

Non-coding RNAs participate in epigenetic regulation through several mechanisms. Long non-coding RNAs (lncRNAs), defined as transcripts longer than 200 nucleotides that do not encode proteins, can recruit chromatin-modifying complexes to specific genomic loci. The classic example is XIST, which is expressed from the future inactive X chromosome and coats that chromosome in cis, recruiting the Polycomb repressive complex PRC2 to deposit H3K27me3 and initiate X-chromosome inactivation.

Small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs) can direct DNA methylation and histone modification to complementary sequences, particularly in plants and in the germline of animals. In the fission yeast Schizosaccharomyces pombe, the RNAi pathway is required for the establishment and maintenance of heterochromatin at centromeres.

Mechanisms of Epigenetic Regulation

Writers, Readers, and Erasers

Epigenetic marks are dynamically regulated by three classes of enzymes. Writers deposit the marks. DNMTs write DNA methylation; HATs write histone acetylation; HMTs write histone methylation. Erasers remove the marks. Ten-eleven translocation (TET) enzymes oxidize 5mC to 5-hydroxymethylcytosine (5hmC) and further oxidation products, ultimately leading to replacement of the modified cytosine with unmodified cytosine through base excision repair. HDACs remove acetyl groups; KDMs remove methyl groups. Readers are proteins that recognize specific epigenetic marks and translate them into functional outcomes. The bromodomain is a reader module that binds acetylated lysine residues; the chromodomain, Tudor domain, and PHD finger are reader modules that bind methylated lysine residues.

The interplay between writers, readers, and erasers creates a dynamic regulatory system. For example, the Polycomb repressive complex PRC2 contains the HMT EZH2, which writes H3K27me3, and the protein EED, which reads H3K27me3. This creates a positive feedback loop: once PRC2 deposits H3K27me3, the mark recruits more PRC2, spreading the repressive mark along the chromatin fiber. Conversely, the Trithorax group proteins, which write H3K4me3, counteract Polycomb silencing and maintain active gene expression.

Chromatin Structure and Accessibility

The packaging of DNA into chromatin is not uniform. At one extreme is heterochromatin—dense, transcriptionally silent, and enriched in H3K9me3 and DNA methylation. At the other extreme is euchromatin—less dense, transcriptionally permissive, and enriched in histone acetylation and H3K4me3. Between these extremes lies a spectrum of intermediate states.

Chromatin accessibility is a key determinant of gene expression. Transcription factors and RNA polymerase II cannot bind to DNA that is tightly wrapped around nucleosomes. Regulatory elements such as enhancers and promoters must be accessible for transcription to occur. The relationship between enhancers and promoters is discussed in detail in the article on the Difference Between Enhancer and Promoter.

ATP-dependent chromatin remodelers, such as the SWI/SNF complex, use the energy of ATP hydrolysis to slide, eject, or restructure nucleosomes, thereby changing DNA accessibility. These remodelers are frequently mutated in cancer, underscoring the importance of chromatin dynamics in normal cellular function.

Heritability and Stability

Mitotic Inheritance

For an epigenetic mark to be biologically meaningful, it must be faithfully propagated through cell division. DNA methylation is the best-understood example of mitotic inheritance. During S phase, DNMT1 is recruited to hemimethylated CpG sites—where the parental strand is methylated but the newly synthesized daughter strand is not—and methylates the daughter strand. This semiconservative mechanism ensures that the methylation pattern is copied to both daughter cells.

Histone modifications are more challenging to inherit, because histones are displaced during replication and new histones must be deposited. The current model holds that parental histones, carrying their modifications, are distributed to both daughter strands and serve as templates for the modification of newly deposited histones. The details of this process remain an active area of research.

Meiotic Inheritance and Transgenerational Epigenetics

Whether epigenetic marks can be inherited across generations—through meiosis—is a contentious question. In mammals, two major waves of epigenetic reprogramming occur during development. The first occurs in the preimplantation embryo, where DNA methylation is largely erased and then re-established. The second occurs in primordial germ cells, where the methylation pattern is erased and reset according to the sex of the embryo. These reprogramming events are thought to erase most epigenetic marks inherited from the parents.

However, some loci escape reprogramming. Imprinted genes retain their parent-of-origin-specific methylation, and certain transposable elements maintain their methylation. There is also evidence for transgenerational epigenetic inheritance at a small number of loci in mammals, although the mechanisms remain poorly understood and the phenomenon is far less robust than in plants, where DNA methylation patterns can be stably inherited across many generations.

Methods to Study Genetic and Epigenetic Differences

Genetic Analysis Tools

The gold standard for detecting genetic variation is DNA sequencing. Sanger sequencing, the first-generation method, can read individual DNA fragments up to approximately 800–1000 base pairs and is still used for targeted analysis of specific genes. Next-generation sequencing (NGS) technologies parallelize millions of sequencing reactions, allowing whole-genome sequencing at a cost that has fallen from billions of dollars per genome in 2001 to under $1,000 today. Whole-exome sequencing, which targets the ~1–2% of the genome that codes for proteins, is a cost-effective alternative for identifying coding mutations.

For known variants, genotyping arrays can interrogate hundreds of thousands to millions of single-nucleotide polymorphisms (SNPs) simultaneously. Polymerase chain reaction (PCR) followed by Sanger sequencing remains a standard approach for validating candidate mutations. The choice between PCR and quantitative PCR depends on the question: PCR amplifies DNA for downstream analysis, while qPCR measures the amount of DNA or RNA in real time. The technical differences are covered in the article on the Difference Between PCR and qPCR.

Epigenetic Profiling Methods

Bisulfite sequencing is the gold standard for detecting DNA methylation. Treatment of DNA with sodium bisulfite deaminates unmethylated cytosines to uracil, while 5-methylcytosine is resistant to deamination. After PCR amplification, uracils are read as thymines, allowing methylated and unmethylated cytosines to be distinguished by sequencing. Whole-genome bisulfite sequencing (WGBS) provides single-base resolution of methylation across the entire genome, but at high cost. Reduced representation bisulfite sequencing (RRBS) enriches for CpG-rich regions, reducing the sequencing burden.

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) identifies the genomic locations of histone modifications or DNA-binding proteins. Cells are cross-linked with formaldehyde, chromatin is sheared by sonication, and an antibody specific to the modification of interest is used to immunoprecipitate the associated DNA fragments. The DNA is then purified and sequenced. ChIP-seq for H3K27ac, for example, identifies active enhancers and promoters; ChIP-seq for H3K27me3 identifies Polycomb-repressed regions.

Assay for transposase-accessible chromatin using sequencing (ATAC-seq) maps chromatin accessibility genome-wide. The hyperactive Tn5 transposase preferentially inserts sequencing adapters into accessible chromatin. After sequencing, regions of high read density indicate open chromatin. ATAC-seq requires only 50,000–100,000 cells and can be completed in a single day, making it the method of choice for profiling regulatory landscapes.

Examples and Clinical Implications

Cancer Epigenetics

Cancer is driven by both genetic and epigenetic alterations. The genetic component includes mutations in oncogenes such as KRAS and EGFR, and in tumor suppressors such as TP53 and RB1. The epigenetic component includes global DNA hypomethylation, which can activate oncogenes and promote genomic instability, and focal hypermethylation of CpG islands at tumor suppressor promoters, which silences their expression.

The CDKN2A locus, which encodes the tumor suppressors p16^INK4a and p14^ARF, is frequently silenced by promoter methylation in many cancer types. The MLH1 gene, which is essential for DNA mismatch repair, is silenced by promoter methylation in a subset of colorectal cancers, producing the CpG island methylator phenotype (CIMP). These epigenetic changes are not mutations—the DNA sequence of the silenced gene is normal—but they have the same functional consequence: loss of tumor suppressor activity.

The distinction between genetic and epigenetic alterations in cancer has therapeutic implications. Mutations are difficult to reverse, but epigenetic silencing can be reversed by drugs. The nucleoside analog 5-azacitidine and its deoxy derivative decitabine are incorporated into DNA during replication, where they covalently trap DNMT1, leading to its degradation and passive demethylation of the genome. These agents are approved for the treatment of myelodysplastic syndromes and acute myeloid leukemia. Histone deacetylase inhibitors such as vorinostat and romidepsin are approved for cutaneous T-cell lymphoma.

Genomic Imprinting

Genomic imprinting is an epigenetic phenomenon in which a subset of genes is expressed from only one parental allele. The choice of which allele is silenced is determined by parent-of-origin-specific DNA methylation marks established in the germline. Approximately 100–200 imprinted genes have been identified in humans, many of which regulate growth and development.

The IGF2/H19 locus is a classic example. IGF2 encodes insulin-like growth factor 2, a fetal growth factor, and is expressed only from the paternal allele. H19 encodes a long non-coding RNA and is expressed only from the maternal allele. The imprinting is controlled by an imprinting control region (ICR) located between the two genes. On the maternal chromosome, the ICR is unmethylated and binds the insulator protein CTCF, which blocks access of the IGF2 promoter to a downstream enhancer. On the paternal chromosome, the ICR is methylated, CTCF cannot bind, and the enhancer activates IGF2 expression.

Loss of imprinting at this locus causes Beckwith-Wiedemann syndrome, characterized by overgrowth and predisposition to Wilms tumor. The underlying defect is epigenetic—abnormal methylation of the ICR—rather than genetic.

Epigenetic Therapies

Beyond cancer, epigenetic drugs are being explored for a range of conditions. In sickle cell disease and beta-thalassemia, drugs that increase expression of fetal hemoglobin (HbF) are of clinical interest. Hydroxyurea, which is used clinically, may act in part through epigenetic mechanisms. The DNMT inhibitor decitabine has been shown to reactivate HbF expression by demethylating the promoter of the HBG gene, which encodes the gamma-globin subunit of fetal hemoglobin.

In psychiatric and neurological disorders, HDAC inhibitors have shown promise in animal models of depression, addiction, and neurodegenerative disease, although clinical translation has been slow. The rationale is that these disorders involve stable changes in gene expression that are maintained by epigenetic marks, and that resetting these marks might restore normal function.

Common Pitfalls and Misconceptions

Epigenetics is Not Lamarckian

A common error is to equate epigenetics with Lamarckian inheritance—the idea that traits acquired during an organism's lifetime can be passed to offspring. While some epigenetic marks can be inherited across generations, the phenomenon is limited, context-dependent, and mechanistically distinct from Lamarck's original proposal. Most epigenetic marks are reset during gametogenesis and early embryonic development. The vast majority of what an organism experiences during its lifetime—diet, stress, exercise—does not produce heritable epigenetic changes in its offspring.

Not All Epigenetic Marks Are Permanent

Students often assume that epigenetic modifications are stable for the life of the organism. In reality, many epigenetic marks are highly dynamic. Histone acetylation turns over on a timescale of minutes to hours. DNA methylation at regulatory regions can be rapidly altered in response to signaling pathways. The enzymes that write and erase these marks are themselves regulated by cellular signals. Epigenetic marks are best understood as a dynamic regulatory layer, not a static imprint.

Distinguishing Correlation from Causation

A third pitfall is the tendency to infer causation from correlation in epigenetic studies. When a differentially methylated region is found in a disease state, it is tempting to conclude that the methylation change causes the disease. But the methylation change could be a consequence of the disease, a bystander effect, or a marker of cell-type composition differences between cases and controls. Establishing causality requires functional experiments—for example, targeted demethylation or methylation of a specific locus and measurement of the phenotypic consequence.

Epigenetics and Mutations Are Not Mutually Exclusive

It is also important to recognize that epigenetic and genetic changes interact. Mutations in epigenetic regulators—such as DNMT3A, TET2, EZH2, and IDH1/2—are among the most common mutations in hematologic malignancies. These mutations alter the epigenetic landscape, leading to aberrant gene expression. Conversely, epigenetic silencing can mask the effects of mutations, and epigenetic changes can influence the mutation rate by altering DNA repair and replication fidelity. The relationship between the two is discussed further in the article on the Difference Between Epigenetics and Mutation.

Frequently Asked Questions

What is the main difference between genetics and epigenetics?

Genetics concerns the DNA sequence itself—the order of nucleotides that encodes genes. Epigenetics concerns chemical modifications to the DNA and its associated proteins that alter gene expression without changing the sequence. Genetic changes are permanent alterations to the genome; epigenetic changes are potentially reversible modifications that regulate how the genome is read.

Can epigenetic changes be inherited?

Yes, but with important caveats. During mitosis, DNA methylation patterns are faithfully copied to daughter cells by DNMT1, so epigenetic marks are inherited through somatic cell division. Across generations, most epigenetic marks are erased and reset during gametogenesis and early embryogenesis, but a small number of loci—particularly imprinted genes—retain parent-of-origin-specific marks. Transgenerational epigenetic inheritance beyond the immediate offspring remains controversial in mammals.

Are epigenetic changes permanent?

No. Unlike genetic mutations, which are permanent alterations to the DNA sequence, epigenetic modifications are reversible. HDAC inhibitors can remove acetyl groups; TET enzymes can oxidize 5-methylcytosine, leading to demethylation; and chromatin remodelers can change nucleosome positioning. This reversibility is the basis for epigenetic therapies.

How do epigenetic changes affect gene expression?

Epigenetic modifications affect gene expression by altering chromatin structure and the accessibility of DNA to the transcriptional machinery. DNA methylation at promoters generally represses transcription by recruiting methyl-binding proteins that block activator binding and promote heterochromatin formation. Histone acetylation loosens chromatin and promotes transcription. Histone methylation can either activate or repress transcription depending on the specific residue modified. Non-coding RNAs can recruit chromatin-modifying complexes to specific genomic loci.

What techniques are used to study epigenetics?

Bisulfite sequencing detects DNA methylation at single-base resolution. ChIP-seq identifies the genomic locations of histone modifications and DNA-binding proteins. ATAC-seq maps chromatin accessibility. Hi-C maps three-dimensional chromatin interactions. Each technique provides a different view of the epigenetic landscape, and integrating multiple approaches is often necessary for a complete picture.

Do epigenetic changes cause mutations?

Epigenetic changes do not directly alter the DNA sequence, but they can influence the rate of mutation. DNA methylation, for example, increases the rate of C-to-T transitions, because 5-methylcytosine can undergo spontaneous deamination to thymine. Epigenetic silencing of DNA repair genes can also increase mutation rates by impairing the cell's ability to correct DNA damage. Conversely, mutations in epigenetic regulators can cause widespread epigenetic changes.

Can lifestyle factors change your epigenetics?

Yes. Diet, exercise, smoking, alcohol consumption, and chronic stress have all been associated with changes in DNA methylation and histone modification. For example, smoking is associated with decreased DNA methylation at the AHRR gene, and this change can persist for years after smoking cessation. However, the functional consequences of most lifestyle-associated epigenetic changes are not well understood, and causality is difficult to establish.

Key Takeaways

Genetics is the study of DNA sequence variation; epigenetics is the study of heritable changes in gene expression that do not involve sequence changes.
Genetic changes include point mutations, insertions, deletions, and chromosomal rearrangements; they are permanent and are the substrate for evolution.
Epigenetic changes include DNA methylation, histone modifications, and non-coding RNA-mediated regulation; they are reversible and provide phenotypic plasticity.
DNA methylation at CpG islands in promoters is associated with transcriptional repression; DNMT1 maintains methylation patterns during replication.
Histone acetylation promotes open chromatin and transcription; histone methylation can activate or repress transcription depending on the residue modified.
Epigenetic marks are written by enzymes such as DNMTs, HATs, and HMTs; read by proteins such as bromodomain and chromodomain proteins; and erased by TET enzymes, HDACs, and KDMs.
Cancer is driven by both genetic mutations and epigenetic alterations; epigenetic changes are pharmacologically reversible, making them attractive therapeutic targets.
Common student errors include assuming all epigenetic marks are permanent, equating epigenetics with Lamarckian inheritance, and inferring causation from correlation in epigenetic association studies.