Forensic Serology Questions and Answers

Answer: C

The key bases disputed paternity on the Hardy-Weinberg principle.

Paternity testing compares the genetic markers of mother, child and alleged father. The child must carry an obligate paternal allele at each locus; if the man lacks it, he is excluded. If he has it, the strength of the evidence is expressed as a paternity index and probability of paternity, which require the frequency of that allele in the population. Those frequencies, and the genotype probabilities built from them, are calculated on the assumption of Hardy-Weinberg equilibrium.

The other options are unrelated: Nysten's law describes the order in which rigor mortis appears, Casper's formula compares the rate of decomposition in air, water and soil, and Locard's principle concerns the exchange of trace evidence on contact.

Answer: B

The sequence is isolation, amplification, visualisation and statistical interpretation.

In STR-based DNA profiling, DNA is first extracted (isolated) from the biological sample and quantified. The target loci are then amplified by PCR. The amplified fragments are separated and detected, usually by capillary electrophoresis with fluorescent labels, which visualises the profile as peaks. Finally the profile is compared with reference samples and, if they match, the significance is assessed statistically, for example as a random match probability.

Interpretation cannot come before visualisation, and amplified DNA cannot be visualised before it is amplified, so the other sequences are out of order.

Answer: C

The FDP test is used to identify menstrual blood.

Menstrual blood contains fibrin degradation products (FDP) at high levels, because fibrinolysis in the uterine lining breaks down fibrin, which is why menstrual blood usually does not clot. Immunological tests for FDP, such as latex agglutination, can therefore distinguish menstrual blood from ordinary venous blood, which matters in sexual assault cases where a woman claims the blood on clothing came from injury.

Semen is identified by acid phosphatase, PSA (P30) and sperm, vomit by its gastric contents and acidity, and faecal matter by urobilinogen and food residues.

Answer: D

The item that does not act as a PCR inhibitor is violet blue dye, as the key has it.

PCR inhibitors are substances co-extracted with DNA that interfere with the polymerase or the DNA itself. Well-known examples in forensic samples are haem (haematin) from blood, melanin (printed 'Melamin') from hair and skin, indigo dye from denim, humic acid from soil, collagen from bone, and calcium.

Haem, melanin and indigo are therefore all established inhibitors that forensic DNA laboratories must deal with, for example by extra purification or dilution. The violet blue dye of the last option is not among the standard inhibitors listed in forensic DNA texts.

Answer: D

All three, (I), (II) and (III), are correct.

When a questioned DNA profile is compared with a reference profile, the examiner's conclusion takes one of three forms. Matching (inclusion): the profiles are the same at all compared loci, and a statistical weight is attached. Exclusion: the profiles differ, so the reference person cannot be the source. Inconclusive: the profile is partial, degraded, a complex mixture or affected by artefacts, so neither inclusion nor exclusion can be stated.

Standard interpretation guidelines, such as those of SWGDAM, recognise all three. Options listing only two leave out a legitimate category.

Answer: C

The inheritance of blood groups was shown by von Dungern and Hirszfeld (printed 'Dungern and Hirzfield').

Karl Landsteiner discovered the ABO blood groups in 1900-01, but the demonstration that they are inherited according to Mendelian rules came from Emil von Dungern and Ludwik Hirszfeld in 1910, from family studies. Their work laid the foundation for using blood groups in disputed paternity cases. Earlier, in 1908, Epstein and Ottenberg had suggested that blood groups might be inherited.

Landsteiner is the tempting choice because he discovered the groups, and Lutheran is the name of a blood group system, not a scientist.

Answer: D

The sex of a biological sample is determined by amelogenin typing.

The amelogenin gene, which codes for a tooth enamel protein, occurs on both the X and Y chromosomes, with a 6-base-pair deletion in the X copy. PCR amplification gives a 106 bp product from the X and a 112 bp product from the Y with the usual primers. A female shows one peak (X), a male two (X and Y). Amelogenin is included in almost every STR kit.

Mitochondrial DNA is inherited from the mother and says nothing about sex. RFLP typing and PCR are general methods; PCR is used for amelogenin, but by itself it is not the sex test.

Answer: C

Anti-H lectin (printed 'Section') is prepared from Ulex europaeus seeds.

The seed extract of gorse, Ulex europaeus, contains a lectin that reacts specifically with the H antigen, the precursor substance of the ABO system. It is used to identify group O and to detect the Bombay phenotype, which lacks H.

Dolichos biflorus gives anti-A1 lectin. Semecarpus anacardium is the marking nut, whose juice causes skin lesions, and Ricinus communis is the castor plant, source of the toxin ricin. Neither provides anti-H.

Answer: A

With the FTA method, the DNA stays on the paper.

FTA cards are treated with chemicals that lyse cells, denature proteins and protect DNA from nucleases and bacteria. The DNA binds to the paper matrix, where it stays stable at room temperature. For analysis, a small punch is washed to remove inhibitors and put straight into the PCR, so the DNA is not eluted off.

The other options describe what FTA avoids: separate elution, a conventional extraction step and the loss of the DNA's usefulness.

Answer: A

Minisatellites are short non-coding repetitive sequences present throughout the chromosome.

Minisatellites, or variable number tandem repeats (VNTRs), are repeat units about 10 to 60 base pairs long, repeated in tandem many times at loci scattered across the genome. The number of repeats varies greatly between people, which made them the basis of Alec Jeffreys's first DNA fingerprinting.

They are non-coding, so 'coding repetitive regions' and 'sequences within the genes' are wrong, and they are not defined by the secondary constriction of chromosomes.