Genotype is read from a gel electrophoresis image by counting the number and position of DNA bands that appear after staining. In the simplest case, a homozygous individual shows one band at a given position, while a heterozygous individual shows two bands at different positions, each representing a different allele. The specific technique you use to prepare the DNA before loading it onto the gel determines exactly what those band patterns mean and how reliably you can call a genotype from them.
The Core Principle Behind Every Method
Gel electrophoresis separates DNA fragments by size. You load your sample into a well at one end of a gel slab, apply an electric field, and smaller fragments migrate faster through the gel matrix than larger ones. After the run, you stain the gel so the DNA becomes visible, and each distinct fragment size shows up as a horizontal band. To turn that separation into genotype information, you need some way to make different alleles produce fragments of different sizes. That is where the variety of methods comes in.
The most direct scenario arises when two alleles already differ in length. If you amplify a region that contains a small insertion or deletion, a heterozygous sample produces two bands on the gel, one for each allele size. A sample homozygous for either allele shows just one band. Researchers working with CRISPR-edited organisms exploit this routinely: wild-type alleles and alleles carrying small insertions or deletions (indels) migrate differently, so heterozygous animals are “immediately identified as they produce two distinct bands, while homozygous wild-type or mutant alleles yield a single band.”1PubMed Central. A simple genotyping method to detect small CRISPR-Cas9 induced indels by agarose gel electrophoresis That two-versus-one-band logic is the backbone of nearly every gel-based genotyping approach, though the way you engineer the size difference varies.
PCR-RFLP for Single-Nucleotide Differences
When two alleles differ by only a single nucleotide, the fragments are the same length, so standard gel electrophoresis cannot tell them apart on its own. One classic workaround is PCR-RFLP, which stands for restriction fragment length polymorphism. You first amplify the region of interest by PCR, then digest the product with a restriction enzyme that recognizes a sequence present in one allele but not the other. If the enzyme cuts, you get two smaller fragments. If it does not cut, you get one intact fragment.
Reading the gel after a restriction digest follows a predictable pattern. An individual homozygous for the allele that contains the enzyme’s recognition site shows two lower bands (the two pieces). An individual homozygous for the allele that lacks the site shows one higher band (uncut). A heterozygote shows all three bands: the uncut fragment from one allele and both cut fragments from the other. You compare these patterns against an expected “in silico digest,” meaning you predict how many bands each genotype should produce based on the known sequence.2PubMed Central. Determining if DNA Stained with a Cyanine Dye Can Be Digested with Restriction Enzymes If you see more bands than expected, the digest may be partial; fewer bands than expected suggests incomplete cutting.
This technique has been used to genotype organisms across a wide range of fields. In parasitology, for instance, researchers have genotyped Trichomonas vaginalis by amplifying the actin gene with nested PCR and then digesting the product with several restriction enzymes, visualizing the resulting fragment patterns on agarose gels.3PubMed. Determination of Trichomonas vaginalis Genotypes Using PCR-Restriction Fragment Length Polymorphism (RFLP) Different genotypes of the parasite produce different banding patterns after digestion because their sequences differ at the enzyme recognition sites.
Tetra-Primer ARMS-PCR
A faster alternative for single-nucleotide changes skips the restriction enzyme step entirely. Tetra-primer ARMS-PCR (amplification-refractory mutation system) uses four primers in a single reaction. Two outer primers amplify a control band that appears regardless of genotype, confirming the PCR worked. Two inner primers are each designed to match only one allele at the variant site. Because each allele-specific primer sits at a different distance from its outer partner, the two allele-specific products differ in size. On the gel, a homozygote for allele A shows the control band plus one allele-specific band. A homozygote for allele B shows the control band plus a different-sized allele-specific band. A heterozygote shows the control band plus both allele-specific bands.
This approach has been validated in both plant and animal genetics. In barley, tetra-primer ARMS-PCR was used to genotype five single-nucleotide polymorphisms across 132 cultivated varieties, with the authors describing the results as unambiguous and reliable for low- to moderate-throughput genotyping.4PubMed. Genotyping single nucleotide polymorphisms in barley by tetra-primer ARMS-PCR In mice, the same method was applied to differentiate wild-type animals from those carrying a mutation in the leptin gene, with the different genotype patterns “clearly visible and distinguishable on 1.5% agarose gel.”5PubMed Central. Tetra-Primer Amplification-Refractory Mutation System (ARMS)-PCR for Genotyping Mouse Leptin Gene Mutation The appeal here is simplicity: one PCR, one gel, and you have your answer. No digestion step, no post-PCR enzyme handling.
Microsatellites and Repeat-Length Polymorphisms
Some genotyping targets are not single-nucleotide changes but differences in the number of short tandem repeats at a locus. These microsatellite markers, also called STRs, vary in repeat count between individuals, making alleles differ by multiples of the repeat unit length (often two to four base pairs per repeat). After PCR amplification, different alleles migrate to different positions on the gel, and a heterozygous individual shows two bands at that locus.
Resolving microsatellite alleles on a gel can be more demanding than resolving larger size differences. Standard agarose gels sometimes lack the resolution to separate alleles that differ by only a few base pairs. Higher-resolution options include polyacrylamide gels or capillary electrophoresis systems. One study comparing detection methods for a human microsatellite found that combining PCR with fluorophore labeling and detection on a capillary sequencer provided the resolution needed to detect small size differences that standard agarose could not resolve.6PubMed Central. Electrophoretic techniques applied to the detection and analysis of the human microsatellite DG10s478 If you are working with microsatellites and your alleles differ by only two or four base pairs, plan to use a gel system with the resolving power to tell them apart.
Choosing the Right Gel and Stain
The gel matrix and concentration matter for how well you can distinguish alleles. Agarose gels at concentrations around 1.5 to 2 percent work well for fragments in the range of a few hundred base pairs, which covers most standard PCR-based genotyping. For very large DNA fragments, as in pulsed-field gel electrophoresis (PFGE), the electric field alternates direction periodically, allowing separation of fragments in the range of tens to hundreds of kilobases.7PubMed Central. Pulse Field Gel Electrophoresis PFGE is more common for bacterial strain typing than for standard allele-level genotyping, but it illustrates how the gel format gets adapted to the size range you care about.
Staining is what makes the bands visible. Ethidium bromide under ultraviolet light has been the default for decades, but it is a known mutagen, which has driven labs toward safer alternatives. SYBR Safe, for example, performed statistically equivalently to ethidium bromide in gene disruption experiments, with success rates that were essentially interchangeable between the two dyes.8PubMed. SYBR safeTM efficiently replaces ethidium bromide in Aspergillus fumigatus gene disruption Newer dyes such as Midori Green Direct, excited by blue or cyan LED light instead of UV, have reached the same detection sensitivity as the traditional ethidium bromide/UV system.9PubMed Central. Development of highly sensitive and low-cost DNA agarose gel electrophoresis detection systems, and evaluation of non-mutagenic and loading dye-type DNA-staining reagents The choice of stain does not change how you read the genotype; it changes whether you need UV protection equipment and how safely you can handle spent gels.
Artifacts That Can Fool You
A clean gel image with crisp, well-separated bands makes genotyping straightforward. Real gels are not always clean. Several common artifacts can lead to a wrong genotype call if you are not watching for them.
- Stutter bands: During PCR amplification of microsatellite loci, the polymerase sometimes slips, producing a minor product one repeat unit shorter than the true allele. These “stutter” bands appear as faint shadows just below the main band. The proportion of stutter product increases as the number of uninterrupted core repeat units grows.10PubMed Central. Sequence analysis and characterization of stutter products at the tetranucleotide repeat locus vWA In a heterozygote where the two alleles differ by only one repeat, a stutter band from the larger allele can overlap with the smaller allele’s true band, making band intensity misleading.
- Partial digestion: In PCR-RFLP, if the restriction enzyme does not cut every molecule in the sample, you see a mix of cut and uncut fragments. A homozygous sample that should show only two small bands will also show a faint uncut band at the top, mimicking the three-band pattern of a heterozygote. Running a known homozygous control alongside your unknowns catches this quickly.
- Allelic dropout: Sometimes one allele fails to amplify during PCR altogether, so a true heterozygote appears to be homozygous. This can happen when a polymorphism sits within a primer binding site, preventing the primer from annealing to that allele. In one documented case, a single nucleotide change within a primer binding site caused allelic dropout in a methylation-specific PCR assay, producing a false-positive result for Prader-Willi syndrome.11PubMed Central. Allelic dropout can cause false-positive results for Prader-Willi and Angelman syndrome testing Broader research has shown that allelic dropout is a general phenomenon wherever differential DNA methylation and certain secondary structures in the template coincide, potentially affecting amplification at multiple imprinted human loci.12PubMed Central. Allelic Dropout During Polymerase Chain Reaction due to G-Quadruplex Structures and DNA Methylation Is Widespread at Imprinted Human Loci
- Nonspecific bands: Primers can bind to unintended regions of the genome, producing extra bands that do not correspond to any real allele. These are usually distinguishable because they appear in every sample at the same size, regardless of genotype, but in a cluttered gel they can be confused with allele-specific bands.
The practical takeaway is that every genotyping gel should include controls: a known homozygous sample for each allele (if available), a known heterozygote, and a no-template negative control. Without these, you are interpreting bands in a vacuum, and the artifacts described above can silently lead you to the wrong genotype call.
Reading Codominant Versus Dominant Markers
Not every gel-based marker system lets you distinguish all three genotypes. Codominant markers, like microsatellites or PCR-RFLP, produce a unique banding pattern for each genotype: homozygote A, homozygote B, and heterozygote AB. You can fully determine the genotype from the gel. Dominant markers, by contrast, produce only a present-or-absent signal. A band is either there or it is not, and you cannot tell whether a band-present individual is homozygous or heterozygous for that allele. RAPD (random amplified polymorphic DNA) markers, which use short arbitrary primers, are a classic example of dominant markers: a band appears if the primer sites are intact, but a heterozygote and a homozygote for the amplified allele look identical.
This distinction matters for the level of genotypic detail you can extract. If you need to tell heterozygotes from homozygotes, you need a codominant system. If you only need to detect the presence or absence of a particular allele (for example, screening for a transgene in a genetically modified line), a dominant marker is simpler and often sufficient.
Denaturing Gels for Finer Discrimination
Standard agarose gels separate DNA by size alone, which means two alleles of identical length but different sequence cannot be distinguished. Denaturing gradient gel electrophoresis (DGGE) solves this by running the gel under conditions where double-stranded DNA gradually unwinds as it migrates through an increasing concentration of chemical denaturant. Because the melting behavior of DNA depends on its sequence, two fragments of the same length but different sequence will denature at different points and stop migrating at different positions. This lets you detect sequence variants that produce no size difference at all.
DGGE has been used clinically for scanning genes for mutations. One example involved scanning all exons of the QDPR gene (linked to a rare metabolic disorder) in a prenatal diagnostic setting, where the method allowed rapid and simultaneous analysis of the entire gene for sequence variants.13PubMed Central. Evaluation of a fetus at risk for dihydropteridine reductase deficiency by direct mutation analysis using denaturing gradient gel electrophoresis On a DGGE gel, a heterozygous sample produces four bands: the two homoduplexes (each allele paired with itself) and two heteroduplexes (mismatched pairings between the two alleles), each migrating to a different position. A homozygous sample shows only one band. The extra complexity of the banding pattern makes heterozygote detection very sensitive, but the gels are more technically demanding to pour and run than standard agarose.
Practical Tips for Calling Genotypes Confidently
Reading a gel image in a textbook is clean. Reading a real gel in the lab involves judgment. A few practical habits improve accuracy:
- Always include a size ladder: A DNA ladder (molecular weight marker) loaded in at least one lane lets you confirm that your bands are at the expected sizes. Without it, you are guessing whether that band is 300 base pairs or 350.
- Run controls every time: Known genotype controls catch problems that unknowns cannot. If your known heterozygote control shows only one band, you know something went wrong with the PCR or the digest before you even look at your unknowns.
- Watch band intensity: In a heterozygote, both allele bands should be roughly equal in brightness. If one is dramatically fainter, you may be seeing partial allelic dropout or preferential amplification of one allele over the other. Equal intensity supports a genuine heterozygous call.
- Photograph immediately: Bands can diffuse over time, especially in low-concentration agarose gels. Capture your image promptly after staining for the sharpest resolution.
- Consider replicate runs: For any genotype call that has clinical or diagnostic consequences, running the sample a second time (ideally with a different primer set or enzyme) protects against artifacts from a single reaction.
Parentage and Population-Level Genotyping
Beyond calling a genotype at a single locus, gel electrophoresis has historically been used to generate multi-locus profiles for parentage testing and population studies. Multi-locus DNA fingerprinting, which produces a complex pattern of many bands at once, was widely used in paternity and relatedness analysis before single-locus microsatellite panels became standard. A simulation study of multilocus fingerprinting found that even though the assumption of band independence used in statistical calculations was an oversimplification, it had only marginal effects on paternity decisions in practice.14PubMed. Multilocus DNA fingerprinting: the independence problem in quantitative paternity testing
Multi-locus fingerprinting has been applied far beyond humans. Researchers used it to confirm parentage in giant kelp, showing that multi-locus DNA fingerprint profiles were consistent with known parent-offspring relationships in lab-reared individuals.15Journal of Phycology. GENETIC VARIABILITY AND PARENTAGE IN MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) USING MULTI‐LOCUS DNA FINGERPRINTING In these applications, you are not calling a single-locus genotype so much as comparing overall banding-pattern similarity between individuals. Shared bands indicate shared alleles, and the proportion of shared bands estimates genetic relatedness.
When Gel-Based Genotyping Falls Short
Gel electrophoresis remains one of the most accessible and inexpensive genotyping tools available, which is why it persists in teaching labs, field stations, and smaller research groups. But it has real limitations. Resolution on standard agarose caps out around five to ten base pairs of difference for fragments in the few-hundred-base-pair range. Throughput is low compared to automated systems: a single gel handles dozens of samples at most, and every gel must be poured, loaded, run, stained, and photographed by hand.
Capillary electrophoresis systems, which separate DNA in thin glass tubes filled with polymer rather than on a slab gel, offer much higher resolution and throughput. Capillary gradient gel electrophoresis, for instance, can achieve fine molecular discrimination across a broad size range, with results captured automatically by a detector as fragments pass through a laser.16PubMed Central. Capillary Gradient Gel Electrophoresis Modern forensic and clinical labs almost universally use capillary platforms for STR genotyping, because the sizing precision (down to a fraction of a base pair) and automated data analysis eliminate many of the judgment calls that slab gels require.
At the highest throughput end, microarray-based SNP chips and next-generation sequencing bypass electrophoresis altogether, genotyping hundreds of thousands of variants simultaneously. These platforms are the standard for genome-wide association studies and direct-to-consumer genetic testing. But for a lab that needs to genotype one or two loci in a few dozen samples, a PCR and an agarose gel still get the job done in an afternoon for a fraction of the cost. The key is matching the method to the question: if you need to call a known variant at a single locus, gel electrophoresis is often perfectly adequate. If you need genome-wide coverage or single-base-pair precision across many loci, it is time to move to a platform designed for that scale.