DNA Polymorphism and Fingerprinting

What creates a molecular DNA profile

Although human genomes share most of their sequence, many genomic positions differ among individuals. A DNA profile measures selected variable regions rather than sequencing an entire genome. Differences can involve individual nucleotides, short insertions or deletions and copy numbers of repeated DNA segments. Comparing several independently inherited markers provides a characteristic pattern of inherited variation that can help distinguish biological samples.

Repetitive DNA, satellite DNA and tandem repeats

Repetitive DNA is a broad category containing sequences found multiple times. Satellite DNA refers to particular repeat-rich DNA fractions historically distinguished by their behaviour during density-gradient separation; not all repetitive DNA is classified as satellite DNA. Tandem repeats consist of short sequence units arranged immediately one after another. Variable number tandem repeats (VNTRs), especially minisatellite loci in the historical method, differ in repeat-copy count between chromosomes and individuals. The copy number affects the length of the DNA fragment containing the repeat.

Genetic polymorphism is a population concept

A polymorphism describes genetic variants occurring in populations at appreciable frequency. Older descriptions sometimes use a frequency threshold of 1% for an allele, but a measured DNA sequence variant can exist below that level and still be relevant to analysis. Variants in noncoding regions often have less direct effects on protein sequence and can accumulate, but it is incorrect to assume that every noncoding change has no biological effect.

Traditional VNTR profiling with a labelled probe

The classical method developed in the 1980s isolates DNA, digests it with restriction enzymes, separates fragments by gel electrophoresis and transfers them onto a membrane by Southern blotting. A labelled probe complementary to a VNTR-containing region binds selected fragments. Detection reveals a pattern of bands representing fragment sizes. If samples have different VNTR repeat counts, their bands can occur at different gel positions. The image must be interpreted with controls, sample quality and the measured marker set in mind.

Modern profiling and PCR amplification

Contemporary forensic identification often amplifies selected short tandem repeat (STR) loci by polymerase chain reaction and detects fragment lengths using instrument-based methods. PCR can make sufficient copies from a small amount of DNA, but low-template samples, contamination and mixtures can complicate interpretation. Microsatellites, frequently used as STR markers, contain short tandem motifs; the older Southern-VNTR approach often targeted longer minisatellite repeats. The two technologies rely on related principles of length variation but are not identical laboratory workflows.

Interpreting banding patterns

A match at one region is not proof of a unique identity, because unrelated people can share alleles at that locus. Using multiple informative loci and appropriate statistical calculations gives more discriminating power. Biological relatives share more sequence variants on average; identical twins often share essentially the same ordinary inherited DNA profile, although rare later mutations can arise. A laboratory result supports an evidential comparison, not an automatic legal or personal conclusion.

Beyond identification

DNA variation is useful in parentage analysis when combined with appropriate genetic inheritance models, in wildlife and biodiversity studies, and in population history research. Ethical handling, consent, database security and chain-of-custody procedures are essential wherever genetic profiles may affect individuals. The scientific method of measuring inherited variation is distinct from assumptions about behaviour, identity or medical status.

How repeat count changes a DNA fragment

At a VNTR locus, a repeated unit is present several times in succession. If the repeat is 20 bp long and two alleles contain six and ten copies, their repeat regions contain 120 bp and 200 bp respectively. If their flanking sequences are otherwise identical, they differ in total length by 80 bp. Restriction fragments containing these alleles may therefore travel to different positions during electrophoresis. The actual band position depends on the full fragment length, not just the repeat count; the flanking regions and the placement of enzyme cutting sites matter.

In a traditional Southern-blot approach, DNA fragments are separated according to size and then transferred to a membrane for probe hybridisation. Only fragments containing sequences sufficiently complementary to the labelled probe produce the detected pattern. Modern STR workflows frequently amplify selected small repeat loci using PCR primers that bind the flanking regions, followed by sizing of the amplified products. A PCR product's size thus includes both repeat and flanking DNA, and validated reference markers are used to infer allele repeat numbers.

Why matching patterns need statistical reasoning

Two individuals may share a particular repeat length by chance, especially at a locus with few common alleles. Additional loci increase the discriminating power, but calculations should account for allele frequencies and the relevant population structure rather than simply multiplying guessed probabilities. A biological relative also has a higher probability of sharing alleles because of inheritance. Identical twins typically have almost indistinguishable inherited standard profiles. DNA-profile similarity is therefore an evidence statement requiring a specified marker panel and interpretation method, not a direct measurement of personal identity.

Laboratory practice also matters. Mixed samples can generate more than two alleles at a locus, degraded material may cause incomplete amplification, and contamination can produce misleading peaks. Analysts use controls, validated thresholds and repeat testing where appropriate. These limitations do not negate the value of DNA profiling, but they explain why a scientific conclusion should state the quality and scope of the evidence being compared.

A simplified sequence-identity estimate illustrates the scale of human genetic differences. If two genomes are described as 99.9% identical at an aligned set of approximately 3.0×10⁹ single-base positions, the different fraction is 0.1% = 0.001. Multiplying 3.0×10⁹ positions by 0.001 gives approximately 3.0×10⁶ differing positions. This is a model based on aligned single-base comparisons: real genome-to-genome variation also includes structural changes, gaps, repeated sequences and differences in how regions are compared. A modern profile does not need to inspect all three million illustrative differences; selected polymorphic markers are sufficient for many types of comparison.

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