Forensic Serology Questions and Answers

Answer: A

The method is differential extraction.

Sperm heads are protected by disulphide-rich protein, so they resist lysis. In differential extraction the swab is first digested with proteinase K and detergent without a reducing agent: the vaginal epithelial cells burst, and their DNA goes into the female fraction. The intact sperm are then spun down and lysed with dithiothreitol (DTT) added, giving a male fraction. This separates the victim's and the assailant's DNA.

Microwave-assisted, reflux and Soxhlet extraction are chemical extraction methods for organic compounds, not ways of separating cell types.

Answer: D

mtDNA markers are useful for identifying a victim by comparing samples of maternal relative DNA.

Mitochondrial DNA is inherited only from the mother, without recombination, so a person shares the same mtDNA sequence (in the hypervariable regions HV1 and HV2) with the mother, siblings and other maternal relatives. When no direct reference sample of a missing person exists, mtDNA from a maternal relative can be compared with remains, which also preserve mtDNA well because each cell holds many copies.

Paternity testing needs inheritance from the father, which mtDNA cannot show. 'Race recognition' is not a forensic use of these markers, and species identification relies on conserved mitochondrial genes rather than the human polymorphic markers the question describes.

Answer: A

FUT3 belongs to the Lewis blood group system.

The FUT3 gene encodes a fucosyltransferase (the Lewis enzyme) that adds fucose to precursor chains, producing the Le^a and Le^b antigens. The antigens are made in secretions and adsorbed onto red cells from plasma, which is unusual among blood groups.

The other systems have different genes: Kell is encoded by KEL, Duffy by ACKR1 (formerly DARC), and MNS by GYPA and GYPB.

Answer: D

The Diego system has 21 antigens, according to the figure the key uses.

The Diego system (ISBT symbol DI, system number 010) is carried on band 3, the red-cell anion exchanger AE1 encoded by SLC4A1. It includes the high-incidence antigen Di(b) and the low-incidence Di(a), which is common in East Asian and South American Indigenous populations. Standard references such as the NCBI Blood Groups and Red Cell Antigens text count 21 antigens.

The ISBT list has grown since then, and recent reviews give 22 or 23, so the number depends on the year of the source. Of the options, 25 and 49 are too many and 02 is far too few, so 21 is the only one that matches the published figure.

Answer: C

Teratospermia means sperm with abnormal morphology.

The prefix comes from the Greek teras, a monster or malformation. In a semen analysis it describes a sample in which the proportion of normally shaped sperm falls below the reference value (abnormal heads, midpieces or tails).

The other terms are separate conditions: a low sperm count is oligospermia, no sperm in the ejaculate is azoospermia, and immobile sperm is asthenospermia (with dead sperm, necrospermia).

Answer: D

The assertion is incorrect and the reason is correct. Blood-stained cloth should be dried in shade at ordinary room temperature, away from direct heat and sunlight, so a room heater is the wrong way to dry it. Heat does break down the blood stain and denature its proteins, which is why the reason is a true statement.

Option (A), (B) and (C) therefore all fail: (A) calls the assertion correct, (B) says the reason is wrong, and (C) says the assertion is correct while the reason is not. Only (D), assertion incorrect and reason correct, matches the facts.

Answer: B

The item that is not a component of the Kastle-Meyer test is glacial acetic acid. The reagent is made from phenolphthalein (reduced to colourless phenolphthalin by zinc dust) in potassium hydroxide solution, and then hydrogen peroxide is added to the sample. A pink colour shows haemoglobin's peroxidase-like action.

Phenolphthalein, zinc dust and potassium hydroxide are all in the reagent. Glacial acetic acid is used in a different blood test, Teichmann's crystal test for haemin, so it is the odd one out.

Answer: B

Menstrual blood is confirmed by detecting fibrin degradation products (FDP). Menstrual blood has already clotted and been dissolved by fibrinolysis in the uterus, so it contains FDP, and does not clot, unlike blood from a wound.

Isoenzyme markers and protein markers are used to type blood or to tell species and are not specific to menstrual blood in routine practice. Restriction enzymes are used in DNA profiling to cut DNA, which identifies a person and not the source of the stain. Blood from a wound clots because fibrin forms, and in menstrual discharge the fibrin is broken down in the uterus, so the stain has a distinctive chemistry, which is also why the discharge stays fluid.

Answer: C

The complementary base pairs in DNA are A with T, held by two hydrogen bonds, and G with C, held by three. The booklet marks the three-bond pair with the sign of identity, so the correct choice reads A = T and G ≡ C.

The first choice pairs A with G, both purines, and T with C, both pyrimidines, which cannot be complementary. The second pairs A with C and G with T, which do not match in size or hydrogen bonding. 'All of the above' cannot be right when two of the three are wrong. In DNA the pairing is the reason that the two strands copy each other exactly, and it is the basis of DNA profiling.

Answer: C

Seminal fluid is produced by the seminal vesicles, the prostate and Cowper's gland (bulbourethral gland). The seminal vesicles add most of the volume and fructose, the prostate adds acid phosphatase, and Cowper's glands add a clear lubricating fluid.

The adrenal, pituitary and thyroid glands are endocrine glands that release hormones into the blood and play no part in making seminal fluid. In the laboratory, a seminal stain is shown by tests such as the acid phosphatase test and the Florence test, and by finding spermatozoa under the microscope, and the glands that make the fluid explain why those tests work.