Forensic Serology Questions and Answers

Answer: B

The key gives 6 months.

Acid phosphatase is an enzyme secreted by the prostate in large amounts, and its activity in a stain is the basis of the presumptive test for semen. Enzyme activity decays with time, faster when warm, so the question asks for the time in which half of the activity is lost when semen is kept at body temperature, 37 °C.

I could not find a published half life for acid phosphatase in semen at 37 °C, so I cannot confirm the figure of 6 months, and I found nothing that shows it wrong. The key is the University's; the other options (6 weeks, 12 and 24 months) are not supported by anything I found either.

Answer: B

The molecular weight of prostate specific antigen is about 30 kDa.

PSA is a glycoprotein secreted by the prostate into seminal fluid. In forensic serology it was first described as p30, a name that comes from its molecular weight of about 30 kilodaltons (measured values run to about 33-34 kDa). Its detection, for example by immunochromatographic test strips, confirms the presence of semen even when no sperm are found.

25, 35 and 40 kDa are not the values given for PSA; the p30 name is the easy way to remember the correct one.

Answer: C

Isoamyl alcohol is added to reduce foaming.

In organic extraction the lysed sample is shaken with phenol-chloroform-isoamyl alcohol, usually in the ratio 25:24:1. Isoamyl alcohol acts as an antifoaming agent during mixing, and it also helps the aqueous and organic phases separate cleanly, so the DNA in the upper aqueous phase can be drawn off.

It does not concentrate DNA; that is done later by ethanol precipitation. Extracting proteins out of the aqueous phase is the job of phenol, which denatures them and takes them into the organic phase. Quality improves as a result of the whole procedure, not because of the isoamyl alcohol.

Answer: D

The control region of the mitochondrial genome is the D loop.

The D (displacement) loop is a non-coding stretch of about 1,100 base pairs where a short third strand of DNA displaces one strand of the double helix. It contains the origin of heavy-strand replication and the promoters for transcription. Its hypervariable regions HV1 and HV2 are the sequences compared in forensic mitochondrial DNA typing of hair shafts, bones and teeth.

H and L refer to the heavy and light strands of mtDNA, not to a loop, and 'A loop' is not a mitochondrial feature.

Answer: B

The key gives ESD (esterase D).

Red cell enzymes such as PGM, ESD, GLO and EAP are typed by separating their isoenzymes on a gel and then locating the bands with a stain specific to each enzyme. For PGM and GLO the usual stain is a coupled reaction ending in a coloured formazan, which is visible in ordinary light. Esterase D is an enzyme that splits esters, so it can be located with a substrate that gives a fluorescent product, and its bands are then seen under ultraviolet light.

I could not confirm the point from a source that I could open, and EAP is also an enzyme that acts on esters, so the key rests on the usual textbook statement for ESD; nothing I found shows another option to be the answer.

Answer: B

The multisystem was developed by Wraxall and Stolorow.

Brian Wraxall and Mark Stolorow (1986) described a single electrophoretic run that separated and typed several polymorphic enzymes from one small bloodstain at once, among them phosphoglucomutase, esterase D and glyoxalase I. It saved scarce stain material and became widely used in crime laboratories before DNA typing.

Brian Culliford wrote the earlier standard manual on typing individual bloodstain proteins and enzymes one system at a time, and Rothwell and Brinkmann are not credited with the multisystem.

Answer: A

The ABO group of saliva is determined by the absorption inhibition technique.

About 80% of people are secretors, whose saliva contains water-soluble A, B and H substances. In absorption inhibition, the extract is mixed with a known antiserum: if the matching blood group substance is present it neutralises the antibody, which then fails to agglutinate indicator red cells. The loss of agglutination identifies the group substance, and it works well on fluids like saliva that hold soluble antigen.

Absorption elution is preferred for dried bloodstains and hair, where the antigen is bound to cells. Lattes' technique detects the antibodies in a bloodstain rather than antigens in saliva, and mixed agglutination is suited to antigens fixed on fibres and cells.

Answer: D

Prostatic secretion is acidic in nature.

The prostate contributes about a quarter to a third of the seminal volume, a thin milky fluid with a pH of about 6.5. It is rich in citric acid, acid phosphatase, zinc and PSA, and the high acid phosphatase is the basis of the classic presumptive test for semen.

The seminal vesicles add a larger, alkaline fluid, which is why whole semen is slightly alkaline (pH about 7.2-8.0). Taken alone, the prostatic secretion is not alkaline, neutral or variable.

Answer: D

Serum is the clear fluid left after removal of fibrinogen.

When blood clots, soluble fibrinogen is converted into a mesh of fibrin that traps the cells. The clot retracts and squeezes out a clear straw-coloured fluid: serum. Serum is therefore plasma minus fibrinogen and the other clotting factors used up in the clot.

Gamma globulins (antibodies) remain in serum, which is why serum is used to test for antibodies. Agglutinogens are the antigens on red cells and agglutinins are the antibodies in plasma; removing either does not turn plasma into serum.

Answer: A

In seminal stain examination the DNA comes from the sperm.

A spermatozoon carries its DNA in the nucleus of its head. Differential extraction exploits this: the victim's epithelial cells are lysed first, and the tough sperm heads are then broken with a reducing agent such as DTT to obtain the male DNA separately.

The seminal fluid itself is a largely cell-free secretion; in a man with no sperm, any DNA has to come from the few epithelial and white cells it carries. The sperm tail holds no nucleus, and prostate-specific antigen is a protein, not a source of DNA.