DNA fingerprinting is a technique used for the detection of

Correct — B, (b) Disputed parentage.

The discrimination here is one line long: DNA fingerprinting IDENTIFIES A PERSON, OR A RELATIONSHIP BETWEEN PERSONS. It does not diagnose a disease. Three of the four options name diseases; only one names an identity question.

WHY IT SETTLES PARENTAGE. Some 99·9 per cent of the human genome is the same in any two people. The technique deliberately ignores all of it and reads instead the short stretches of non-coding DNA that repeat over and over — minisatellites, or variable number tandem repeats, and in modern practice the shorter STRs, short tandem repeats. The NUMBER of repeats at each such site varies enormously from person to person, so the combined pattern across a dozen or so sites is effectively unique to an individual.

And because those sites are inherited in the ordinary Mendelian way, the pattern is not merely unique — it is HERITABLE. Every band in a child's profile must have come from one parent or the other. A child's profile is therefore, band by band, half the mother's and half the father's, and a man whose profile cannot supply the bands the mother did not is excluded. That is exactly what a parentage dispute asks, and no other technique answers it so directly.

THE HISTORY, briefly. The method was devised by Alec Jeffreys at the University of Leicester in 1984, and its very first applications were an immigration case and a paternity case before it was ever used to convict anyone. In India the technique was established by Lalji Singh at the Centre for Cellular and Molecular Biology in Hyderabad, and the Centre for DNA Fingerprinting and Diagnostics was set up in the same city.

The stem prints DNA and AIDS in capitals and Alzheimer's with an apostrophe, and ends without punctuation, as this booklet's short definitional items do throughout.

Why the others are wrong

• (a)Alzheimer's disease — A disease, and one diagnosed by entirely different means — clinical assessment of memory and cognition, brain imaging, and markers in cerebrospinal fluid. There IS genetics in Alzheimer's disease: the APOE ε4 allele raises risk, and rare early-onset familial forms are caused by mutations in specific genes. But finding those means SEQUENCING a named gene to see what it says. DNA fingerprinting does the opposite — it deliberately reads non-coding repeat regions that code for nothing and therefore carry no information about health at all. That is a feature, not a limitation: it is precisely why a profile can be held on a database for identification without disclosing anything medical about the person it belongs to.
• (c)AIDS — The most tempting of the three, because HIV testing genuinely does involve nucleic acids and the words sound adjacent. The distinction is whose DNA is being read. AIDS is caused by infection with HIV, and it is detected by looking for the VIRUS or the body's response to it — antibody tests by ELISA with a confirmatory test, and nucleic acid amplification to measure viral load. DNA fingerprinting reads the HOST's own genome, at sites that are the same in a healthy person and a sick one. A person's DNA profile is unchanged by any infection they acquire, which is what makes it usable as an identifier for life.
• (d)Yellow fever — A viral disease transmitted by Aedes mosquitoes and confined to parts of Africa and South America, diagnosed by serology or by detecting viral nucleic acid, and prevented by a long-established live attenuated vaccine. It has no connection with human identification of any kind. Its presence on the page is a reminder of how the option set is built: three diseases of three different types — neurodegenerative, immunological and infectious — surrounding the one option that is not a disease at all. A candidate who has fixed on the idea that DNA fingerprinting is about IDENTITY rather than illness does not have to know anything about any of the three.

DNA FINGERPRINTING, or DNA profiling, identifies an individual from the pattern of variation in the repetitive, non-coding parts of their genome.

THE PRINCIPLE. Human genomes are about 99·9 per cent identical, so nothing useful for identification lies in the genes themselves. The variation is concentrated in stretches of DNA where a short sequence repeats head to tail many times over — minisatellites or VNTRs, and the shorter microsatellites or STRs used in modern kits. What varies between people is the NUMBER of repeats at each site. Read a dozen or more such sites at once and the combined pattern is, in practice, unique to one person; identical twins are the standing exception, because they arose from a single fertilised egg.

THE METHOD, in outline. The original technique digested the DNA with a restriction enzyme, separated the fragments by size using gel electrophoresis, transferred them to a membrane by SOUTHERN BLOTTING and revealed the repeat-containing fragments with a labelled probe, producing the barcode-like image the word 'fingerprint' comes from. Modern practice amplifies a standard panel of STR sites by PCR and sizes them by capillary electrophoresis, which needs far less material and works on degraded samples.

INHERITANCE IS WHAT MAKES IT ANSWER PARENTAGE. Repeat sites obey ordinary Mendelian inheritance, so a child carries one allele from each parent at every site. A child's profile can be read as half its mother's and half its father's, and a man who cannot account for the paternal half is excluded absolutely. Exclusion is the stronger result; inclusion is stated as a probability.

WHAT IT IS USED FOR. Parentage and maternity disputes, forensic identification of suspects and of victims, identification of the dead after disasters, immigration and inheritance disputes, wildlife forensics, and pedigree verification in plant and animal breeding. India regulates the use of DNA evidence through the courts and the Centre for DNA Fingerprinting and Diagnostics at Hyderabad, an autonomous institute founded for this purpose.

WHAT IT IS NOT. It is not a diagnostic test. Diagnosing an inherited disorder means sequencing a particular gene to read its message; a DNA profile deliberately avoids messages, sampling only sites that code for nothing.

Key facts

• DNA fingerprinting establishes IDENTITY or a biological RELATIONSHIP; it is not a diagnostic test for any disease.
• It reads variation in non-coding repetitive DNA — minisatellites or VNTRs originally, short tandem repeats (STRs) in modern kits — where the number of repeats differs greatly between individuals.
• Human genomes are about 99·9 per cent identical, so the coding sequence carries almost nothing useful for telling two people apart.
• Repeat sites are inherited in the ordinary Mendelian way, so every band in a child's profile must come from one parent or the other — which is what settles a parentage dispute.
• Identical twins, having arisen from one fertilised egg, share a DNA profile; no one else does.
• The technique was devised by Alec Jeffreys at the University of Leicester in 1984, and its earliest uses were an immigration case and a paternity case.
• In India it was established by Lalji Singh at the Centre for Cellular and Molecular Biology, Hyderabad; the Centre for DNA Fingerprinting and Diagnostics was founded in the same city.
• Applications extend to forensic identification, disaster victim identification, inheritance and immigration disputes, wildlife forensics and pedigree verification in breeding.

Common traps

• Treating DNA fingerprinting as a way of diagnosing genetic disease. Diagnosis reads a gene's message; profiling deliberately reads non-coding sites that carry no message.
• Confusing reading the HOST's genome with detecting a PATHOGEN's nucleic acid, which is what HIV and other infection tests do.
• Assuming the profile can change — with age, illness, diet or treatment. It is fixed from conception and identical in every nucleated cell of the body.
• Forgetting that identical twins share a profile, which is the single standing exception to individual uniqueness.
• Mixing up the founders: Alec Jeffreys devised the technique in Britain in 1984; Lalji Singh established it in India at the CCMB in Hyderabad.

• practice — not a real PYQ

  DNA fingerprinting depends chiefly on variation between individuals in

  • (a)the coding sequences of structural genes
  • (b)the number of repeats in short non-coding tandem-repeat regions
  • (c)the total number of chromosomes
  • (d)the sequence of ribosomal RNA genes

  Answer(b) the number of repeats in short non-coding tandem-repeat regions — the coding sequence is nearly identical between any two people, and the chromosome number is the same in all normal individuals, so neither can tell them apart. The usable variation lies in how many times a short sequence repeats at each of a panel of non-coding sites, and it is that pattern which the profile records.

• practice — not a real PYQ

  Two individuals are found to have indistinguishable DNA profiles. They are most likely to be

  • (a)brother and sister
  • (b)parent and child
  • (c)identical twins
  • (d)first cousins

  Answer(c) identical twins — monozygotic twins develop from a single fertilised egg and so carry the same genome, which is the one situation in which two people share a profile. Siblings, cousins and a parent and child share only some alleles: a child matches each parent at exactly one allele per site, which shows relationship but produces a clearly different overall pattern.