Genetics Terms Starting With E
Genetics Glossary: E
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Electrophoresis
/ eh-lek-troh-foh-REE-sis / · Greek: elektron (amber) + phoresis (carrying)
Electrophoresis is a laboratory technique that separates molecules such as DNA, RNA, or proteins by size and charge as they migrate through a gel matrix under an electric field.
In agarose gel electrophoresis, DNA fragments migrate toward the positive electrode at rates inversely proportional to their size; a 500 base pair fragment travels roughly twice as fast as a 1,000 base pair fragment. Visualization uses intercalating dyes such as ethidium bromide, which fluoresces orange when bound to double-stranded DNA under ultraviolet light, or SYBR Green, which fluoresces approximately 1,000-fold brighter than ethidium bromide with lower toxicity. Capillary electrophoresis achieves higher resolution than slab gels and automates detection using laser-induced fluorescence, making it the standard method for DNA fingerprinting in forensic laboratories and paternity testing.
Gel electrophoresis was first used to separate hemoglobin variants in the 1950s by Linus Pauling and colleagues, who identified sickle-cell hemoglobin as a distinct molecular form, marking one of the earliest demonstrations of a molecular basis for genetic disease.
Electrophoresis does not identify the sequence of DNA fragments. It separates them by size only; sequencing requires additional analytical steps.
In forensic DNA analysis, electrophoresis of the 20 CODIS core short tandem repeat markers from a crime scene sample generates a profile whose allele frequencies combine to produce a random match probability of less than one in a quadrillion for unrelated individuals. Each marker migrates to a position determined by the number of repeat units it contains, producing a pattern unique to each person.
Epigenetics
/ ep-ee-jeh-NET-iks / · Greek: epi (upon, above) + genesis (origin)
Epigenetics is the study of heritable changes in gene expression that do not involve alterations to the underlying DNA sequence, mediated by chemical modifications to DNA and histones.
Key epigenetic mechanisms include DNA methylation at cytosine residues, post-translational modification of histone proteins, and regulation by non-coding RNAs. Epigenetic marks can be reset between generations, but some transmit from parent to offspring in a process called transgenerational epigenetic inheritance. Environmental exposures, diet, stress, and aging all influence the epigenome, linking an organism’s environmental history to its gene expression patterns.
In mammals, DNA methylation at CpG dinucleotides is strongly associated with gene silencing and is one of the best-characterized epigenetic marks.
Identical twins have identical DNA sequences but develop increasingly different epigenomes as they age, explaining why they can differ in disease susceptibility and physical characteristics despite sharing the same genome.
Epigenetics does not involve changes to the DNA sequence itself. Epigenetic modifications regulate which genes are expressed, not what the genes encode.
In honeybee (Apis mellifera) colonies, queen bees and worker bees develop from genetically identical larvae, yet they differ dramatically in body size, reproductive capacity, and lifespan. Differential methylation of developmental genes, triggered by royal jelly feeding of queen-destined larvae, drives these differences without altering a single nucleotide in the DNA sequence.
Epigenomics
/ ep-ih-jeh-NOH-miks / · Greek epi, upon; Greek genea, birth; Greek nomos, law
Epigenomics is the genome-wide study of all epigenetic modifications including DNA methylation, histone modifications, and chromatin accessibility that regulate gene expression without altering the DNA sequence itself.
Unlike genomics, which catalogs fixed DNA sequences, epigenomics maps dynamic chemical marks across the entire genome that change in response to development, environment, aging, and disease. Technologies including whole-genome bisulfite sequencing, ChIP-seq, ATAC-seq, and Hi-C allow researchers to profile the complete epigenome of a cell type at single-base resolution. The NIH Roadmap Epigenomics project generated reference epigenomes for over 100 human cell and tissue types, revealing how the same genome produces more than 200 distinct cell identities.
These reference maps have become indispensable for identifying regulatory regions disrupted in cancer and other diseases.
A single human body carries essentially one genome in nearly every cell, yet epigenomic profiling reveals that a liver cell and a neuron use strikingly different subsets of that genome, with tens of thousands of regulatory regions switching between active and silent states depending on cell type.
Building Blocks of Nucleic Acids →Epigenomic marks are not always permanent. Many epigenetic modifications are dynamic and reversible, which is why epigenetic drug therapies such as DNMT inhibitors can alter gene-expression states in cancer cells.
Arabidopsis thaliana, a small flowering plant widely used in plant biology research, carries heritable DNA methylation patterns that differ between ecotypes adapted to different climates. Genome-wide bisulfite sequencing of these ecotypes has identified thousands of differentially methylated regions spanning roughly 5 to 10 percent of the genome, linking epigenomic variation to differences in stress tolerance and flowering time.
Epistasis
/ eh-PIS-tuh-sis / · Greek: epistasis (a stopping)
Epistasis is the interaction between two or more genes in which the alleles at one locus mask or modify the phenotypic expression of alleles at a different locus.
Epistasis differs from dominance, which describes the relationship between alleles at the same gene; epistasis describes interactions between alleles at different genes. When one gene masks the expression of another, the masking gene is called epistatic and the masked gene is called hypostatic. Classic epistatic ratios in the F2 generation, such as 9:3:4 or 12:3:1, deviate from the standard 9:3:3:1 dihybrid ratio and were among the first clues that genes do not always act independently.
Coat color genetics in Labrador retrievers is a well-documented example, where homozygosity at the E locus suppresses pigment deposition regardless of genotype at the B locus.
Most human traits are influenced by epistatic interactions among dozens or hundreds of genes, explaining why simple Mendelian ratios rarely apply to complex phenotypes like height or cardiovascular disease risk.
Epistasis is not the same as any interaction between two genes. The term has a specific technical meaning: one gene masking the expression of another, not simply two genes influencing the same trait additively.
In Labrador retrievers, a dog homozygous recessive at the E locus produces a yellow coat regardless of its genotype at the B locus, which otherwise determines black versus brown pigmentation. Breeders have confirmed this pattern across thousands of litters, and molecular analysis shows that the E locus controls whether melanocytes deposit any pigment at all.
Autosomal Recessive Inheritance →Euploidy
/ YOO-ploy-dee / · Greek: eu (well, true) + ploos (fold) + eidos (form)
Euploidy is the condition in which a cell or organism contains a chromosome number that is an exact multiple of the haploid chromosome set for that species.
Euploid cells may be haploid, diploid, triploid, tetraploid, or higher polyploids, but each condition involves a complete set of chromosomes with no extra or missing individual chromosomes. Euploidy contrasts with aneuploidy, in which individual chromosomes are gained or lost rather than whole sets. Many crop plants are euploid polyploids, including bread wheat, which is hexaploid with 42 chromosomes representing six complete sets, and cultivated strawberry, which is octoploid with 56 chromosomes.
Polyploidy in plants often arises from failures of cell division during meiosis or mitosis that produce unreduced gametes.
Polyploid euploidy has driven much of plant evolution and crop domestication. Modern bread wheat (Triticum aestivum) is a hexaploid euploid containing six complete sets of chromosomes derived from three ancestral grass species that hybridized over the past few hundred thousand years.
Euploidy is not synonymous with diploidy. Diploidy is one specific type of euploidy, and organisms can be haploid, triploid, or tetraploid euploids as well.
Seedless watermelons are triploid euploids created by crossing tetraploid and diploid plants, producing a plant with three full chromosome sets. The triploid condition, with 33 total chromosomes, prevents normal pairing during meiosis and makes balanced gamete formation nearly impossible, which is why the fruit develops without seeds.
Stages of Meiosis →Exon
/ EK-son / · Greek: exo (outside) + -on (unit)
Exon is a nucleotide sequence in a gene that is retained in the mature messenger RNA after splicing and contributes to the protein-coding or functional sequence of the transcript.
Exons are interrupted by non-coding intron sequences in eukaryotic genes, and the pre-mRNA must be spliced to remove introns and join exons before translation can occur. Alternative splicing of exons from a single gene can produce multiple distinct protein isoforms, greatly expanding the protein repertoire from a limited number of genes. Mutations in splice sites at exon-intron boundaries are a major cause of human genetic disease, accounting for roughly 10 percent of all disease-causing mutations cataloged in human genetics databases.
The average human protein-coding gene contains about 8 to 9 exons, though genes like titin contain over 360.
Although the human genome contains about 20,000 protein-coding genes, alternative splicing of their exons is estimated to produce over 100,000 distinct protein isoforms, meaning most of the proteome's diversity comes from rearranging existing exon sequences rather than from unique genes.
Exons are not exclusively protein-coding. Some exons encode functional non-coding RNA sequences and regulatory elements that do not direct protein synthesis.
Translation Biology →In the dystrophin gene, which at 2.4 million base pairs is one of the largest human genes, skipping of specific exons using antisense oligonucleotides can restore the reading frame in certain Duchenne muscular dystrophy patients. Clinical trials have shown that exon 51 skipping, which applies to roughly 13 percent of Duchenne patients, can partially restore dystrophin protein production and slow disease progression.