DNA polymorphisms, linkage and DNA fingerprinting
Small DNA differences between people are inherited markers: they follow genes through families and can identify a person from a drop of blood.
Key facts
- A polymorphism is a DNA variant common in the population (more than 1%), usually harmless.
- SNP: a single-base difference, the most frequent kind (millions in each genome).
- Microsatellites (STRs) are short repeats of 2–6 bases whose number of copies varies between people; VNTRs (minisatellites) are longer repeats.
- RFLP: a variant that creates or removes a restriction-enzyme site, so the DNA is cut into fragments of different length.
- DNA fingerprinting (profiling) compares the STR pattern at many loci; the combination is practically unique except in identical twins.
In detail
- Each person inherits one allele of each marker from each parent, so profiles are used to test paternity and identify remains.
- Linkage analysis: loci close together on a chromosome are inherited together more often than chance, because recombination rarely separates them; following a marker through a family tracks a nearby disease gene even before the gene is known.
- Genome-wide association studies compare SNPs in people with and without a disease to find regions linked to common, multifactorial disorders.
- The Human Genome Project (completed in 2003) produced a reference sequence of about 3 billion base pairs, with about 20 000 protein-coding genes.
- Microsatellite instability (changing STR lengths within a tumour) is a sign of defective mismatch repair.
- Also called
- RFLP, STR and SNP, DNA profiling
Clinical notes
- Forensic medicine and paternity testing use STR panels; the weight of the evidence is the probability of a chance match.
- Testing colorectal and endometrial cancers for microsatellite instability screens for Lynch syndrome and helps choose immunotherapy.
- Before direct sequencing, RFLP linkage in a family with an affected child allowed prenatal diagnosis.
Test yourself
Think of the answer, then tap the question to see it.
What makes STR profiles useful for identifying a person?
The number of repeats at each locus varies between people; combining many loci gives a pattern that is practically unique.
What is the basis of linkage analysis?
Loci close together on a chromosome are rarely separated by recombination, so a marker travels with a nearby disease gene in a family.
What does microsatellite instability in a tumour suggest?
Defective DNA mismatch repair, as in Lynch syndrome.
Connections
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Microsatellite (National Human Genome Research Institute (NHGRI), 2026); Restriction Fragment Length Polymorphism (RFLP) (National Human Genome Research Institute (NHGRI), 2026); DNA Fingerprinting (National Human Genome Research Institute (NHGRI), 2026); Linkage (National Human Genome Research Institute (NHGRI), 2026); Human Genome Project (National Human Genome Research Institute (NHGRI), 2026).
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Representative model, for educational purposes only.