Forensic Serology Questions and Answers

Answer: C

The term is teratozoospermia.

Teratozoospermia describes semen in which sperm are present but an abnormally high proportion have abnormal morphology, such as defects of the head, midpiece or tail. The number of sperm may be normal.

The other terms describe different conditions. Aspermia is the absence of ejaculate, oligospermia is a low sperm count, and necrozoospermia means the sperm are dead or immotile. None of them refers to abnormal shape in sperm that are present in large numbers.

Answer: B

The key's figure is 3,00,000 times.

Luminol reacts with an oxidant in the presence of the haem in blood, which acts as a catalyst, and emits a blue chemiluminescent glow visible in the dark. It is one of the most sensitive presumptive tests for blood, so it is used to reveal stains that have been cleaned or diluted and are invisible to the eye.

Forensic texts put its sensitivity at dilutions of the order of hundreds of thousands of times, and the key takes 3,00,000 (three lakh). Dilutions of 300, 3,000 or 30,000 times are well within the reach of ordinary presumptive tests and understate luminol's sensitivity.

Answer: A

Cut hair is not used for nuclear DNA profiling.

Nuclear DNA is found in the cells of the root and its sheath. The hair shaft itself is made of keratinised dead cells with little or no nuclear DNA. A hair that has been cut has no root, so it can usually be analysed only for mitochondrial DNA, which survives in the shaft.

A plucked hair in the growing phase carries a root with plenty of cellular material, and fallen or naturally shed hairs may still carry some root tissue, so they can sometimes give a nuclear profile. Kinky hair describes the hair's shape, not whether it has a root.

Answer: B

The test for urine is the Jaffe test, spelled 'Jaffer' in the paper.

Urine contains a high concentration of creatinine. In the Jaffe reaction, creatinine reacts with picric acid in alkaline solution to give an orange-red colour, so a stain giving a strong Jaffe reaction is presumptively urine.

The Kastle-Meyer (phenolphthalein) test is a presumptive test for blood, the Teichmann test is a crystal test confirming blood (haemin crystals), and the Phadebas test detects amylase and is used for saliva.

Answer: A

Such STRs are called simple repeats.

Short tandem repeats are classified by the structure of their repeat units. In a simple repeat, every unit has the same length and the same sequence, for example the four-base unit AATG repeated over and over, as at the locus TPOX.

A compound repeat contains two or more different simple repeats next to each other, and a complex repeat has several repeat blocks of different lengths together with variable intervening sequences, as at D21S11. 'Complex hypervariable' repeats are the most irregular of all, so they do not fit identical units.

Answer: B

The key's answer is ELISA.

Neutralization, precipitation and agglutination are the classical types of antigen-antibody reaction: an antibody neutralises a toxin or virus, links soluble antigens into a visible precipitate, or clumps particulate antigens such as red cells.

ELISA (enzyme-linked immunosorbent assay) is a laboratory technique that uses an antigen-antibody binding step and an enzyme label to measure an antigen or antibody. It relies on the reaction but is an assay method rather than one of the kinds of reaction, which is the distinction the question draws.

Answer: B

Forensic STR loci are chosen to be genetically not linked.

The strength of a DNA profile comes from multiplying the frequencies of the alleles at each locus (the product rule). That multiplication is valid only if the loci are inherited independently of one another, which requires them to lie on different chromosomes or far apart on the same one.

Linked loci are inherited together, so multiplying their frequencies would overstate the rarity of a profile. Size matters for other reasons, such as using shorter amplicons for degraded DNA, but it is not the selection rule the question asks about.

Answer: A

The key's answer is starch into glucose.

Salivary amylase (ptyalin) begins the digestion of starch in the mouth by breaking its alpha-1,4 glycosidic bonds. Strictly, the main product is maltose, a disaccharide made of two glucose units, along with dextrins, rather than free glucose, but maltose is not among the options, and the key takes the glucose-based sugar as the product.

Starch is not turned into glycogen (that is synthesis in the liver and muscle) or into fats, and starch is already a carbohydrate, so 'into carbohydrates' says nothing. In forensic work, amylase activity is the basis of presumptive tests for saliva stains.

Answer: D

The test that is not for semen is Schiff's test.

Schiff's reagent (decolourised fuchsin) is a chemical test for aldehydes, which restore its magenta colour. It has no role in identifying seminal stains.

The Florence test gives brown crystals of choline periodide with iodine in potassium iodide, and Barberio's test gives yellow needles of spermine picrate with picric acid; both are classical crystal tests for semen. The ammonium molybdate test is also listed in forensic texts among the chemical tests applied to seminal stains, so of the four, Schiff's is the odd one out.

Answer: A

P-30 is secreted by the prostate gland (printed 'Prostrate').

P30 is the forensic name for prostate-specific antigen (PSA), a glycoprotein of molecular weight about 30 kDa produced by the epithelial cells of the prostate and found in high concentration in seminal fluid. Because it is present whether or not the man has sperm, testing for P30 (by immunodiffusion or rapid immunochromatographic cards) is used to confirm semen in sexual assault cases involving vasectomised or azoospermic men.

Salivary and parotid glands produce amylase, the marker for saliva, and the pancreas produces digestive enzymes and hormones. None of them produces P30.