Forensic Serology Questions and Answers

Answer: A

Faeces stains are identified from the odour, the presence of undigested matter and vegetable fibres, and from stercobilin, the pigment that gives faeces its brown colour. It can be shown by spectroscopy or a chemical test.

Choline iodide crystals are the test for seminal fluid (Florence test), and rhombic crystals are not the faecal marker. Since only stercobilin belongs, 'All of the above' is wrong. A chemical test for urobilinogen and stercobilin, together with the smell and the fibres seen under a microscope, builds the identification, as no single feature on its own proves that a stain is faecal.

Answer: A

ABO grouping rests on antigens on the surface of the red cells. Antigen A, antigen B, both or neither is present, and the matching antibodies are in the plasma.

Plasma proteins, red cell enzymes and the nuclear chromatin are not the basis of ABO typing. Mature red cells also have no nucleus. Landsteiner discovered the system in 1901, and the blood group of a stain can be worked out from the antigens on the cells or from the antibodies in the serum, which is the forensic use of this knowledge.

Answer: A

Conventional electrophoretic techniques resolve the allelic products of the PGM locus (phosphoglucomutase) into phenotypes 1, 1-2 (the heterozygote) and 2, that is, the two homozygotes and the heterozygote between them. Option A is the only one that lists these three in this form. Options B, C and D repeat a phenotype or give a combination, 1-1 or 2-2, in the place of the heterozygote, so they do not give the three distinct patterns. So A is correct.

Answer: A

The choline of seminal fluid originates from the seminal vesicle. The Florence test, which gives brown needle-like crystals of choline periodide with iodine in potassium iodide, depends on free choline set free from the phosphorylcholine of the seminal vesicle secretion. The Sertoli cells of the testis support sperm formation and the Leydig cells produce testosterone, so neither is a source of choline. The prostate contributes acid phosphatase, citric acid and the P30 protein. So A is correct.

Answer: A

Both statements are correct and the reason explains the assertion. Foetal and adult haemoglobin differ in the globin part of the molecule, and they are separated by the alkali denaturation (Apt) test: adult haemoglobin is denatured by alkali and changes colour, while foetal haemoglobin resists alkali and stays pink. So (R) gives the reason for (A), and option A is correct. Options B and D call a correct statement incorrect, and C calls both incorrect.

Answer: D

The test used to find the species of origin of a blood stain is the immunoprecipitation test, the precipitin test. An antiserum made against the blood protein of one species forms a visible precipitate when it meets blood of that species, so the origin is told. The Teichmann test only shows that the stain is blood by forming haemin crystals, acid phosphatase is a test for semen, and fluorescence is used to find stains. So D is correct.

Answer: B

Amelogenin typing is used to determine sex from a biological specimen. The amelogenin gene occurs on both the X and Y chromosomes and the two copies differ in size, so a PCR product of one size shows a female (XX) and two sizes show a male (XY). Y-STR typing shows only the male line and gives no result for a female, mitochondrial STR typing is used for maternal lineage, and PCR typing is a general method and not a sex test. So B is correct.

Answer: C

The key credits the determination of the richest concentration of choline in the spermatic fluid to Fletcher et al, 1935. Florence, whose name is attached to the choline periodide test for semen, is the person who introduced the test and not the one who measured the concentration, so option D is not the keyed answer. Jhonston and Davis and Wilson are other names in the list, and the key does not support them. The keyed answer is C.

Answer: C

The human genome contains about 6 x 10^9 base pairs per diploid genome, that is, about 3 x 10^9 in each haploid set, which is why the key gives 6 x 10^9. The values 4, 5 and 7 x 10^9 are not the figure for a diploid cell. So C is correct. The haploid human genome has about 3 x 10^9 base pairs, which is the number found in a sperm or egg cell, and a body cell has two sets, so the diploid amount is double that figure. A DNA content of this size also explains why a single cell carries about six picograms of DNA, the value that forensic laboratories use when they estimate how much DNA a sample may contain.

Answer: C

The most common means of identification of a faeces sample is by the detection of urobilinogen (stercobilinogen), the pigment that comes from bile and gives faeces its colour. Ptyalin is the salivary amylase, glycogen is a store of carbohydrate in liver and muscle, and creatin is a muscle constituent, so none of these is the usual marker of faeces. The answer is C. Faeces are identified by finding the pigments that come from the breakdown of bile and by the microscopic remains of food and the bacteria. Urobilinogen is the colourless chromogen that changes into a brown pigment in the gut, and its detection by the Ehrlich aldehyde test is the usual means in a stain.